Structure of avian immune system Dr Fares El-Khayat

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.