hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes. The classes Trematoda and Cestoda contain parasitic forms Phylum Aschelminthes(蛔线虫门) The nematodes, or roundworm s. are elongate lindrical worms, frequently attenuated at both ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free-livinge. g, Caenorhabditis elegans), a large number of species parasitize humans, animals, and plants. Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species Phylum Acanthocephala(棘头虫门) The thorny- headed worms are all endoparasite organisms. While thorny-headed worms are widely distributed among wild and domestic animal only three genera have been reported in human beings including Macracanthorhynchus hirudinaceusy(猪巨吻棘头虫 Phylum Arthropoda(节肢动物门) The phylum is subdivided into a number if classes. many of which are of medical importance. The classes main include the Class arachnida( E=) and Class Insecta(EE s!). The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites. Certain ticks and mites many transmit diseases Insects have three pairs of legs and a body divided into three distinct parts: Insects head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs etc MORPHOLOGY(形态学) The protozoa are small, unicellular organisms which are morphologically and fu complete. A single cell carries out all the functions such as digestion, respiration, excretion The helminths are larger organisms. A particular function such as reproduction, digestion or excretion is performed by a group of special cells Arthropods are segmented and bilaterally symmetrical (XfFk BJ) animals with a body enclosed in a stiff, chitinous(甲壳质) covering or exoskeleton(外骨骼) and bearing paired jointed appendages. The digestive system is well developed. Sexes are separate LIFE CYCLE(生活史) The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stage of the parasite pleted in a single host such as man. Change of host is required only to propagate the parasite in the community(eg, E histolytica溶组织阿米巴, Trichuris trichiura H s, etc). Some of the parasites require two different hosts to complete their various stage of development(e.g, Schistosoma japonicum日本血吸虫etc). In a complex life cle many parasites require two different hosts, one definitive host and one intermediate host to complete their life cycle(e. g, Schistosoma species require man as definitive host and snail as intermediate hosts). Few of the parasite require two different intermediate hosts apart from a single definitive host(e.g, Paragonimus westerman卫氏并殖吸虫 requires snails as the first intermediate host and fresh water fish and crabs as the second intermediate host, apart from man and the fish eating mammals as the definitive host
5 hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes. The classes Trematoda and Cestoda contain parasitic forms only. Phylum Aschelminthes(蛔线虫门) The nematodes, or roundworms, are elongate, cylindrical worms, frequently attenuated at both ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free-living(e.g., Caenorhabditis elegans), a large number of species parasitize humans, animals, and plants. Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species. Phylum Acanthocephala(棘头虫门) The thorny-headed worms are all endoparasite organisms. While thorny-headed worms are widely distributed among wild and domestic animal, only three genera have been reported in human beings including Macracanthorhynchus hirudinaceus(猪巨吻棘头虫). Phylum Arthropoda(节肢动物门) The phylum is subdivided into a number if classes, many of which are of medical importance. The classes main include the Class Arachnida(蛛形纲) and Class Insecta(昆虫纲). The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites. Certain ticks and mites many transmit diseases. Insects have three pairs of legs and a body divided into three distinct parts: Insects head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs etc. MORPHOLOGY（形态学） The protozoa are small, unicellular organisms which are morphologically and functionally complete. A single cell carries out all the functions such as digestion, respiration, excretion, reproduction, etc. The helminths are larger organisms. A particular function such as reproduction, digestion or excretion is performed by a group of special cells. Arthropods are segmented and bilaterally symmetrical（对称的）animals with a body enclosed in a stiff, chitinous（甲壳质） covering or exoskeleton（外骨骼） and bearing paired, jointed appendages. The digestive system is well developed. Sexes are separate. LIFE CYCLE（生活史） The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stage of the parasite are completed in a single host such as man. Change of host is required only to propagate the parasite in the community (e.g., E.histolytica 溶组织阿米巴, Trichuris trichiura 鞭虫, etc). Some of the parasites require two different hosts to complete their various stage of development (e.g., Schistosoma japonicum 日本血吸虫, etc). In a complex life cycle many parasites require two different hosts, one definitive host and one intermediate host to complete their life cycle(e.g., Schistosoma species require man as definitive host and snail as intermediate hosts). Few of the parasite require two different intermediate hosts apart from a single definitive host(e.g., Paragonimus westermani 卫氏并殖吸虫 requires snails as the first intermediate host and fresh water fish and crabs as the second intermediate host, apart from man and the fish eating mammals as the definitive host.)
TRANSMISSION OF PARASITES It depend upon: Source or reservoir of infection, and Mode of transmission Source ofinfection(传染源 1)Humans Humans is the source or reservoir in a majority of parasitic infections(e. g taeniasis, amoebiasis, etc). The condition in which the infection is transmitted from one infected man to another man is called anthroponosis(人类传染病) 2)Animal In many of the parasitic diseases, animals act as the source of infection. The condition where infection is transmitted from animals to humans is called zoonoses人兽共患病 (eg, hydatid disease包虫病) Mode of transmission(传播方式) Transmission of infection fiom one host to another; cause by a certain form of the parasite is known as the infective stage. The infective stage of various parasites many be transmitted fiom one host to another in the following ways 1)Oral route Ingestion of food, water and vegetable: The infection is transmitted orally by ingestion of food, water or vegetables contaminated by the faeces that contain the infective stages of the parasite. This mode of transmission is referred to as faecal-oral route(e.g, cysts of Giardia intestinalis贾第虫 and Entamoeba histolytica溶组织阿米巴; ova of Ascaris lumbricoides蛔虫, Trichuris trichura鞭虫 and Enterobius vermicularis蛲虫) ngestion of raw or undercooked meat: The infection is transmitted orally also by ingestion of raw or undercooked meat harbouring the infective stage of the parasite(e. g, pork containing cysticercus cellulosae猪囊尾蚴, the larval stage of Taenia solium猪带绦虫) Ingestion of raw or uncooked fish and crab: Infection is transmitted by ingestion of raw or infective stage of the parasite(e. g, crab or cray fish containing the metacercariae of Paragonimus esterman卫氏并殖吸虫i, fish harbouring the metacercariae囊蚴 of Clonorchis sinensis华支睾 吸虫/肝吸虫,etc) Ingestion of raw or under cooked water plants: Infection can be transmitted bt eating raw or under cooked water plants harbouring the infective form of the parasite(e.g, water chest nuts, etc containing metacercariae of Fasciolopsis buski布氏姜片虫 and Fasciola hepatica肝片形吸虫) 2) Penetration of the skin and mucous membrane The infection is transmitted by A)Penetration of the intact skin by filariformlarvae2i#h)of hookworm, Sreongyloides stercoralis on coming in contact with faecally polluted soil, and B) Piercing the skin by cercariae of Schistosoma japonicum日本血吸虫, S mansoni曼氏 血吸虫andS. haematobium埃及血吸虫 on coming in contact with infected water. 3)Inoculation by an arthropod vector The infection also can be transmitted by A) ne blood by Anopheles( 疟原虫) B) Inoculation into the skin by mosquitoes( vectors for Wuchereria bancrofti斑氏丝虫 Brugia malayi马来丝虫 Trichomonas is transmitted by sexual contact. Frequently, Entamoeba also is transmitted by sexual contact among homosexuals(同性恋) HOST-PARASITE EXISTENCE Establishment of the parasite in its host is referred to as an infection The outcome of the infection is highly variable. It may be(a) sub-clinical latent infection,(b) clinical disease or(c) carrier(携带者) 6
6 TRANSMISSION OF PARASITES It depend upon: Source or reservoir of infection, and Mode of transmission. Source of infection（传染源） 1) Humans Humans is the source or reservoir in a majority of parasitic infections(e.g., taeniasis, amoebiasis, etc). The condition in which the infection is transmitted from one infected man to another man is called anthroponoses(人类传染病). 2) Animal In many of the parasitic diseases, animals act as the source of infection. The condition where infection is transmitted from animals to humans is called zoonoses 人兽共患病 (e.g., hydatid disease 包虫病). Mode of transmission（传播方式） Transmission of infection from one host to another, cause by a certain form of the parasite is known as the infective stage. The infective stage of various parasites many be transmitted from one host to another in the following ways. 1) Oral route Ingestion of food, water and vegetable: The infection is transmitted orally by ingestion of food, water or vegetables contaminated by the faeces that contain the infective stages of the parasite. This mode of transmission is referred to as faecal-oral route(e.g., cysts of Giardia intestinalis 贾第虫 and Entamoeba histolytica 溶组织阿米巴； ova of Ascaris lumbricoides 蛔虫, Trichuris trichura 鞭虫 and Enterobius vermicularis 蛲虫). Ingestion of raw or undercooked meat: The infection is transmitted orally also by ingestion of raw or undercooked meat harbouring the infective stage of the parasite(e.g., pork containing cysticercus cellulosae 猪囊尾蚴, the larval stage of Taenia solium 猪带绦虫). Ingestion of raw or uncooked fish and crab: Infection is transmitted by ingestion of raw or under cooked fish and crab containing the infective stage of the parasite(e.g., crab containing the infective stage of the parasite(e.g., crab or cray fish containing the metacercariae of Paragonimus westerman卫氏并殖吸虫i, fish harbouring the metacercariae 囊蚴of Clonorchis sinensis华支睾 吸虫/肝吸虫, etc). Ingestion of raw or under cooked water plants: Infection can be transmitted bt eating raw or under cooked water plants harbouring the infective form of the parasite(e.g., water chest nuts, etc., containing metacercariae of Fasciolopsis buski 布氏姜片虫 and Fasciola hepatica 肝片形吸虫). 2) Penetration of the skin and mucous membrane The infection is transmitted by A) Penetration of the intact skin by filariformlarvae（丝状蚴）of hookworm, Sreongyloides stercoralis on coming in contact with faecally polluted soil, and B) Piercing the skin by cercariae of Schistosoma japonicum 日本血吸虫, S. mansoni 曼氏 血吸虫 and S. haematobium 埃及血吸虫 on coming in contact with infected water. 3) Inoculation by an arthropod vector The infection also can be transmitted by A) Inoculation into the blood by Anopheles(vector for Plasmodium 疟原虫). B) Inoculation into the skin by mosquitoes(vectors for Wuchereria bancrofti 斑氏丝虫, Brugia malayi 马来丝虫 etc). 4) Sexual contact Trichomonas is transmitted by sexual contact. Frequently, Entamoeba also is transmitted by sexual contact among homosexuals（同性恋）. HOST-PARASITE EXISTENCE Establishment of the parasite in its host is referred to as an infection. The outcome of the infection is highly variable. It may be(a) sub-clinical latent infection, (b) clinical disease or (c) carrier（携带者）
The disease is the clinical manifestation of the infection which shows the active pre and replication of the parasite causing damage in the host. It may be mild, severe, fulminant(W tE J ), and in some cases may even cause death of the host The person who is infected with the parasite but without any clinical or sub clinical disea is referred to as a carrier PARASITIC ZOONOSES(人兽共患病) These are the infections which are naturally transmitted between the vertebrate animals and man. The condition usually includes those infections in which the proof of strong circumstantial evidence of transmission between the man and animals are documented PATHOGENESIS AND PATHOLOGY(发病机制与病理学 Pathogenesis(发病机制) of the parasitic diseases is a dynamic process and depends on the complex interaction of a variety of host and parasitic factors Host factors The host factor include 1) Nutritional status of the host, whether malnutrition or under nutrition 2) Immune response to parasitic infection 3) Immune status of the host whether there is immuno-suppression or not 4)The presence or absence of the co-existing disease or other physiological conditions such as pregnancy, 5) The age and level of the immunity at the time of infection Parasitic factors The parasitic factors include 1) Site of the attachment of the parasite and the size of the parasite. 2) Number of invading parasites, and 3)Parasite strain(pathogenic or non-pathogenic) and the growth, development and multiplication of parasites inside the human body and their metabolic products The parasites can cause disease in man in various ways as follows: trauma by adult worm larva, and egg(e. g, hookworm cause oozing of the blood at the site of attachment); Invasion and destruction of host cell( Plasmodium and Toxoplasma are obligate intracellular parasites of man they produce several enzymes which cause digestion and necrosis of host cells); Inflammatory reaction(many of the parasite induce inflammatory reactions in the host leading to the formation of various pathological lesions); Toxin( parasites like bacteria also produce toxins but they appear to have a minimal role in the pathogenesis of the disease processes; Allergic manifestation( many of the metabolic and excretory products of the parasites absorbed in the circulation, produce a variety of immunological and allergic manifestations in the sensitized hosts). Various pathogenic mechanisms in parasitic diseases are summarized in the below table I-1-2 The parasitic infections usually are designate by generic names of the parasites en asis or-osis.(e.g, Schistosoma infection is called as schistosomiasis) Table 1-1-2 Pathogenic Mechanisms in Parasitic Diseases Mechanism Parasitic diseases Trauma Strongyloidiasis, enterobiasis, taeniasis, clonorchiasis, schistosomiasis and hookworm infection
7 The disease is the clinical manifestation of the infection which shows the active presence and replication of the parasite causing damage in the host. It may be mild, severe, fulminant（爆发 性的）,and in some cases may even cause death of the host. The person who is infected with the parasite but without any clinical or sub clinical diseases is referred to as a carrier. PARASITIC ZOONOSES(人兽共患病) These are the infections which are naturally transmitted between the vertebrate animals and man. The condition usually includes those infections in which the proof of strong circumstantial evidence of transmission between the man and animals are documented. PATHOGENESIS AND PATHOLOGY（发病机制与病理学） Pathogenesis（发病机制） of the parasitic diseases is a dynamic process and depends on the complex interaction of a variety of host and parasitic factors. Host factors The host factor include: 1) Nutritional status of the host, whether malnutrition or under nutrition. 2) Immune response to parasitic infection 3) Immune status of the host whether there is immuno-suppression or not. 4) The presence or absence of the co-existing disease or other physiological conditions such as pregnancy, and 5) The age and level of the immunity at the time of infection. Parasitic factors The parasitic factors include: 1) Site of the attachment of the parasite and the size of the parasite. 2) Number of invading parasites, and 3) Parasite strain(pathogenic or non-pathogenic) and the growth, development and multiplication of parasites inside the human body and their metabolic products. The parasites can cause disease in man in various ways as follows: trauma by adult worm, larva, and egg(e.g., hookworm cause oozing of the blood at the site of attachment); Invasion and destruction of host cell(Plasmodium and Toxoplasma are obligate intracellular parasites of man , they produce several enzymes which cause digestion and necrosis of host cells); Inflammatory reaction(many of the parasite induce inflammatory reactions in the host leading to the formation of various pathological lesions); Toxin(parasites like bacteria also produce toxins but they appear to have a minimal role in the pathogenesis of the disease processes; Allergic manifestation(many of the metabolic and excretory products of the parasites absorbed in the circulation, produce a variety of immunological and allergic manifestations in the sensitized hosts). Various pathogenic mechanisms in parasitic diseases are summarized in the below table Ⅰ-Ⅰ-2. The parasitic infections usually are designate by generic names of the parasites ending with -iasis or –osis. (e.g., Schistosoma infection is called as schistosomiasis) TableⅠ-Ⅰ-2 Pathogenic Mechanisms in Parasitic Diseases Mechanism Parasitic diseases Trauma Strongyloidiasis, enterobiasis, taeniasis,clonorchiasis, schistosomiasis and hookworm infection
Invasion and destruction of host cell Malaria, leishmaniasis, trypanosomiasis, Inflammation Trichnellosis, lymphatic filariasis, paragonimiasis, Amoebiasis, Chaga's disease and sleeping sickness Alergic manifestation Schistosomiasis, hydatid disease HOST IMMUNITY The host resistance or immunity in parasitic infections refers to the resistance offered by the host towards the injury caused by the parasites and their products. It may be classified into: a Innate, and b)Acquired immunity Innate immunity(先天性免疫) t is the inherited but non- immune type of the host defence against a parasite,eg, Haemoglobin- s thalassaemia(地中海贫血)andg! mucose-6- phosphate dehydrogenase(葡萄糖6-磷酸脱氢酶,G6PD) deficient erythrocytes(红细胞)are resistant against Plasmodium falciparum恶性疟原虫; persons with Duffy- negative genes(Dufy 阴性基因) are resistant to malaria,etc. Acquired immunity(获得性免疫) t may be a)Non- specific b)Specif. 1)Non-specific or acquired immunity It confers protective immunity against many elminthic infections. It is developed during exposure of persons to antigenically unrelated micro-organisms, microbial extracts or some sythetic products. Non-specific immunity has been shown to supplement the specific acquired immunity. These appear to be mediated by macrophages or their active products and also by interferons 2)Specific acquired immunity It is mediated by both humoral and cell mediated Immunities a) Humoral immunity It is mediated through the production of specific antibodies These antibodies are serum proteins and gamma globulins in nature. These antibodies may be protective or non-protective. The antibodies may offer protection in following ways O The antibodies prevent the parasites from attaching and penetrating the host cells by binding the specific sites on the surface of parasites the The antibodies neutralize parasite toxins and inactivate parasite enzymes by binding with determinants of parasitic antigens 8 The secretory IgA antibodies found in various body secretions prevent attachment of some protozoal parasites in the gut wall epithelium In a few parasitic infections(e.g, trypanosomiasis), the parasites are killed by lysis of antibody-coated cells mediated by the complement, and 6 The antibody dependent cell mediated cytotoxicity ADCC) helps in the killing of a few helminths coated by specific antibodies. It is an important mechanism by which the parasites are killed. This is mediated mainly by the lymphocytes and to some extent by neutrophils, eosinophils and macrophages. The antibodies are mainly of IgG, IgE and bind specifically to the parasites b)Cell mediated immunity (CMI) It is mediated through T cell which are cytotoxic. The CMI offers protection against many parasitic infections in following ways O Cytotoxic T lymphocytes alter the osmotic permeability of parasitic cells causing
8 Invasion and destruction of host cell Malaria, leishmaniasis,trypanosomiasis, toxoplasmosis and amoebiasis Inflammation Trichnellosis, lymphatic filariasis, paragonimiasis, Amoebiasis Toxin Amoebiasis, Chaga’s disease and sleeping sickness Allergic manifestation Schistosomiasis, hydatid disease HOST IMMUNITY The host resistance or immunity in parasitic infections refers to the resistance offered by the host towards the injury caused by the parasites and their products. It may be classified into: a) Innate, and b) Acquired immunity. Innate immunity（先天性免疫） It is the inherited but non-immune type of the host defence against a parasite., e.g., Haemoglobin-S thalasaemia（地中海贫血） and glucose- 6- phosphate dehydrogenase （葡萄糖 6-磷酸脱氢酶，G6PD）deficient erythrocytes（红细胞） are resistant against Plasmodium falciparum 恶性疟原虫; persons with Duffy-negative genes（Duffy 阴性基因）are resistant to malaria, etc. Acquired immunity（获得性免疫） It may be a) Non-specific or acquired immunity, or b)Specific acquired immunity. 1) Non-specific or acquired immunity It confers protective immunity against many protozoal and helminthic infections. It is developed during exposure of persons to antigenically unrelated micro-organisms, microbial extracts or some sythetic products. Non-specific immunity has been shown to supplement the specific acquired immunity. These appear to be mediated by macrophages or their active products and also by interferons. 2) Specific acquired immunity It is mediated by both humoral and cell mediated immunities. a) Humoral immunity It is mediated through the production of specific antibodies. These antibodies are serum proteins and gamma globulins in nature. These antibodies may be protective or non-protective. The antibodies may offer protection in following ways: ① The antibodies prevent the parasites from attaching and penetrating the host cells by binding the specific sites on the surface of parasites. ② The antibodies neutralize parasite toxins and inactivate parasite enzymes by binding with the determinants of parasitic antigens. ③ The secretory IgA antibodies found in various body secretions prevent attachment of some protozoal parasites in the gut wall epithelium. ④ In a few parasitic infections(e.g., trypanosomiasis), the parasites are killed by lysis of antibody-coated cells mediated by the complement, and ⑤ The antibody dependent cell mediated cytotoxicity(ADCC) helps in the killing of a few helminths coated by specific antibodies. It is an important mechanism by which the parasites are killed. This is mediated mainly by the lymphocytes and to some extent by neutrophils, eosinophils and macrophages. The antibodies are mainly of IgG, IgE and bind specifically to the parasites. b) Cell mediated immunity(CMI) It is mediated through T cell which are cytotoxic. The CMI offers protection against many parasitic infections in following ways. ① Cytotoxic T lymphocytes alter the osmotic permeability of parasitic cells causing
swelling and disruption of cells there by lysing the cells(e. g, Plasmodium falciparum infection in (2 Activated macrophages kill in various ways such as producing enzymes or activated substances(e. g, Toxoplasma, nania. the schistosomule of Schistosoma species or by producing hydrogen peroxide( Leishmania species etc) 3 Natural killer cells These cells appear to be helpful in the initial host resistance against many parasitic infections IMMUNE RESPONES ox The immune response of man against parasitic infections are variable. It m Protective, or b)Harmful to host Protective immune response 1)Sterilizing immunity The sterilizing or complete immunity is associated with the clinical cure, complete elimination of the parasite from the host and life long resistance against ubsequent infection. It occurs rarely in humans. It occurs only in the cutaneous leishmaniasis 2)Incomplete immunity It is associate with the clinical recovery from the disease and the development of immunity to specific challenge with the parasite. The parasites always persist in the host, even though relatively at a low level. This incomplete immunity also known as premunition(带虫免疫) typically is found in many protozoal infections(e.g, malaria,as concomitant immunity(伴随免疫” typically is found in helminthic infection( schistosomiasis 3)Absence of an effective immunity It is seen after complete clinical cure from tryp Harmful immune responses In this condition, immune regulatory system shows a negative effect by inhibiting protective immune responses and instead produces harmful effects in the host. It is manifested development of hypersensitivity reactions This is of four reaction: Type I, Il, Ill, IV hypersensitivity ① Anaphylactic reaction(过敏反应) It is type I hypersensitivity reaction. It is involved in the pathogenesis of tropical pulmonary eosinophilia, Loefflers pneumonia, swimmers'itch and anaphylactic reaction of ruptured hydatid cyst inside the body. The skin manifestations of tl anaphylactic reactions characteristically are seen during the phase of invasion of the skin by the arvae of of hookworm(钩虫), Strongyloid(类圆线虫), Schistosoma and other parasites ② Cytoxic(细胞毒) t is type ll reaction. t is responsible for a) anaemia in malaria, b) chronic myocarditis(心肌炎) and megacolon(巨结肠) in Chagas' disease,c) quinine(奎 宁) induced massive haemolysis(溶血) and haemoglobinuria(血蛋白尿) in malaria Immune complex mediated(免疫复合物介导) This is type Ill reaction and responsible for development of glomerulonephritis(肾小球肾炎) seen in malaria. It is also responsible for immune complex mediated nephritis in leishmaniasis, trichinosis and ④ Delayed hypersensitive(迟发性变态反应) This is type IV reacti responsible for development of pathological lesions in a) Schistosoma species infections, b)local lymphatic inflammation in filariasis, c)inflammation of muscle tissue around Trichinella and d) survival and proliferation of parasites
9 swelling and disruption of cells there by lysing the cells(e.g., Plasmodium falciparum infection in man). ② Activated macrophages kill parasites in various ways such as producing enzymes or activated substances(e.g., Toxoplasma, Leishmania, the schistosomule of Schistosoma species or by producing hydrogen peroxide(Leishmania species etc). ③ Natural killer cells These cells appear to be helpful in the initial host resistance against many parasitic infections. IMMUNE RESPONES The immune response of man against parasitic infections are variable. It may be: a) Protective, or b) Harmful to host. Protective immune response 1) Sterilizing immunity The sterilizing or complete immunity is associated with the clinical cure, complete elimination of the parasite from the host and life long resistance against subsequent infection. It occurs rarely in humans. It occurs only in the cutaneous leishmaniasis. 2) Incomplete immunity It is associate with the clinical recovery from the disease and the development of immunity to specific challenge with the parasite. The parasites always persist in the host, even though relatively at a low level. This incomplete immunity also known as “premunition(带虫免疫)” typically is found in many protozoal infections(e.g., malaria), as “concomitant immunity(伴随免疫)” typically is found in helminthic infection(schistosomiasis). 3) Absence of an effective immunity It is seen after complete clinical cure from infections(e.g.,amoebiasis, visceral leishmaniasis, American trypanosomiasis). Harmful immune responses In this condition, immune regulatory system shows a negative effect by inhibiting protective immune responses and instead produces harmful effects in the host. It is manifested development of hypersensitivity reactions. This is of four reaction: Type I, II, III, IV hypersensitivity. ① Anaphylactic reaction(过敏反应) It is type I hypersensitivity reaction. It is involved in the pathogenesis of tropical pulmonary eosinophilia, Loefflers pneumonia, swimmers’ itch and anaphylactic reaction of ruptured hydatid cyst inside the body. The skin manifestations of the anaphylactic reactions characteristically are seen during the phase of invasion of the skin by the larvae of of hookworm（钩虫）, Strongyloide（类圆线虫）, Schistosoma and other parasites. ② Cytoxic（细胞毒） It is type II reaction. It is responsible for a) anaemia in malaria, b) chronic myocarditis（心肌炎） and megacolon（巨结肠） in Chagas’ disease ， c) quinine（奎 宁） induced massive haemolysis（溶血） and haemoglobinuria（血蛋白尿） in malaria . ③ Immune complex mediated（免疫复合物介导） This is type III reaction and is responsible for development of glomerulonephritis（肾小球肾炎） seen in malaria. It is also responsible for immune complex mediated nephritis in leishmaniasis, trichinosis and schistosomiasis. ④ Delayed hypersensitivity（迟发性变态反应） This is type IV reaction and is responsible for development of pathological lesions in a) Schistosoma species infections, b) local lymphatic inflammation in filariasis, c) inflammation of muscle tissue around Trichinella and d) survival and proliferation of parasites