《免疫学》（英文版） Chapter 16 Hypersensitive Reactions

366 paRI I Immune Effector mechanisms (a)FcERI: High-affinity IgE receptor Low-affinity IgE receptor Soluble(s CD23 ig-lik COOH Extracellular H2N NH, y lavage aPlasma &membrane Cytoplasm COOHCOOH ∠mAM COOHCOOH FIGURE 16-4 Schematic diagrams of the high-affinity FCERI and CD3 complex of the T-cell receptor.(b)The low-affinity receptor is un- low-affinity FCERll receptors that bind the Fc region of igE (a) Each y usual because it is oriented in the membrane with its NH2-terminus chain of the high-affinity receptor contains an ITAM, a motif also pre- directed toward the cell interior and its COoH-terminus directed to- sent in the Ig-a/Ig-B heterodimer of the B-cell receptor and in the ward the extracellular space classes of fcer been identified, designated FcERI and fceril, brane receptors that have this motif are CD3 and the asso- which are expressed by different cell types and differ by 1000- ciated s chains of the T-cell receptor complex (see Figure fold in their affinity for igE 10-10)and the Ig-a/Ig-B chains associated with membran immunoglobulin on B cells(see Figure 11-7). The ITAM HIGH-AFFINITY RECEPTOR(FCeRI) Mast cells and baso- motif on these three receptors interacts with protein tyrosine phils express FceRl, which binds IgE with a high affinity(Kp kinases to transduce an activating signal to the cell. aller 1-2 X 10 M). The high affinity of this receptor enables mediated crosslinkage of the bound ige results in aggrega- it to bind ige despite the low serum concentration of Ige tion of the FceRI receptors and rapid tyrosine phosphoryla (1 x10-7)Between 40,000 and 90,000 FceRI molecules have tion, which initiates the process of mast-cell degranulation been shown to be present on a human basopl The role of FcERI in anaphylaxis is confirmed by experiments The FcERI receptor contains four polypeptide chains: an conducted in mice that lack FcERI. These mice have normal c and a B chain and two identical disulfide -linked y chains levels of mast cells but are resistant to localized and systemic (Figure 16-4a). The external region of the a chain contains anaphylaxis two domains of 90 amino acids that are homologous with the immunoglobulin-fold structure, placing the molecule in the LOW-AFFINITY RECEPTOR(FCeRII) The other IgE recep- immunoglobulin superfamily (see Figure 4-19). FcERI inter- tor, designated FcERIl (or CD23), is specific for the CH3/ acts with the CH3/CH3 and CH4/CH4 domains of the ige Ch3 domain of ige and has a lower affinity for ige (Kp molecule via the two Ig-like domains of the a chain. The p 1x 10-M)than does FcERI (Figure 16-4b). The FcErll chain spans the plasma membrane four times and is thought receptor appears to play a variety of roles in regulating the to link the a chain to the y homodimer. The disulfide-linked intensity of the igE response. Allergen crosslinkage of igE y chains extend a considerable distance into the cytoplasm. bound to FceRII has been shown to activate B cells, alveolar Each y chain has a conserved sequence in its cytosolic do- macrophages, and eosinophils. When this receptor is blocked main known as an immunoreceptor tyrosine-based activa- with monoclonal antibodies, IgE secretion by b cells is tion motif (ITAM). As described earlier, two other mem- diminished. a soluble form of FcERIl (or sCD23), which is

classes of FcR been identified, designated FcRI and FcRII, which are expressed by different cell types and differ by 1000- fold in their affinity for IgE. HIGH-AFFINITY RECEPTOR (FCRI) Mast cells and baso￾phils express FcRI, which binds IgE with a high affinity (KD = 1–2  10–9 M). The high affinity of this receptor enables it to bind IgE despite the low serum concentration of IgE (1  10–7). Between 40,000 and 90,000 FcRI molecules have been shown to be present on a human basophil. The FcRI receptor contains four polypeptide chains: an  and a chain and two identical disulfide-linked  chains (Figure 16-4a). The external region of the  chain contains two domains of 90 amino acids that are homologous with the immunoglobulin-fold structure, placing the molecule in the immunoglobulin superfamily (see Figure 4-19). FcRI inter￾acts with the CH3/CH3 and CH4/CH4 domains of the IgE molecule via the two Ig-like domains of the  chain. The chain spans the plasma membrane four times and is thought to link the  chain to the  homodimer. The disulfide-linked  chains extend a considerable distance into the cytoplasm. Each  chain has a conserved sequence in its cytosolic do￾main known as an immunoreceptor tyrosine-based activa￾tion motif (ITAM). As described earlier, two other mem￾brane receptors that have this motif are CD3 and the asso￾ciated  chains of the T-cell receptor complex (see Figure 10-10) and the Ig-/Ig- chains associated with membrane immunoglobulin on B cells (see Figure 11-7). The ITAM motif on these three receptors interacts with protein tyrosine kinases to transduce an activating signal to the cell. Allergen￾mediated crosslinkage of the bound IgE results in aggrega￾tion of the FcRI receptors and rapid tyrosine phosphoryla￾tion, which initiates the process of mast-cell degranulation. The role of FcRI in anaphylaxis is confirmed by experiments conducted in mice that lack FcRI. These mice have normal levels of mast cells but are resistant to localized and systemic anaphylaxis. LOW-AFFINITY RECEPTOR (FCRII) The other IgE recep￾tor, designated FcRII (or CD23), is specific for the CH3/ CH3 domain of IgE and has a lower affinity for IgE (KD = 1  10–6M) than does FcRI (Figure 16-4b). The FcRII receptor appears to play a variety of roles in regulating the intensity of the IgE response. Allergen crosslinkage of IgE bound to FcRII has been shown to activate B cells, alveolar macrophages, and eosinophils. When this receptor is blocked with monoclonal antibodies, IgE secretion by B cells is diminished. A soluble form of FcRII (or sCD23), which is 366 PART III Immune Effector Mechanisms NH2 Ig-like domains Extracellular space Plasma membrane Cytoplasm ITAM S S COOH COOH COOH COOH NH2 α β S S γ γ H2N NH2 S S NH2 Soluble CD23 S S S S S S COOH Proteolytic cleavage (a) FcεRI: High-affinity IgE receptor (b) FcεRII (CD23): Low-affinity IgE receptor FIGURE 16-4 Schematic diagrams of the high-affinity FcRI and low-affinity FcRII receptors that bind the Fc region of IgE. (a) Each  chain of the high-affinity receptor contains an ITAM, a motif also pre￾sent in the Ig-/Ig- heterodimer of the B-cell receptor and in the CD3 complex of the T-cell receptor. (b) The low-affinity receptor is un￾usual because it is oriented in the membrane with its NH2-terminus directed toward the cell interior and its COOH-terminus directed to￾ward the extracellular space.

Hypersensitive Reactions CHAPTER 16 generated by autoproteolysis of the membrane receptor, has (a) Allergen crosslinkage of (c)Chemical crosslinkage ingly, atopic individuals have higher levels of CD23 on their Ab nd lg been shown to enhance igE production by B cells. Interest- Crosslinking chemical s andn macrophages and higher levels of scD23 in their serum than do nonatopic individuals. IgE Crosslinkage Initiates Degranulation F The biochemical events that mediate degranulation of mast 5 cells and blood basophils have many features in common Mast cell For simplicity, this section presents a general overview of (d)Crosslinkage of IgE mast-cell degranulation mechanisms without calling atten tion to the slight differences between mast cells and baso b)Antibody crosslinkage receptors by of IgE phils. Although mast-cell degranulation generally is initiated Anti-receptor by allergen crosslinkage of bound igE, a number of other stimuli can also initiate the process, including the anaphyla toxins(C3a, C4a, and C5a)and various drugs. This section focuses on the biochemical events that follow allergen crosslinkage of bound igE. RECEPTOR CROSSLINKAGE (e) Enhanced Ca2+ influx IgE-mediated degranulation begins when an allergen cross- Anti-idiotype Ab tha links ige that is bound (fixed) to the Fc receptor on the sur- increases membrane face of a mast cell or basophil. In itself, the binding of igE to permeability to Ca2+ FcERI apparently has no effect on a target cell. It is only after Ca2+ allergen crosslinks the fixed igE-receptor complex that de- Ionophore granulation proceeds. The importance of crosslinkage is in- dicated by the inability of monovalent allergens, which can- not crosslink the fixed IgE, to trigger degranulation. granulation is crosslinkage of two or more FceRI mole- FIGURE 16-5 Schematic diagrams of mechanisms that can trigger cules--with or without bound IgE. Although crosslinkage is degranulation of mast cells. Note that mechanisms(b)and (c)do not normally effected by the interaction of fixed IgE with diva- require allergen: mechanisms (d)and (e)require neither allergen nor lent or multivalent allergen, it also can be effected by a vari- gE and mechanism(e)does not even require receptor crosslinkage ty of experimental means that bypass the need for allergen and in some cases even for IgE (Figure 16-5) Intracellular Events Also Regulate last-Cell Degranulation intracellular stores in the endoplasmic reticulum(see Figure 16-6). The Ca increase eventually leads to the formation of The cytoplasmic domains of the B and y chains of fceRI are arachidonic acid, which is converted into two classes of age of the FcERI receptors activates the associated PTKs. ure 16-6). The increase of Ca2+also promotes the assembly resulting in the phosphorylation of tyrosines within the of microtubules and the contraction of microfilaments, both ITAMs of the y subunit as well as phosphorylation of resi- of which are necessary for the movement of granules to the dues on the p subunit and on phospholipase C. These phos- plasma membrane. The importance of the Ca increase in phorylation events induce the production of a number of mast-cell degranulation is highlighted by the use of drugs second messengers that mediate the process of degranulation such as disodium cromoglycate(cromolyn sodium), that (Figure 16-6) block this influx as a treatment for allergies Within 15 s after crosslinkage of FceRl, methylation of Concomitant with phospholipid methylation and Ca in various membrane phospholipids is observed, resulting in an crease, there is a transient increase in the activity of membrane- increase in membrane fluidity and the formation of Ca+ bound adenylate cyclase, with a rapid peak of its reaction prod channels. An increase of Ca+ reaches a peak within 2 min of uct, cyclic adenosine monophosphate( cAMP), reached about FcERI crosslinkage( Figure 16-7). This increase is due both to 1 min after crosslinkage of FcERI(see Figure 16-7). The effects of the uptake of extracellular Ca and to a release of Ca* from cAMP are exerted through the activation of cAMP-dependent

generated by autoproteolysis of the membrane receptor, has been shown to enhance IgE production by B cells. Interest￾ingly, atopic individuals have higher levels of CD23 on their lymphocytes and macrophages and higher levels of sCD23 in their serum than do nonatopic individuals. IgE Crosslinkage Initiates Degranulation The biochemical events that mediate degranulation of mast cells and blood basophils have many features in common. For simplicity, this section presents a general overview of mast-cell degranulation mechanisms without calling atten￾tion to the slight differences between mast cells and baso￾phils. Although mast-cell degranulation generally is initiated by allergen crosslinkage of bound IgE, a number of other stimuli can also initiate the process, including the anaphyla￾toxins (C3a, C4a, and C5a) and various drugs. This section focuses on the biochemical events that follow allergen crosslinkage of bound IgE. RECEPTOR CROSSLINKAGE IgE-mediated degranulation begins when an allergen cross￾links IgE that is bound (fixed) to the Fc receptor on the sur￾face of a mast cell or basophil. In itself, the binding of IgE to FcRI apparently has no effect on a target cell. It is only after allergen crosslinks the fixed IgE-receptor complex that de￾granulation proceeds. The importance of crosslinkage is in￾dicated by the inability of monovalent allergens, which can￾not crosslink the fixed IgE, to trigger degranulation. Experiments have revealed that the essential step in de￾granulation is crosslinkage of two or more FcRI mole￾cules—with or without bound IgE. Although crosslinkage is normally effected by the interaction of fixed IgE with diva￾lent or multivalent allergen, it also can be effected by a vari￾ety of experimental means that bypass the need for allergen and in some cases even for IgE (Figure 16-5). Intracellular Events Also Regulate Mast-Cell Degranulation The cytoplasmic domains of the and  chains of FcRI are associated with protein tyrosine kinases (PTKs). Crosslink￾age of the FcRI receptors activates the associated PTKs, resulting in the phosphorylation of tyrosines within the ITAMs of the  subunit as well as phosphorylation of resi￾dues on the subunit and on phospholipase C. These phos￾phorylation events induce the production of a number of second messengers that mediate the process of degranulation (Figure 16-6). Within 15 s after crosslinkage of FcRI, methylation of various membrane phospholipids is observed, resulting in an increase in membrane fluidity and the formation of Ca2+ channels. An increase of Ca2+ reaches a peak within 2 min of FcRI crosslinkage (Figure 16-7). This increase is due both to the uptake of extracellular Ca2+ and to a release of Ca2+ from intracellular stores in the endoplasmic reticulum (see Figure 16-6). The Ca2+ increase eventually leads to the formation of arachidonic acid, which is converted into two classes of potent mediators: prostaglandins and leukotrienes(see Fig￾ure 16-6). The increase of Ca2+ also promotes the assembly of microtubules and the contraction of microfilaments, both of which are necessary for the movement of granules to the plasma membrane. The importance of the Ca2+ increase in mast-cell degranulation is highlighted by the use of drugs, such as disodium cromoglycate (cromolyn sodium), that block this influx as a treatment for allergies. Concomitant with phospholipid methylation and Ca2+ in￾crease, there is a transient increase in the activity of membrane￾bound adenylate cyclase, with a rapid peak of its reaction prod￾uct, cyclic adenosine monophosphate (cAMP), reached about 1 min after crosslinkage of FcRI (see Figure 16-7).The effects of cAMP are exerted through the activation of cAMP-dependent Hypersensitive Reactions CHAPTER 16 367 (a) Allergen crosslinkage of cell-bound IgE (b) Antibody crosslinkage of IgE (c) Chemical crosslinkage of IgE (d) Crosslinkage of IgE receptors by anti-receptor antibody (e) Enhanced Ca2+ influx by ionophore that increases membrane permeability to Ca2+ IgE Fc receptor IgE Allergen Mast cell Anti-isotype Ab Anti-idiotype Ab Crosslinking chemical Anti-receptor Ab Ionophore Ca2+ FIGURE 16-5 Schematic diagrams of mechanisms that can trigger degranulation of mast cells. Note that mechanisms (b) and (c) do not require allergen; mechanisms (d) and (e) require neither allergen nor IgE; and mechanism (e) does not even require receptor crosslinkage.

368 paRI I Immune Effector mechanisms FceRI FCeRI ③ PMT (MTIPE ATP cAMP( transient个 Protein kinase⑥ Protein kinase inactive Swollen.么 Mediators (e.g, histamine) Arachidonic acid doplasmic reticulum Leukotriene A,< >Prostaglandin D? LTB4 LTCA LTD4 SRS-A LTEA Secretion Secretion FIGURE 16-6 Diagrammatic overview of biochemical events in face of the plasma membrane causes an increase in membrane fluidity mast-cell activation and degranulation. Allergen crosslinkage of bound and facilitates the formation of Ca+ channels. The resulting influx of IgE results in FCERI aggregation and activation of protein tyrosine ki- Ca activates phospholipase A2, which promotes the breakdown of nase(PTk).(1)PTK then phosphorylates phospholipase C, which con- PC into lysophosphatidylcholine(lyso PC)and arachidonic acid verts phosphatidylinositol-4, 5 bisphosphate(PIP2)into diacylglycerol(5) Arachidonic acid is converted into potent mediators: the leuko- (DAG)and inositol triphosphate(IP3).(2)DAG activates protein ki- trienes and prostaglandin D. (6)FceRI crosslinkage also activates the nase C(PKC), which with Ca* is necessary for microtubular assembly membrane adenylate cyclase, leading to a transient increase of CAMP and the fusion of the granules with the plasma membrane. IP3 is a po- within 15 S. A later drop in cAMP levels is mediated by protein kinase tent mobilizer of intracellular Castores. (3)Crosslinkage of FcERI also and is required for degranulation to proceed. (7)cAMP-dependent pro- ctivates an enzyme that converts phosphatidylserine(PS)into phos. tein kinases are thought to phosphorylate the granule-membrane pro- phatidylethanolamine(PE). Eventually, PE is methylated to form phos- teins, thereby changing the permeability of the granules to water and phatidylcholine(PC)by the phospholipid methyl transferase enzymes I Ca. The consequent swelling of the granules facilitates fusion with the and ll(PMT I and Ii). (4)The accumulation of PC on the exterior sur- plasma membrane and release of the mediators protein kinases, which phosphorylate proteins on the granule Several Pharmacologic Agents Mediate membrane, thereby changing the permeability of the granules Type I Reactions of the granules facilitates their fusion with the plasma mem- The clinical manifestations of type I hypersensitive reactions brane,releasing their contents. The increase in cAMP is tran- are related to the biological effects of the mediators released sient and is followed by a drop in camP to levels below base- during mast-cell or basophil degranulation. These mediators line(see Figure 16-7). This drop in cAMP appears to be are pharmacologically active agents that act on local tissues necessary for degranulation to proceed; when cAMP level as well as on populations of secondary effector cells, includ increased by certain drugs, the degranulation process is ing eosinophils, neutrophils, T lymphocytes, monocytes, and blocked. Several of these drugs are given to treat allergic disor- platelets. The mediators thus amplifying termin ders and are considered later in this section effector mechanism, much as the complement system sery

protein kinases, which phosphorylate proteins on the granule membrane, thereby changing the permeability of the granules to water and Ca2+ (see Figure 16-6). The consequent swelling of the granules facilitates their fusion with the plasma mem￾brane, releasing their contents. The increase in cAMP is tran￾sient and is followed by a drop in cAMP to levels below base￾line (see Figure 16-7). This drop in cAMP appears to be necessary for degranulation to proceed; when cAMP levels are increased by certain drugs, the degranulation process is blocked. Several of these drugs are given to treat allergic disor￾ders and are considered later in this section. Several Pharmacologic Agents Mediate Type I Reactions The clinical manifestations of type I hypersensitive reactions are related to the biological effects of the mediators released during mast-cell or basophil degranulation. These mediators are pharmacologically active agents that act on local tissues as well as on populations of secondary effector cells, includ￾ing eosinophils, neutrophils, T lymphocytes, monocytes, and platelets. The mediators thus serve as an amplifying terminal effector mechanism, much as the complement system serves 368 PART III Immune Effector Mechanisms Swollen granule Allergen IgE Adenylate cyclase PMT II Phospho￾lipase C PKC PKC S S S S PIP2 DAG Active Inactive Ca2+ Ca2+ Ca2+ cAMP (transient) ATP Protein kinase inactive Protein kinase active IP3 Endoplasmic reticulum PC PE PS PMT I Lyso PC Phospho￾Degranulation Fusogens Microtubules and microfilaments Arachidonic acid Ca2+ Ca2+ Granule Prostaglandin D2 (PGD2) Leukotriene A4 LTB4 LTC4 LTD4 LTE4 SRS-A Secretion Secretion lipase A2 Mediators (e.g., histamine) PTK PTK PTK 1 2 6 3 4 7 5 PKC FCεRI FCεRI FIGURE 16-6 Diagrammatic overview of biochemical events in mast-cell activation and degranulation. Allergen crosslinkage of bound IgE results in FcRI aggregation and activation of protein tyrosine ki￾nase (PTK). (1) PTK then phosphorylates phospholipase C, which con￾verts phosphatidylinositol-4,5 bisphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3). (2) DAG activates protein ki￾nase C (PKC), which with Ca2+ is necessary for microtubular assembly and the fusion of the granules with the plasma membrane. IP3 is a po￾tent mobilizer of intracellular Ca2+ stores. (3) Crosslinkage of FcRI also activates an enzyme that converts phosphatidylserine (PS) into phos￾phatidylethanolamine (PE). Eventually, PE is methylated to form phos￾phatidylcholine (PC) by the phospholipid methyl transferase enzymes I and II (PMT I and II). (4) The accumulation of PC on the exterior sur￾face of the plasma membrane causes an increase in membrane fluidity and facilitates the formation of Ca2+ channels. The resulting influx of Ca2+ activates phospholipase A2, which promotes the breakdown of PC into lysophosphatidylcholine (lyso PC) and arachidonic acid. (5) Arachidonic acid is converted into potent mediators: the leuko￾trienes and prostaglandin D2. (6) FcRI crosslinkage also activates the membrane adenylate cyclase, leading to a transient increase of cAMP within 15 s. A later drop in cAMP levels is mediated by protein kinase and is required for degranulation to proceed. (7) cAMP-dependent pro￾tein kinases are thought to phosphorylate the granule-membrane pro￾teins, thereby changing the permeability of the granules to water and Ca2+ . The consequent swelling of the granules facilitates fusion with the plasma membrane and release of the mediators

Hypersensitive Reactions CHAPTER 16 369 FIGURE 16-7 Kinetics of major bio- chemical events that follow crosslinkage of bound igE on cultured human ba ex IgE Curves are shown for phospholipid methylation(solid blue). CAMP produc. (solid black ) Ca + influx(dashed blue), and histamine release(dashed black). In control experiments with e anti-IgE Fab fragments, no significant ine release changes were observed. /Adapted from T. Ishizaka et al., 1985, Int. Arch. Allergy 2宝 Appl ImmunoL. 77: 137) Anti-IgE Fab 2 Time. min as an amplifier and effector of an antigen-antibody interac- receptors on various target cells. Three types of histamine re- tion. When generated in response to parasitic infection, these ceptors-designated H1, H2, and Hy-have been identified; mediators initiate beneficial defense processes, including these receptors have different tissue distributions and medi vasodilation and increased vascular permeability, which ate different effects when they bind histamine brings an influx of plasma and inflammatory cells to attack Most of the biologic effects of histamine in allergic reac the pathogen. On the other hand, mediator release induced tions are mediated by the binding of histamine to H, recep by inappropriate antigens, such as allergens, results in unnec- tors. This binding induces contraction of intestinal and bron essary increases in vascular permeability and inflammation chial smooth muscles, increased permeability of venules, and hose detrimental effects far outweigh any beneficial effect. increased mucus secretion by goblet cells. Interaction of his The mediators can be classified as either primary or sec- tamine with H2 receptors increases vasopermeability and ondary (Table 16-3). The primary mediators are produced dilation and stimulates exocrine glands. Binding of hista before degranulation and are stored in the granules. The mine to H2 receptors on mast cells and basophils suppresses most significant primary mediators are histamine, proteases, degranulation; thus, histamine exerts negative feedback on eosinophil chemotactic factor, neutrophil chemotactic fac- the release of mediators tor, and heparin. The secondary mediators either are synthe- sized after target-cell activation or are released by the break- LEUKOTRIENES AND PROSTAGLANDINS down of membrane phospholipids during the degranulation As secondary mediators, the leukotrienes and prostaglandins process. The secondary mediators include platelet-activating are not formed until the mast cell undergoes degranulation factor,leukotrienes, prostaglandins, bradykinins, and various and the enzymatic breakdown of phospholipids in the cytokines. The differing manifestations of type I hypersens- plasma membrane. An ensuing enzymatic cascade generates tivity in different species or different tissues partly reflect the prostaglandins and the leukotrienes(see Figure 16-6).It variations in the primary and secondary mediators present. therefore takes a longer time for the biological effects of these The main biological effects of several of these mediators are mediators to become apparent. Their effects are more pro- described briefly in the next sections nounced and longer lasting, however, than those of histamine leukotrienes mediate bronchoconstriction, increased vas- HISTAMIN cular permeability, and mucus production. Prostaglandin D2 Histamine, which is formed by decarboxylation of the amino causes bronchoconstriction. acid histidine, is a major component of mast-cell granules, The contraction of human bronchial and tracheal smooth accounting for about 10% of granule weight. Because it is muscles appears at first to be mediated by histamine, but tored-preformed-in the granules, its biological effects are within 30-60 s, further contraction is mediated by the leuko- observed within minutes of mast-cell activation. Once re- trienes and prostaglandins. Being active at nanomole levels, leased from mast cells, histamine initially binds to specific the leukotrienes are as much as 1000 times more potent

as an amplifier and effector of an antigen-antibody interac￾tion. When generated in response to parasitic infection, these mediators initiate beneficial defense processes, including vasodilation and increased vascular permeability, which brings an influx of plasma and inflammatory cells to attack the pathogen. On the other hand, mediator release induced by inappropriate antigens, such as allergens, results in unnec￾essary increases in vascular permeability and inflammation whose detrimental effects far outweigh any beneficial effect. The mediators can be classified as either primary or sec￾ondary (Table 16-3). The primary mediators are produced before degranulation and are stored in the granules. The most significant primary mediators are histamine, proteases, eosinophil chemotactic factor, neutrophil chemotactic fac￾tor, and heparin. The secondary mediators either are synthe￾sized after target-cell activation or are released by the break￾down of membrane phospholipids during the degranulation process. The secondary mediators include platelet-activating factor, leukotrienes, prostaglandins, bradykinins, and various cytokines. The differing manifestations of type I hypersensi￾tivity in different species or different tissues partly reflect variations in the primary and secondary mediators present. The main biological effects of several of these mediators are described briefly in the next sections. HISTAMINE Histamine, which is formed by decarboxylation of the amino acid histidine, is a major component of mast-cell granules, accounting for about 10% of granule weight. Because it is stored—preformed—in the granules, its biological effects are observed within minutes of mast-cell activation. Once re￾leased from mast cells, histamine initially binds to specific receptors on various target cells. Three types of histamine re￾ceptors—designated H1, H2, and H3—have been identified; these receptors have different tissue distributions and medi￾ate different effects when they bind histamine. Most of the biologic effects of histamine in allergic reac￾tions are mediated by the binding of histamine to H1 recep￾tors. This binding induces contraction of intestinal and bron￾chial smooth muscles, increased permeability of venules, and increased mucus secretion by goblet cells. Interaction of his￾tamine with H2 receptors increases vasopermeability and dilation and stimulates exocrine glands. Binding of hista￾mine to H2 receptors on mast cells and basophils suppresses degranulation; thus, histamine exerts negative feedback on the release of mediators. LEUKOTRIENES AND PROSTAGLANDINS As secondary mediators, the leukotrienes and prostaglandins are not formed until the mast cell undergoes degranulation and the enzymatic breakdown of phospholipids in the plasma membrane. An ensuing enzymatic cascade generates the prostaglandins and the leukotrienes (see Figure 16-6). It therefore takes a longer time for the biological effects of these mediators to become apparent. Their effects are more pro￾nounced and longer lasting, however, than those of histamine. The leukotrienes mediate bronchoconstriction, increased vas￾cular permeability, and mucus production. Prostaglandin D2 causes bronchoconstriction. The contraction of human bronchial and tracheal smooth muscles appears at first to be mediated by histamine, but, within 30–60 s, further contraction is mediated by the leuko￾trienes and prostaglandins. Being active at nanomole levels, the leukotrienes are as much as 1000 times more potent as Hypersensitive Reactions CHAPTER 16 369 45Ca uptake, cpm × 10–3/106 cells ( ) Histamine release, % ( ) 8 6 4 2 50 30 10 Methylation cAMP Ca2+ uptake Anti-IgE Fab Histamine release 1 2 3 5 8 10 Time, min [3H] Methyl incorporation, cpm × 10–3/106 cells ( ) cAMP, pmol/106 cells ( ) 6 4 2 6 5 4 3 2 FIGURE 16-7 Kinetics of major bio￾chemical events that follow crosslinkage of bound IgE on cultured human ba￾sophils with F(ab )2 fragments of anti￾IgE. Curves are shown for phospholipid methylation (solid blue), cAMP produc￾tion (solid black), Ca2+ influx (dashed blue), and histamine release (dashed black). In control experiments with anti–IgE Fab fragments, no significant changes were observed. [Adapted from T. Ishizaka et al., 1985, Int. Arch. Allergy Appl. Immunol. 77:137.]

370 paRI I Immune Effector mechanisms TABLE 16-3 Principal mediators involved in type I hypersensitivity Mediat Effects PRIMARY Histamine, heparin Increased vascular permeability; smooth-muscle contraction serotonin Increased vascular permeability; smooth-muscle contraction Eosinophil chemotactic factor(ECF-A) eosinophil chemotaxis Neutrophil chemotactic factor (NCF. eutrophil chemotaxis Proteases Bronchial mucus secretion; degradation of blood-vessel basement membrane generation of complement split products SECONDARY Platelet-activating factor Platelet aggregation and degranulation; contraction of pulmonary smooth muscles Leukotrienes(slow reactive substance of anaphylaxis, SRS-A Increased vascular permeability: contraction of pulmonary smooth muscles Prostaglandins Vasodilation contraction smooth muscles; platelet aggregation B Increased vascular perme ooth-muscle contraction IL-1 and TNF-c Systemic anaphylaxis; increased expression of CAMs on venular endothelial cells IL-2, IL-3, IL-4, IL-5, IL-6, TGF-B, and GM-CSF Various effects(see Table 12-1) bronchoconstrictors than histamine is, and they are also reaction. This was the response observed by portier and more potent stimulators of vascular permeability and mucus Richet in dogs after antigenic challenge. Systemic anaphy secretion. In humans, the leukotrienes are thought to con laxis can be induced in a variety of experimental animals and tribute to the prolonged bronchospasm and buildup of mu- is seen occasionally in humans. Each species exhibits charac Is seen in asthmatics teristic symptoms, which reflect differences in the distribu tion of mast cells and in the biologically active contents of CYTOKINES their granules. The animal model of choice for studying sys- Adding to the complexity of the type I reaction is the variety temic anaphylaxis has been the guinea pig Anaphylaxis can of cytokines released from mast cells and eosinophils. Some be induced in guinea pigs with ease, and its symptoms closely of these may contribute to the clinical manifestations of type I hypersensitivity. Human mast cells secrete IL-4, IL-5, IL-6, Active sensitization in guinea pigs is induced by a single and TNF-a These cytokines alter the local microenviron- injection of a foreign protein such as egg albumin. After an ment, eventually leading to the recruitment of inflammatory incubation period of about 2 weeks. the animal is usually cells such as neutrophils and eosi inophils. IL-4 increases ige challenged with an intravenous injection of the same pro production by B cells. IL-5 is especially important in the tein. Within 1 min, the animal becomes restless, its respira- recruitment and activation of eosinophils. The high concen- tion becomes labored, and its blood pressure drops.As the trations of TNF-a secreted by mast cells may contribute to smooth muscles of the gastrointestinal tract and bladder shock in systemic anaphylaxis. (This effect may parallel the contract, the guinea pig defecates and urinates. Finally bron chiole constriction results in death by asphyxiation within role of TNF-a in bacterial septic shock and toxic-shock syn- 2-4 min of the injection. These events all stem from the sys- drome described in Chapter 12. temic vasodilation and smooth-muscle contraction brought on by mediators released in the course of the reaction. Post Type I Reactions Can Be Systemic mortem examination reveals that massive edema, shock, and or localized bronchiole constriction are the major of death The clinical manifestations of type I reactions can range from Systemic anaphylaxis in humans is characterized by a sim- ilar sequence of events. a wide range of antigens have been life-threatening conditions, such as systemic anaphylaxis and shown to trigger this reaction in susceptible humans, includ asthma, to hay fever and eczema, which are merely annoying. ing the venom from bee, wasp, hornet, and ant stings; drugs, such as penicillin, insulin, and antitoxins; and seafood and SYSTEMIC ANAPHYLAXIS If not treated quickly, these re be fatal. Epi- Systemic anaphylaxis is a shock-like and often fatal state nephrine is the drug of choice for systemic anaphylactic reac- hose onset occurs within minutes of a type I hypersensitive tions. Epinephrine counteracts the effects of mediators such

bronchoconstrictors than histamine is, and they are also more potent stimulators of vascular permeability and mucus secretion. In humans, the leukotrienes are thought to con￾tribute to the prolonged bronchospasm and buildup of mu￾cus seen in asthmatics. CYTOKINES Adding to the complexity of the type I reaction is the variety of cytokines released from mast cells and eosinophils. Some of these may contribute to the clinical manifestations of type I hypersensitivity. Human mast cells secrete IL-4, IL-5, IL-6, and TNF- These cytokines alter the local microenviron￾ment, eventually leading to the recruitment of inflammatory cells such as neutrophils and eosinophils. IL-4 increases IgE production by B cells. IL-5 is especially important in the recruitment and activation of eosinophils. The high concen￾trations of TNF- secreted by mast cells may contribute to shock in systemic anaphylaxis. (This effect may parallel the role of TNF- in bacterial septic shock and toxic-shock syn￾drome described in Chapter 12.) Type I Reactions Can Be Systemic or Localized The clinical manifestations of type I reactions can range from life-threatening conditions, such as systemic anaphylaxis and asthma, to hay fever and eczema, which are merely annoying. SYSTEMIC ANAPHYLAXIS Systemic anaphylaxis is a shock-like and often fatal state whose onset occurs within minutes of a type I hypersensitive reaction. This was the response observed by Portier and Richet in dogs after antigenic challenge. Systemic anaphy￾laxis can be induced in a variety of experimental animals and is seen occasionally in humans. Each species exhibits charac￾teristic symptoms, which reflect differences in the distribu￾tion of mast cells and in the biologically active contents of their granules. The animal model of choice for studying sys￾temic anaphylaxis has been the guinea pig. Anaphylaxis can be induced in guinea pigs with ease, and its symptoms closely parallel those observed in humans. Active sensitization in guinea pigs is induced by a single injection of a foreign protein such as egg albumin. After an incubation period of about 2 weeks, the animal is usually challenged with an intravenous injection of the same pro￾tein. Within 1 min, the animal becomes restless, its respira￾tion becomes labored, and its blood pressure drops. As the smooth muscles of the gastrointestinal tract and bladder contract, the guinea pig defecates and urinates. Finally bron￾chiole constriction results in death by asphyxiation within 2–4 min of the injection. These events all stem from the sys￾temic vasodilation and smooth-muscle contraction brought on by mediators released in the course of the reaction. Post￾mortem examination reveals that massive edema, shock, and bronchiole constriction are the major causes of death. Systemic anaphylaxis in humans is characterized by a sim￾ilar sequence of events. A wide range of antigens have been shown to trigger this reaction in susceptible humans, includ￾ing the venom from bee, wasp, hornet, and ant stings; drugs, such as penicillin, insulin, and antitoxins; and seafood and nuts. If not treated quickly, these reactions can be fatal. Epi￾nephrine is the drug of choice for systemic anaphylactic reac￾tions. Epinephrine counteracts the effects of mediators such 370 PART III Immune Effector Mechanisms TABLE 16-3 Principal mediators involved in type I hypersensitivity Mediator Effects PRIMARY Histamine, heparin Increased vascular permeability; smooth-muscle contraction Serotonin Increased vascular permeability; smooth-muscle contraction Eosinophil chemotactic factor (ECF-A) Eosinophil chemotaxis Neutrophil chemotactic factor (NCF-A) Neutrophil chemotaxis Proteases Bronchial mucus secretion; degradation of blood-vessel basement membrane; generation of complement split products SECONDARY Platelet-activating factor Platelet aggregation and degranulation; contraction of pulmonary smooth muscles Leukotrienes (slow reactive substance of anaphylaxis, SRS-A) Increased vascular permeability; contraction of pulmonary smooth muscles Prostaglandins Vasodilation; contraction of pulmonary smooth muscles; platelet aggregation Bradykinin Increased vascular permeability; smooth-muscle contraction Cytokines IL-1 and TNF- Systemic anaphylaxis; increased expression of CAMs on venular endothelial cells IL-2, IL-3, IL-4, IL-5, IL-6, TGF-, and GM-CSF Various effects (see Table 12-1)