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Revision of Poultry and Rabbits Biology Dr Fares El-Khayat
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Structure of avian immune system Dr Fares El-Khayat

Structure of avian immune system Dr Fares El-Khayat

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Structure of avian immune system Dr Fares El-Khayat

  1. 1. Structure of Avian Immune System Avian immune system is a highly complex system which is composed of immune organs, immune cells and immune mediators. A- Immune Organs I-Primitive = germinative organs: They include the yolk sac, the fetal liver and the bone marrow. They produce the un-differentiated un-programmed stem cells. II-Primary = central = maturation lymphoid organs: A) Thymus: It appears at the 5th day of the embryonic life and post-hatching, it continues to grow till 3-4 months of age then regresses with onset of sexual maturity. Its peak level of activity occurs in the young age. The maximum mean thymic weight is 16 gm at 17 weeks old in male chicken. It extends from anterior cervical regions into the thorax and some portions may be embedded in the thyroid glands; in close association with parathyroid and ultimobranchial glands. It is composed of seven lobes on each side of the neck in close association with jugular veins and vagus nerves. It is responsible for the maturation and differentiation of stem cells into thymus-dependent or thymus derived lymphocytes or T-cells under the hormonal control of the thymopiotin and thymocin. T-cell has the major role in cell-mediated immunity. Thymus has an endocrine functions hence secrets thymopiotin and thymocin autocoids. B) Bursa of Fabricius = cloacal bursa: It a lymphoepithelial hollow, round or oval sac-like extension of the hindgut located in the caudal body cavity and connected by short duct with the dorsal region of the cloaca. The bursal mucosa has 11-13 longitudinal folds. It attains its maximum weight at 4-10 weeks post-hatching and then regresses with onset of sexual maturity and is fibrotic by 23.5 weeks post-hatching. The natural bursal regression may be determined by adrenal and sex hormones. Sometimes redevelopment of bursal tissues occurs with molting. It is responsible for maturation and differentiation of the stem cells into bursal-dependent or bursal derived lymphocytes or B-lymphocytes under the hormonal control of the urospoetin. B-cell has the major role in antibody-mediated immunity. It has an endocrine-functions, hence it secrets urospoetin autocoid.
  2. 2. III-Secondary = peripheral = seeding lymphoid organs: These are the site to which the maturated B and T-cells are migrated from the primary or maturation lymphoid organs in a process known as immune migration or immune peripheralization. They include: 1-Gut associated lymphoid tissue – GALT GALT is one of the most important components of secondary lymphoid organs which represent all the lymphoid structures and cell aggregation which present in the digestive tracts. In association with the secretory IgA, it is responsible the presence of the digestive local or mucosal immunity which is one of the most important immune mechanisms. GALT includes: -The massive sub-mucosal lymphoid cell aggregation in the digestive tracts. -The esophageal tonsils which are non capsulated lymphoid structure located at the end of the esophagus at its connection with the proventriclus. It is highly prominent in the chickens. -The gastric tubular glands which is embedded in the proventricular wall. -Meckel’s diverticulum which is a pouch at the connection site between the intestine and the umbilical cord. -The lymphoid or annular rings which are present at the end of the jejunum and the beginning of the ileum. They are well developed in the aquatic birds. -Cecal tonsils are one of the most important components of peripheral lymphoid organs which are located at ileocecal junctions. It is not present at hatching and develop shortly afterwards. It is readily identified by 10 days old and its size increases up to 12 wks old, depending on degree of the antigenic stimulation. It is the largest collection of GALT. It contains both T- lymphocytes (50%) and B-lymphocytes (50%) and large numbers of immature and mature plasma cells and with age, the number of B- lymphocytes and plasma cells increases. It involved in the antibody production and cell-mediated immune response. -Peyer's patches, which are located in the intestinal mucosa and structurally similar to the cecal tonsils. Subjacent to the epithelium, there is heavy B- dependent lymphocyte infiltration. Peyer's patches in chickens share several characteristics with mammalian ones. -The lymphoid aggregates which present in the urodeum and proctodeum of the cloaca are also part of the gut associated lymphoid tissue - GALT.
  3. 3. 2-Head-associated lymphoid tissue – HALT HALT is one of the most important components of secondary lymphoid organs which represent all the lymphoid structures and the massive lymphoid cell aggregation which present in the head region. HALT includes: -Harderian (paraocular) gland is located ventral and posteriomedial to the eyeball within the orbit. Its secretory duct opens on the surface of the nictitating membrane and produces a secretion to lubricate the 3rd eye lid. It appears and developed after hatching. It contains numerous plasma cells which produce and secrete primarily IgA and other immunoglobulins. It is the major secondary lymphoid organ of HALT. B-cells comprise 80% of lymphoid cell population while T-cells comprise 20% of lymphoid cell population. Testosterone treatment neither does nor inhibits Harderian gland development, which suggests that this lymphoid organ is relatively bursa- independent. Its stromal elements produce interleukin-7 that influences the proliferation and differentiation of plasma cell. -Conjunctival-associated lymphoid tissue "CALT" is a massive lymphoid cell aggregation located under the mucosa of the conjunctiva. In SPF birds, it is not prominent but it is prominent in poultry esp. turkeys. -Paranasal glands, lachrymal duct and lateral nasal ducts. 3-Bronchial-associated lymphoid tissues - BALT BALT is one of the most important components of secondary, peripheral or seeding lymphoid organs which represent all the lymphoid structures and the massive lymphoid cell aggregation which present in the respiratory tract. BALT in association with the secretory IgA are responsible the presence of the respiratory local or mucosal immunity which is one of the most important immune mechanisms. BALT includes: -Sub-mucosal lymphoid cell aggregation in the respiratory tracts. -Bronchial epithelium whose cells are primarily non-ciliated squamous and then become more columnar ciliated with age. -Lymphoid nodules found in the lung associated with the primary bronchi. 4-Skin-associated lymphoid tissues – SALT SALT is one of the most important components of secondary lymphoid organs which represent all the lymphoid structures and the massive lymphoid cell aggregation which is located under the skin of the whole body. 5-Spleen
  4. 4. It is the graveyard of the body which is located dorsal and to the right of the proventriculus. Accessory spleens may be present. It was found that a maximum spleen to body weight of 0.2% at 10 weeks of age. It is the largest secondary lymphoid organ that is composed of white and red pulps which comprises about 80% of splenic tissue. They are not sharply distinct from each other in chicken. White pulp contains small, T-dependent lymphocytes. Red pulp contains lymphocytes, macrophages, granulocytes and plasma cell. It contains 70% T-lymphocytes and 30% B-lymphocytes. It is the major site for hemopoietic activity in the developing embryo; important for antigen processing and antibody production after hatching. It is the site of phagocytosis of effete erythrocytes after hatching. 6-Mural lymphoid nodules They are lymphoid nodules closely associated with lymph vessels most prominent in the pelvic region of chicken. They are circular, elongated, or oval, non-capsulated and contain diffuse lymphoid tissue within which, are usually found three or four germinal centers. 7-Pineal body or gland It is located between the cerebral hemispheres and the cerebellum. It is essentially the light sensor of the body and may have an immune function. 8-Bone marrow It is essentially a primitive lymphoid organ but after the immune migration, it also acts as a secondary immune organ. As a secondary lymphoid organ, it contains B- lymphocytes, mononuclear cells and T- lymphocytes. 9-Bursa of Fabricius It is essentially a primary lymphoid organ but after the immune migration, it also acts as a secondary lymphoid organ. As a secondary lymphoid organ, it contains B- lymphocytes, mononuclear cells and T- lymphocytes. 10-Thymus It is essentially a primary lymphoid organ but after the immune migration, it also acts as a secondary lymphoid organ. As a secondary lymphoid organ, it contains B- lymphocytes, mononuclear cells and T- lymphocytes. -There are no lymph nodes in birds except for the primitive lymph nodes in the aquatic birds such as cervico-thoracic nodes at the thoracic inlets, which
  5. 5. occur as elongated structures surrounding the terminal part of the cervical lymphatic duct, and the lumber nodes on either side of the abdominal aorta. Percent of B and T-cells in different immune organs: B-cells%T-cells%Organs 8020Harderian gland 5050Esophageal tonsils 8020Meckel’s diverticulum 5050Cecal tonsils 3070Spleen Diagram of avian immune organs B-Immunocommpetent cells I-Avian red blood cells: Birds have nucleated erythrocytes "it contains MHC class-I and may have immune functions". The cell is oval with an oval centrally positioned nucleus. Avian erythrocytes are much larger than mammalian erythrocytes. The size varies among species. It constitutes 3- 3.25 million/cubic mm. Its life span is 35 days" chicken", 35-45 days "pigeon", 42 days "ducks" and 120 days "human".
  6. 6. II-Avian thrombocytes: They are mononuclear cells which are equivalent to the mammalian blood platelets. It is nucleated "has immune function due to the presence of MHC-1". Its number is 25-30 thousands/cubic mm. The avian thrombocytes have a phagocytic activity due to the presence of the lysosomal-like cytoplasmic inclusions and acid phosphatase positive granules. It functions like mammalian platelets in hemostasis. Thrombocytes are smaller and more rounded than mature erythrocytes. Compared to the erythrocyte nuclei, thrombocyte nuclei are more rounded and have a higher nuclear/cytoplasmic ratio. Avian thrombocytes are often mistaken for lymphocytes. The cytoplasm is clear but not homogenous. They contain specific granules in variable number, size and position in the cell. They take a pink to reddish color. They tend to clump, so it is difficult to do a thrombocyte count. Enlargement of the thrombocyte’s cytoplasm indicates a reactive change. Thrombocytes have a phagocytic defense function and the reactive changes are thought to be associated with this function. III-Avian leucocytes: A-Granulocytic leucocytes 1-Heterophils = Microphages: they are polymorphnuclear phagocytic cells which originated in the bone marrow and are equivalent to mammalian neutriphils "hetrophils have no proteolytic enzymes". It is formed in the bone marrow and then transmits to the blood. Its life span is short "few days" and has 4-5 thousand/cubic mm. It has a limited energy and so it is able to do a very limited number of phagocytosis. 2-Eosinophils: they are polymorphnuclear eosinophilic phagocytic cells which originated in the bone marrow and are scanty in number and increase with parasitic infestations. Eosinophils tend to be more irregular than hetrophils. They are typically round and have round granules. Eosinophil cytoplasm is pale blue. Granules may be red, blue or clear. Cell size varies quite a lot. The nucleus of the eosinophil often stains more blue and is more noticeable than the hetrophil nucleus. Eosinophil nuclei are lobed with clumped chromatin that stains purple. 3-Basophils: they are polymorphnuclear basophilic cells, which originated in the bone marrow and are scanty number and may have phagocytic activity. Avian basophils are round with a round nucleus. The nucleus is centrally located and light blue. The cytoplasmic granules stain deeply basophilic and often hide the nucleus. B-Agranulocytic leucocytes 1-Monocyte and macrophage: They originates in bone marrow and then distributed allover the body. Monocyte has smooth contour and always presents in the blood while macrophages has serrated contour and present in the tissues. The skin macrophages is called Langerhan's cell. That of
  7. 7. connective tissue is called histocyte. That of the liver is called Von Kupffer cells. That of the brain is called microglia cell. That of the lung is called alveolar or septal cell. Monocyte-macrophage system is the harmony between these two cells and formerly, it was known as reticuloendothelial system. They are aged cells and their number is 1.5 thousand/cubic mm. Macrophage is potent phagocytic inflammatory cell that predominate during chronic inflammations. It is an important antigen presenting and processing cell and initiates the immune response through their secretion of the IL-1 which stimulates T-helper cells. 2-Lymphocyte: They are the functional units of the immune system. They are round but can sometimes look irregular due to molding around other adjacent cells. The nucleus is round. The amount of cytoplasm may vary from a narrow band to abundant cytoplasm in large lymphocytes. The nuclear to cytoplasmic ratio is high. The cytoplasm is light blue and hyaline. Antigenic stimulation transforms lymphocytes into reactive lymphocytes. There are two principal classes of lymphocytes, T-lymphocytes so named because of their dependence on the thymus for their maturation from the stem cell, and B-lymphocytes, derived from the bursa of the avian species. They differ in their antigen receptors, surface markers and functions. In some avian species lymphocytes are the most common leukocyte. a-T-Lymphocytes It originates in bone marrow and maturates in the thymus gland. It constitutes 70-80% of the total lymphocytic count. It always circulates in blood seeking for antigens. It is responsible for cell-mediated immunity. It contains one of two surface receptors “CD4 which is the receptor of MHC class-II and CD8 which is the receptor of MHC class-I. T-lymphocytes have five sub-populations: 1-T-helper cells: It is the master cell of all immune cells. It secretes certain lymphokines which activate T-helper themselves; activate B-cells helping them to be transformed to plasma cells which produce antibodies and activate T- cytotoxic lymphocytes, helping them to do cytotoxcity and destruct the target pathogen. There two types of T-helper lymphocytes; T-helper-1 and T-helper- 2. T-helper-1 secrets the IL-2, INF-γ and TNF-β and responsible for macrophage activation, B-cell proliferation and transformation. T-helper-2 secrets IL-4, IL-6 and IL-10 which are responsible for the regulation of antibody formation.
  8. 8. 2-T-cytotoxic (Tc) cells: carry surface marker CD8, recognize epitopes of the endogenous antigens as virus-infected cells or cancer cells in association with MHC class I. They lyse these cells that carry particular viral epitope and cancer cells. Theyb internalized these antigens and degraded them into small antigen peptides usually 7-13 amino acid long by a large proteolytic complex called proteasome. They transported these small antigen peptides to the endoplasmic reticulum where the peptides become attached to MHC- I. The peptide-MHC-I complex is transported to the cell surface for possible recognition by antigen-specific T-cytotoxic. 3-T-Suppressor cells: They regulate T-cell or B-cell mediated immune responses. They suppress the activity of them after the immune system destroyed the pathogens. 4-T-delayed cells: They are generally CD4 (rarely may be CD8+) and considered to be a sub-population of Th-cells, and recognize peptides in association with MHC class II (sometimes MHC class I) protein. They secrete a variety of lymphokines which set up inflammatory response and greatly augment the immune response by attracting monocytes, macrophages and other T-cells to the site of inflammation. They activate monocytes, macrophages, and other T-cells to proliferate and secrete additional cytokines activating themselves. It predominates during the chronic inflammation and delayed hypersensitivity.
  9. 9. 5-Memory T-cells: carry CD4 or CD8 and responsible for rapid potent reaction in succeeded infections by the same pathogen. b-B-Lymphocytes, plasma cell and memory B-cell: B-Lymphocytes: It originates in bone marrow and maturates in bursa of Fabricius. It is larger than T-lymphocytes but it is short lived "5-7 days". It constitutes 10-15% of the total lymphocytic count. It is responsible for the antibody-mediated immunity. It contains primitive IgD as a receptor for antigens. Under the effect of IL-2, it is transformed to plasma cell that secret antibodies. There are 1013 B-cell clones. Sometimes, they act as antigen presenting cell. Some of them remain as memory cells. They have antigen-specific binding receptors on their surface, receptors for complement (C3) and receptor for Fc portion of antibodies. Each individual B-lymphocyte and its progeny expresses only one of these antibody genes, hence by the time a bird is hatched there is a vast number of B-lymphocytes expressing different monomeric IgM molecules as surface receptor proteins. The receptors of B-cells are antibody-like molecules that recognize antigens in their native state, rather than peptides bound to MIC protein, hence B cells react directly with viral proteins or even with virions. When the particular clones of B-cells bearing receptors complementary to anyone of the several epitopes on an antigen bind that antigen, they respond, after receiving the appropriate signals from the cells by differentiation into antibody secreting plasma cells. Plasma cells: It originates from the activated B-lymphocytes under the effect of IL-2. It is responsible for the formation of the different types of the antibodies. Each plasma cell can produce 300 antibody molecules/second. Their life span varies from 3 days to 4 weeks. Each plasma cell secret antibody of only a single specificity, corresponding to the particular V domain of the antibody receptor it expresses. Initially this antibody is of the IgM class, few days “about 9-10 days” switch to IgG and IgA. The early antibodies are of low avidity, while later become of high avidity. Memory B-cells: It originates from B-cells under the effect of IL-1 and IL- 5. It has very higher affinity to the previously invading antigens than the original B-cells. It is responsible for rapid potent reaction in succeeded infections by the same pathogen. c-Non B Non T lymphocytes: 1-Killer cell: it is a unique subset of lymphocytes which originates in bone marrow and responsible for antibody-dependent cell-mediated cytotoxcity (ADCC). They can kill the target cells without the participation of the complement if the target cells are coated with specific antibodies. It is very important and active cell in competing against the viral infections.
  10. 10. 2-Natural killer cell (NK) or Null cell It is non-B-non-T lymphocytes or null cells. They constitute about 5% of the total lymphocytic count. They are similar to T-cytotoxic cells in their activity of destroying the permanent virus infected cells and the cancer cells. The basis for their selectivity for virus-infected cells is not known. It is also similar to the killer cells in the activity of antibody-dependent cell-mediated cytotoxcity (ADCC). They display no immunologic specificity for particular viral antigens, no memory, no MHC restriction and no dependence on antibody. They may be an important early defense mechanism, since their activity is greatly enhanced within 1-2 days of viral infection. Virus-induced activation of NK- cells is mediated by interferon, acting synergistically with IL-2. NK cells themselves secrete several cytokines including IFN-y and TNF-. Immunocommpetent cells C-Avian immune mediators Avian Cytokines Avian immune mediators or avian cytokines are activated protein secreted from T-lymphocytes, B-lymphocytes, macrophages, dendertic cells, the natural killer cells and sometimes other cells.
  11. 11. They act as chemical messengers which are responsible for regulating the immune responses. Avian cytokines which are secreted from lymphocytes are known as lymphokines while those which are secreted from the macrophage are called nonokines. There are about 90 known mediators up till now, but the most important types are summarized in the following table: Interferons Interferons are non-antigenic glycoproteins which are produced by certain particular cells in response to viral or non viral inducers. Interferons act on the cells rendering them refractory to the virus infection. Interferons include 3 types; alpha (Type-1 or leucocytic interferon), beta (Type-1 or fibroblast interferon) and gamma (Type-2 or immune interferon) types. Interferons are species specific and not virus specific i.e. they act on the cells and not on the viruses. The abbreviation of interferon is IFN. -interferon is produced by B-lymphocytes or macrophages under the effect of viruses as inducing agents. Β-interferon is produced by fibroblasts or epithelial cells as a response to viruses or nucleotides. γ-interferon is produced by T-lymphocytes under the effect of antigens or mitogens as inducing agents.
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Structure of avian immune system Dr Fares El-Khayat

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