The document summarizes key aspects of immunity in chickens. It discusses innate and acquired immunity, the immune organs including the thymus and bursa of Fabricius, and the two main types of acquired immunity - antibody-mediated and cell-mediated responses. The antibody-mediated response involves B cells producing antibodies that target pathogens, while the cell-mediated response uses T cells to identify and destroy cells infected with pathogens. Together these immune responses provide chickens with protection against disease.

1. IMMUNITY OF CHICKEN
Prepared by: Dr. Muhammad Umer Haider
DVM(Gold Medalist), Mphil Pathology
Technical Head

2. IMMUNITY & IMMUNE SYSTEM
• The ability of the body to fight infection and/or foreign
invaders by producing antibodies or killing infected cells. It is
derived from the latin word ‘immunis’ which means free or
exempt .It is the ability of the body to resist infection.
• The system in the body responsible for maintaining
homeostasis by recognizing harmful from non harmful
organisms and produces an appropriate response.

4. IMMUNE ORGANS
Primary Organs
1) Thymus gland – T-cell system – Cell-
mediated immunity
2) Bursa of Fabricius – B-cell system –
Humoral immunity
3) Bone marrow – Precursor blood cells
4) Yolk sac – Maternal immunity
Peripheral Lymphoid Tissue
1) Harderian gland
2) Caecal tonsilles
3) Spleen
4) GALT

6. TYPES OF IMMUNITY
• It has two components
• Innate immunity
• Acquired or adaptive immunity

8. INNATE IMMUNITY
• This refers to the natural or inherited ability to resist disease.
• Included in this type of immunity are a number of non-specific disease response
mechanisms.It is seen before birth itself .
• This word comes from latin word ‘innatus’ which means to be born with . Birds
have well developed innate defence mechanisms. It is a slow and non-specific
immunity.
• It has three components
1) Barrier to physical attack
2) Chemical defense system
3) Cellular components

9. ACQUIRED IMMUNITY
• This refers to immunity provided by immune cells or antibodies
that are produced in response to exposure to an antigen.
• These cells and proteins circulate through the body in the blood.
• It includes targeted recognition of specific molecular features on
the surface of the pathogen.
• It is also called adaptive immunity

12. IMMUNE RESPONSE
• The primary and most important function of the immune system is
to provide the bird with the ability to resist the entry and harmful
effects of the infectious agents.
• The protection to birds against infection is brought about by both
antibodies and lymphocytes.
• The immunity induced by antibody is referred as ‘humoral
immunity’, likewise the immunity induced by the cells are called as
‘cell mediated immunity.
• These are the two important segment of acquired immune system.

13. CONT..

14. ANTIBODY –MEDIATED IMMUNE
RESPONSE
• The chicken blood has two types Red cells and White cells
• Normally lymphocytes constitute 60% of white blood cells
• Lymphocytes are produced in the bone marrow from (soft tissue
within the bones)from a cell that has the potentiality to develop into
several types of blood cell.
• This cell is known as ‘pluripotent stem cell’ .
• During incubation of the egg , in the developing chick , the
lymphoid stem cells migrate into two different direction through
bloodstream.

15. CONT..
• Bursa and thymus are called primary lymphoid organs because they
regulate the production and differentiation of lymphocytes .
• One set (B-cell precursors) goes into bursa of fabricius and another
set of cells (T- cell precursors ) goes into thymus.
• Mature lymphocytes of chicken blood fall into two major
populations , B- lymphocytes or Bcells and T-lymphocytes or T-
cells.
• B –cell mature in bursa and T- mature in thymus. After maturation
B and T cells migrate to secondary lymphoid organs.

16. •

17. ANTIBODIES
• Antibodies are very small bodies of a very special protein (globulin protein) that
animals release into their circulatory system to fight invading viruses and bacteria.
• Antibodies are short lived.
• They are specific for the antigen to which they attach.
• For example, the antibody against Newcastle disease virus will attach only to the
Newcastle disease virus, not to the infectious bronchitis virus.

18. MODE OF ACTION
• Antibodies do not have the capability to kill disease organisms directly.
• Antibodies perform their function by attaching to disease organisms and
blocking their receptors and prevent them from attaching to their target cell
receptors in the chicken.
• For example: 1. An infectious bronchitis virus which has its receptors
covered with antibodies will not be able to attach to and penetrate its target
cells, the cells lining the trachea.
• 2. The attached antibodies also immobilize the disease organism which
assists their destruction by macrophages.

20. TYPES OF ANTIBODIES
• In chicken 3 main type of antibodies ( Ig ) are produced .
• IgM: First antibodies produced in chicken which is actually five of the Ig’s (IgY ) joined
at the tails in a circular fashion. • The function of IgM is mainly to arrest the entry of
infectious agent.
• IgG: It consists of only one ‘y’ and hence five times smaller than IgM. It causes
clumping of the infectious agents and attach to their surfaces. This facilitates
phagocytosis.
• Ig A: The third important antibody in the chicken. • IgA is produced mainly to protect
body surfaces . This process is known as “immune elimination “. In chicken 30% to 75%
of IgA produced in the intestinal wall diffuse into the blood circulation for reaching the
liver.

22. PARENTAL TRANSMISSION OF ANTIBODIES
• A day old chick has an inadequately developed immune system and
therefore depends on the immunity from the mother.
• Newly hatched chick has temporary immunological protection until
systemic production of antibodies begins IgG in yolk is having equal
antibody titer as hen’s blood.
• IgM and IgA from oviduct secretion are found in the albumin.
• IgM and IgA from the albumin diffuse into the amniotic fluid and entered
into the embryo.
• The function of maternal antibodies is to prevent pathogenic organisms
from causing disease in young chicks.

23. CELL MEDIATED RESPONSE
• Lymphoid stem cells undergo a period of processing in the thymus and differentiate
into mature lymphocytes.
• The differentiated mature cells are called ‘thymus-derived lymphocytes’ , or ‘T-
lymphocytes’ , or simply as ‘T-cells’ . T stands for thymus.
• The processing and differentiation transform them into ‘cytotoxic T-cells’.
• That is they acquire the ability to kill the cells that harbour the infectious agents.
• By the time thymus disappears, which soon after sexual maturity , T-cells populate
various secondary lymphoid organs.

24. PURPOSE OF THE CELL-MEDIATED
IMMUNE RESPONSE
• The B-cell antibody system is meant basically to destroy the infectious agents before they
reach their targets. T-cells system is to deal with the infectious agents after they reach
their target cells.
• The cell-mediated immune response is meant to deal with the infectious agents after they
reach their targets , the cells. T-cells are manufactured in the thymus ,a gland present in
the neck of birds.
• While there is only one kind of antibody-producing B-cell, there are several kinds of T-
cells , each serving different functions.
• These are
• Cytotoxic T-cells-serve to kill cells infected with disease agents.
• Helper T-cells-helps in the mediation of the immune response.
• Suppressor T-cells-control cells of the immune response.

25. MODE OF ACTION
• Those cells in which the infectious agents grow within and cannot be attacked
by antibodies are destroyed by the cell-mediated immune response. •
• Since viruses grow within the cells , virus-infected cells and all those cells in
which bacteria or other infectious agents grow within ( as in avian tuberculosis
) , are destroyed by cytotoxic T-cells to protect the body from any further
damage.
• Cytotoxic T-cells have receptors on their surface for that lock and key match.
Each cell possesses about 10,000 – 20000 exactly similar receptors.
• Because all receptors are exactly the same , one T-cell will bind and kill only
one type of infectious agent.

27. Role Of T-Helper Cells
• That neither the cytotoxic T-cells which kill the infected cells , nor the B-cells which
produce protective antibodies , can function without the signals received from ‘helper T-
cells’.
• Helper T-cells secrete certain chemical substances (cytokines) which act as messenger
molecules.
• This molecules help both B-cells and T-cells in the mediation of antibody and cell-
mediated immune responses, hence the name ‘Helper T-cell’ .
• In other words ,these cells regulate the immune responses.

28. ROLE OF MACROPHAGES
• The B –cells and T-cells do not themselves pick up the infectious agents.
• The infectious agents are first engulfed by a cell called ‘macrophage’.
• It is derived from blood cell known as ‘monocyte ‘.
• Infectious agents are broke down ,processed and then presented to T-cells
and B-cells.
• Macrophages thus acts as ‘antigen-processing’or ‘antigen-presenting cell’

30. PRIMARY IMMUNE RESPONSE
• This is a response to an invader the
First time the invader infects the
body.
• No measurable immune response for
first few days.
• Next 10 – 15 days antibody
production grows steadily
• A more rapid response to an invader
the 2nd time it invades the body.
• Antibody production increases
dramatically and in a much shorter
time period.
SECONDARY IMMUNE RESPONSE
IMMUNE RESPONSE