《微生物生物学》课程教学资源（书籍文献）World of Microbiology and Immunology，Brigham Narins, Editor，Volumes 1 and 2 A-Z.pdf_P36-P40

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Antibody-antigen, biochemical and molecular reactions 25 • • infections could blossom into life-threatening maladies. In the decades following the discovery of penicillin, many naturally occurring antibiotics were discovered and still more were synthesized towards specific targets on or in bacteria. Antibiotics are manufactured by bacteria and various eukaryotic organisms, such as plants, usually to protect the organism from attack by other bacteria. The discovery of these compounds involves screening samples against bacteria for an inhibition in growth of the bacteria. In commercial settings, such screening has been automated so that thousands of samples can be processed each day. Antibiotics can also be manufactured by tailoring a compound to hone in on a selected target. The advent of molecular sequencing technology and three-dimensional image reconstruction has made the design of antibiotics easier. Penicillin is one of the antibiotics in a class known as beta-lactam antibiotics. This class is named for the ring structure that forms part of the antibiotic molecule. Other classes of antibiotics include the tetracyclines, aminoglycosides, rifamycins, quinolones, and sulphonamides. The action of these antibiotics is varied. For example, beta-lactam antibiotics exert their effect by disrupting the manufacture of peptidoglycan, which is main stress-bearing network in the bacterial cell wall. The disruption can occur by blocking either the construction of the subunits of the peptidoglycan or by preventing their incorporation into the existing network. In another example, amonglycoside antibiotics can bind to a subunit of the ribosome, which blocks the manufacture of protein, or can reduce the ability of molecules to move across the cell wall to the inside of the bacterium. As a final example, the quinolone antibiotics disrupt the function of an enzyme that uncoils the double helix of deoxyribonucleic acid, which is vital if the DNA is to be replicated. Besides being varied in their targets for antibacterial activity, different antibiotics can also vary in the range of bacteria they affect. Some antibiotics are classified as narrow-spectrum antibiotics. They are lethal against only a few types (or genera) of bacteria. Other antibiotics are active against many bacteria whose construction can be very different. Such antibiotics are described as having a broad-spectrum of activity. In the decades following the discovery of penicillin, a myriad of different antibiotics proved to be phenomenally effective in controlling infectious bacteria. Antibiotics quickly became (and to a large extent remain) a vital tool in the physician’s arsenal against many bacterial infections. Indeed, by the 1970s the success of antibiotics led to the generally held view that bacterial infectious diseases would soon be eliminated. However, the subsequent acquisition of resistance to many antibiotics by bacteria has proved to be very problematic. Sometimes resistance to an antibiotic can be overcome by modifying the antibiotic slightly, via addition of a different chemical group. This acts to alter the tree-dimensional structure of the antibiotic. Unfortunately, such a modification tends to produce susceptibility to the new antibiotic for a relatively short time. Antibiotic resistance, a problem that develops when antibiotics are overused or misused. If an antibiotic is used properly to treat an infection, then all the infectious bacteria should be killed directly, or weakened such that the host’s immune response will kill them. However, the use of too low a concentration of an antibiotic or stopping antibiotic therapy before the prescribed time period can leave surviving bacteria in the population. These surviving bacteria have demonstrated resistance. If the resistance is governed by a genetic alteration, the genetic change may be passed on to subsequent generations of bacterial. For example, many strains of the bacterium that causes tuberculosis are now also resistant to one or more of the antibiotics routinely used to control the lung infection. As a second example, some strains of Staphylococcus aureus that can cause boils, pneumonia, or bloodstream infections, are resistant to almost all antibiotics, making those conditions difficult to treat. Ominously, a strain of Staphylococcus (which so far has been rarely encountered) is resistant to all known antibiotics. See also Bacteria and bacterial infection; Bacterial genetics; Escherichia coli; Rare genotype advantage ANTIBIOTICS, HISTORY OF DEVELOP- MENT • see HISTORY OF THE DEVELOPMENT OF ANTIBIOTICS ANTIBODY-ANTIGEN, BIOCHEMICAL AND Antibody-antigen, biochemical, and molecular reactions MOLECULAR REACTIONS Antibodies are produced by the immune system in response to antigens (material perceived as foreign. The antibody response to a particular antigen is highly specific and often involves a physical association between the two molecules. This association is governed by biochemical and molecular forces. In two dimensions, many antibody molecules present a “Y” shape. At the tips of the arms of the molecules are regions that are variable in their amino acid sequences, depending upon the antigen and the antibody formed in response. The Ciprofloxacin. womi_A 5/6/03 1:07 PM Page 25

Antibody and antigen WORLD OF MICROBIOLOGY AND IMMUNOLOGY 26 • • arm-tip regions are typically those that bind to the antigen. These portions of the antibody are also known as the antigenic determinants, or the epitopes. There are several different types of biochemical interactions between the antibody’s epitopes and the target regions on the antigen. Hydrogen bonds are important in stabilizing the antibody-antigen association. In addition, other weak interactions (e.g., van der Waals forces, hydrophobic interactions, electrostatic forces) act to tighten the interaction between the regions on the antibody and the antigen. The hydrogen bonds that are important in antigen-antibody bonding form between amino acids of the antibody and the antigen. Water molecules that fill in the spaces between the antibody and the antigen create other hydrogen bonds. The formation of hydrogen bonds between other regions of the antibody and antigen, and the water molecules stabilizes the binding of the immune molecules. The three-dimensional shape of the molecules is also an important factor in binding between an antibody and an antigen. Frequently, the antibody molecule forms a pocket that is the right size and shape to accommodate the target region of the antigen. This phenomenon was initially described as the “lock and key” hypothesis. The exact configuration of the antibody-antigen binding site is dependent on the particular antigen. Some antigens have a binding region that is compact. Such a region may be able to fit into a pocket or groove in the antibody molecule. In contrast, other antigen sites may be bulky. In this case, the binding site may be more open or flatter. These various three dimensional structures for the binding site are created by the sequence of amino acids that comprise the antibody protein. Some sequences are enriched in hydrophobic (water-loving) amino acids. Such regions will tend to form flat sheets, with all the amino acids exposed to the hydrophilic environment. Other sequences of amino acids can contain both hydrophilic and hydrophobic (water-hating) amino acids. The latter will tend to bury themselves away from water via the formation of a helical shape, with the hydrophobic region on the inside. The overall shape of an antibody and antigen depends upon the number of hydrophilic and hydrophobic regions and their arrangement within the protein molecule. The fact that the interaction between an antibody and an antigen requires a specific three-dimensional configuration is exploited in the design of some vaccines. These vaccines consist of an antibody to a region that is present on a so-called receptor protein. Antigens such as toxin molecules recognize the receptor region and bind to it. However, if the receptor region is already occupied by an antibody, then the binding of the antigen cannot occur, and the deleterious effect associated with binding of the antigen is averted. Antibody antigen reactions tend to be irreversible under normal conditions. This is mainly due to the establishment of the various chemical bonds and interactions between the molecules. The visible clumping of the antibody-antigen complex seen in solutions and diagnostic tests such as the Ochterlony test is an example of the irreversible nature of the association. See also Immune system; Immunoglobulins and immunoglobulin deficiency syndromes; Laboratory techniques in immunology; Protein crystallography AAntibody and antigen NTIBODY AND ANTIGEN Antibodies, or Y-shaped immunoglobulins, are proteins found in the blood that help to fight against foreign substances called antigens. Antigens, which are usually proteins or polysaccharides, stimulate the immune system to produce antibodies. The antibodies inactivate the antigen and help to remove it from the body. While antigens can be the source of infections from pathogenic bacteria and viruses, organic molecules detrimental to the body from internal or environmental sources also act as antigens. Genetic engineering and the use of various mutational mechanisms allow the construction of a vast array of antibodies (each with a unique genetic sequence). Specific genes for antibodies direct the construction of antigen specific regions of the antibody molecule. Such antigen-specific regions are located at the extremes of the Yshaped immunglobulin-molecule. Once the immune system has created an antibody for an antigen whose attack it has survived, it continues to produce antibodies for subsequent attacks from that antigen. This longterm memory of the immune system provides the basis for the practice of vaccination against disease. The immune system, with its production of antibodies, has the ability to recognize, remember, and destroy well over a million different antigens. There are several types of simple proteins known as globulins in the blood: alpha, beta, and gamma. Antibodies are gamma globulins produced by B lymphocytes when antigens enter the body. The gamma globulins are referred to as immunoglobulins. In medical literature they appear in the abbreviated form as Ig. Each antigen stimulates the production of a specific antibody (Ig). Antibodies are all in a Y-shape with differences in the upper branch of the Y. These structural differences of amino acids in each of the antibodies enable the individual antibody to recognize an antigen. An antigen has on its surface a combining site that the antibody recognizes from the combining sites on the arms of its Y-shaped structure. In response to the antigen that has called it forth, the antibody wraps its two combining sites like a “lock” around the “key” of the antigen combining sites to destroy it. An antibody’s mode of action varies with different types of antigens. With its two-armed Y-shaped structure, the antibody can attack two antigens at the same time with each arm. If the antigen is a toxin produced by pathogenic bacteria that cause an infection like diphtheria or tetanus, the binding process of the antibody will nullify the antigen’s toxin. When an antibody surrounds a virus, such as one that causes influenza, it prevents it from entering other body cells. Another mode of action by the antibodies is to call forth the assistance of a group of immune agents that operate in what is known as the plasma complement system. First, the antibodies will coat infectious bacteria and then white blood cells will womi_A 5/6/03 1:07 PM Page 26

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Antibody and antigen 27 • • complete the job by engulfing the bacteria, destroying them, and then removing them from the body. There are five different antibody types, each one having a different Y-shaped configuration and function. They are the Ig G, A, M, D, and E antibodies. IgG is the most common type of antibody. It is the chief Ig against microbes. It acts by coating the microbe to hasten its removal by other immune system cells. It gives lifetime or long-standing immunity against infectious diseases. It is highly mobile, passing out of the blood stream and between cells, going from organs to the skin where it neutralizes surface bacteria and other invading microorganisms. This mobility allows the antibody to pass through the placenta of the mother to her fetus, thus conferring a temporary defense to the unborn child. After birth, IgG is passed along to the child through the mother’s milk, assuming that she nurses the baby. But some of the Ig will still be retained in the baby from the placental transmission until it has time to develop its own antibodies. Placental transfer of antibodies does not occur in horses, pigs, cows, and sheep. They pass their antibodies to their offspring only through their milk. This antibody is found in body fluids such as tears, saliva, and other bodily secretions. It is an antibody that provides a first line of defense against invading pathogens and allergens, and is the body’s major defense against viruses. It is found in large quantities in the bloodstream and protects other wet surfaces of the body. While they have basic similarities, each IgA is further differentiated to deal with the specific types of invaders that are present at different openings of the body. Since this is the largest of the antibodies, it is effective against larger microorganisms. Because of its large size (it combines five Y-shaped units), it remains in the bloodstream where it provides an early and diffuse protection against invading antigens, while the more specific and effective IgG antibodies are being produced by the plasma cells. The ratio of IgM and IgG cells can indicate the various stages of a disease. In an early stage of a disease there are more IgM antibodies. The presence of a greater number of IgG antibodies would indicate a later stage of the disease. IgM antibodies usually form clusters that are in the shape of a star. This antibody appears to act in conjunction with B and T-cells to help them in location of antigens. Research continues on establishing more precise functions of this antibody. The antibody responsible for allergic reactions, IgE acts by attaching to cells in the skin called mast cells and basophil cells (mast cells that circulate in the body). In the presence of environmental antigens like pollens, foods, chemicals, and drugs, IgE releases histamines from the mast cells. The histamines cause the nasal inflammation (swollen tissues, running nose, sneezing) and the other discomforts of hay fever or other types of allergic responses, such as hives, asthma, and in rare cases, anaphylactic shock (a life-threatening condition brought on by an allergy to a drug or insect bite). An explanation for the role of IgE in allergy is that it was an antibody that was useful to early man to prepare the immune system to fight parasites. This function is presently overextended in reacting to environmental antigens. The presence of antibodies can be detected whenever antigens such as bacteria or red blood cells are found to agglutinate (clump together), or where they precipitate out of solution, or where there has been a stimulation of the plasma complement system. Antibodies are also used in laboratory tests for blood typing when transfusions are needed and in a number of different types of clinical tests, such as the Wassermann test for syphilis and tests for typhoid fever and infectious mononucleosis. By definition, anything that makes the immune system respond to produce antibodies is an antigen. Antigens are living foreign bodies such as viruses, bacteria, and fungi that cause disease and infection. Or they can be dust, chemicals, pollen grains, or food proteins that cause allergic reactions. Antigens that cause allergic reactions are called allergens. A large percentage of any population, in varying degrees, is allergic to animals, fabrics, drugs, foods, and products for the home and industry. Not all antigens are foreign bodies. They may be produced in the body itself. For example, cancer cells are antigens that the body produces. In an attempt to differentiate its “self” from foreign substances, the immune system will reject an organ transplant that is trying to maintain the body or a blood transfusion that is not of the same blood type as itself. There are some substances such as nylon, plastic, or Teflon that rarely display antigenic properties. For that reason, nonantigenic substances are used for artificial blood vessels, component parts in heart pacemakers, and needles for hypodermic syringes. These substances seldom trigger an immune system response, but there are other substances that are highly antigenic and will almost certainly cause an immune system reaction. Practically everyone reacts to certain chemicals, for example, the resin from the poison ivy plant, the venoms from insect and reptile bites, solvents, formalin, and asbestos. Viral and bacterial infections also generally trigger an antibody response from the immune system. For most people penicillin is not antigenic, but for some there can be an immunological response that ranges from severe skin rashes to death. Another type of antigen is found in the tissue cells of organ transplants. If, for example, a kidney is transplanted, the surface cells of the kidney contain antigens that the new host body will begin to reject. These are called human leukocyte antigens (HLA), and there are four major types of HLA subdivided into further groups. In order to avoid organ rejection, tissue samples are taken to see how well the new organ tissues match for HLA compatibility with the recipient’s body. Drugs will also be used to suppress and control the production of helper/suppressor T-cells and the amount of antibodies. Red blood cells with the ABO antigens pose a problem when the need for blood transfusions arises. Before a transfusion, the blood is tested for type so that a compatible type is used. Type A blood has one kind of antigen and type B another. A person with type AB blood has both the A and B antigen. Type O blood has no antigens. A person with type A blood would require either type A or O for a successful transfusion. Type B and AB would be rejected. Type B blood would womi_A 5/6/03 1:07 PM Page 27

Antibody and antigen WORLD OF MICROBIOLOGY AND IMMUNOLOGY 28 • • be compatible with a B donor or an O donor. Since O has no antigens, it is considered to be the universal donor. Type AB is the universal recipient because its antibodies can accept A, B, AB, or O. One way of getting around the problem of blood types in transfusion came about as a result of World War II. The great need for blood transfusions led to the development of blood plasma, blood in which the red and white cells are removed. Without the red blood cells, blood could be quickly administered to a wounded soldier without the delay of checking for the blood antigen type. Another antigenic blood condition can affect the life of newborn babies. Rhesus disease (also called erythroblastosis fetalis) is a blood disease caused by the incompatibility of Rh factors between a fetus and a mother’s red blood cells. When an Rh negative mother gives birth to an Rh positive baby, any transfer of the baby’s blood to the mother will result in the production of antibodies against Rh positive red blood cells. At her next pregnancy the mother will then pass those antibodies against Rh positive blood to the fetus. If this fetus is Rh positive, it will suffer from Rh disease. Tests for Rh blood factors are routinely administered during pregnancy. Western medicine’s interest in the practice of vaccination began in the eighteenth century. This practice probably originated with the ancient Chinese and was adopted by Turkish doctors. A British aristocrat, Lady Mary Wortley Montagu (1689–1762), discovered a crude form of vaccination taking place in a lower-class section of the city of Constantinople while she was traveling through Turkey. She described her experience in a letter to a friend. Children who were injected with pus from a smallpox victim did not die from the disease but built up immunity to it. Rejected in England by most doctors who thought the practice was barbarous, smallpox vaccination was adopted by a few English physicians of the period. They demonstrated a high rate of effectiveness in smallpox prevention. By the end of the eighteenth century, Edward Jenner (1749–1823) improved the effectiveness of vaccination by injecting a subject with cowpox, then later injecting the same subject with smallpox. The experiment showed that immunity against a disease could be achieved by using a vaccine that did not contain the specific pathogen for the disease. In the nineteenth century, Louis Pasteur (1822–1895) proposed the germ theory of disease. He went on to develop a rabies vaccine that was made from the spinal cords of rabid rabbits. Through a series of injections starting from the weakest strain of the disease, Pasteur was able, after 13 injections, to prevent the death of a child who had been bitten by a rabid dog. There is now greater understanding of the principles of vaccines and the immunizations they bring because of our knowledge of the role played by antibodies and antigens within the immune system. Vaccination provides active immunity because our immune systems have had the time to recognize the invading germ and then to begin production of specific antibodies for the germ. The immune system can continue producing new antibodies whenever the body is attacked again by the same organism or resistance can be bolstered by booster shots of the vaccine. For research purposes there were repeated efforts to obtain a laboratory specimen of one single antibody in sufficient quantities to further study the mechanisms and applications of antibody production. Success came in 1975 when two British biologists, César Milstein (1927– ) and Georges Kohler (1946– ) were able to clone immunoglobulin (Ig) cells of a particular type that came from multiple myeloma cells. Multiple myeloma is a rare form of cancer in which white blood cells keep turning out a specific type of Ig antibody at the expense of others, thus making the individual more susceptible to outside infection. By combining the myeloma cell with any selected antibody-producing cell, large numbers of specific monoclonal antibodies can be produced. Researchers have used other animals, such as mice, to produce hybrid antibodies which increase the range of known antibodies. Monoclonal antibodies are used as drug delivery vehicles in the treatment of specific diseases, and they also act as catalytic agents for protein reactions in various sites of the body. They are also used for diagnosis of different types of diseases and for complex analysis of a wide range of biological substances. There is hope that monoclonal antibodies will be as effective as enzymes in chemical and technological processes, and that they currently play a significant role in genetic engineering research. See also Antibody-antigen, biochemical and molecular reactions; Antibody formation and kinetics; Antibody, monoBinding of an antibody with an antigen, as detected using X-ray crystallography. womi_A 5/6/03 1:07 PM Page 28

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Antibody, monoclonal 29 • • clonal; Antigenic mimicry; Immune stimulation, as a vaccine; Immunologic therapies; Infection and resistance; Infection control; Major histocompatibility complex (MHC) AAntibody formation and kinetics NTIBODY FORMATION AND KINETICS Antibody formation occurs in response to the presence of a substance perceived by the immune system as foreign. The foreign entity is generically called an antigen. There are a myriad of different antigens that are presented to the immune system. Hence, there are a myriad of antibodies that are formed. The formation of innumerable antibodies follows the same general pattern. First, the immune system discriminates between host and non-host antigens and reacts only against those not from the host. However, malfunctions occur. An example is rheumatoid arthritis, in which a host response against self-antigens causes the deterioration of bone. Another example is heart disease caused by a host reaction to a heart muscle protein. The immune response is intended for an antigen of a bacterium called Chlamydia, which possess an antigen very similar in structure to the host heart muscle protein. Another feature of antibody formation is that the production of an antibody can occur even when the host has not “seen” the particular antigen for a long time. In other words, the immune system has a memory for the antigenic response. Finally, the formation of an antibody is a very precise reaction. Alteration of the structure of a protein only slightly can elicit the formation of a different antibody. The formation of antibody depends upon the processing of the incoming antigen. The processing has three phases. The first phase is the equilibration of an antigen between the inside and outside of cells. Soluble antigens that can dissolve across the cell membranes are able to equilibrate, but more bulky antigens that do not go into solution cannot. The second phase of antigen processing is known as the catabolic decay phase. Here, cells such as macrophages take up the antigen. It is during this phase that the antigen is “presented” to the immune system and the formation of antibody occurs. The final phase of antigen processing is called the immune elimination phase. The coupling between antigen and corresponding antibody occurs, and the complex is degraded. The excess antibody is free to circulate in the bloodstream. The antibody-producing cell of the immune system is called the lymphocyte or the B cell. The presentation of a protein target stimulates the lymphocyte to divide. This is termed the inductive or lag phase of the primary antibody response. Some of the daughter cells will then produce antibody to the protein target. With time, there will be many daughter lymphocytes and much antibody circulating in the body. During this log or exponential phase, the quantity of antibody increases rapidly. For a while, the synthesis of antibody is balanced by the breakdown of the antibody, so the concentration of antibody stays the same. This is the plateau or the steady-state phase. Within days or weeks, the production of the antibody slows. After this decline or death phase, a low, baseline concentration may be maintained. The lymphocytes retain the memory of the target protein. If the antigen target appears, as happens in the second vaccination in a series, the pre-existing, “primed” lymphocytes are stimulated to divide into antibody-producing daughter cells. Thus, the second time around, a great deal more antibody is produced. This primed surge in antibody concentration is the secondary or anamnestic (memory) response. The higher concentration of antibody can be maintained for months, years, or a lifetime. Another aspect of antibody formation is the change in the class of antibodies that are produced. In the primary response, mainly the IgM class of antibody is made. In the secondary response, IgG, IgE, or IgA types of antibodies are made. The specificity of an antibody response, while always fairly specific, becomes highly specific in a secondary response. While in a primary response, an antibody may crossreact with antigens similar to the one it was produced in response to, such cross-reaction happens only very rarely in a secondary response. The binding between antibody and antigen becomes tighter in a secondary response as well. See also Antigenic mimicry; History of immunology; Immunoglobulins and immunoglobulin deficiency syndromes; Laboratory techniques in immunology; Streptococcal antibody tests AAntibody, monoclonal NTIBODY, MONOCLONAL The immune system of vertebrates help keep the animal healthy by making millions of different proteins (immunoglobulins) called antibodies to disable antigens (harmful foreign substances such as toxins or bacteria). Scientists have worked to develop a method to extract large amounts of specific antibodies from clones (exact copies) of a cell created by fusing two different natural cells. Those antibodies are called monoclonal antibodies. Antibody research began in the 1930s when the American pathologist Karl Landsteiner found that animal antibodies counteract specific antigens and that all antibodies have similar structures. Research by the American biochemists Rodney R. Porter (1917–1985) and Gerald M. Edelman (1929– ) during the 1950s determined antibody structure, and particularly the active areas of individual antibodies. For their work they received the 1972 Nobel prize in physiology or medicine. By the 1960s, scientists who studied cells needed large amounts of specific antibodies for their research, but several problems prevented them from obtaining these antibodies. Animals can be injected with antigens so they will produce the desired antibodies, but it is difficult to extract them from among the many types produced. Attempts to reproduce various antibodies in an artificial environment encountered some complications. Lymphocytes, the type of cell that produces specific antibodies, are very difficult to grow in the laboratory; conversely, tumor cells reproduce easily and endlessly, but make only their own types of antibodies. A bone marrow tumor called a myeloma interested scientists because it begins womi_A 5/6/03 1:07 PM Page 29