Avian Immunity and its types

Originated from the latin word “immunis” means “exempt” indicating the state of protection from infectious disease. Immunity is a balanced state having adequate biological defenses to fight infections, diseases or other unwanted biological invasions.

Avian Immunity and its types

It mainly defends the body against invaders. An antibody is a protein produced in response to an antigen. Antigens are macromolecules that elicit an immune response in the body. The most common antigens are proteins and polysaccharides.


Two types of immunity (1) Innate immunity (non specific) (2) Adaptive or Acquired immunity (specific)

Further has two types (a) Humoral or Passive immunity (b) Cell mediated or Active immunity


It is the natural resistance with which a person is born.  It provides the first line of defense against infection. It provides resistance through several physical, chemical and cellular approaches. Several soluble molecules contribute to innate immunity and among these components are proteins like lysozyme, interferon proteins and components of complement system.


It is a specific kind of immunity. Not same in all members of a species and have three characteristics like (1) Antigenic specificity (2) Diversity and (3) Immunologic memory.


Naturally acquired active immunity occurs when the person is exposed to a live pathogen, develops the disease and becomes immune as a result of the primary immune response.

Artificially acquired active immunity can be induced by a vaccine, a substance that contains the antigen. A vaccine stimulates a primary response against the antigen without causing symptoms of the disease.


Naturally acquired passive immunity: in which certain antibodies are passed from the maternal into the fetal bloodstream.

Artificially acquired passive immunity: a short-term immunization by the injection of antibodies, such as gamma globulin that are not produced by the recipient’s cells. 

Humoral and cell-mediated immunity

Defend against different types of threats. Acquired immunity includes two branches. The humoral immune response involves the activation and clonal selection of B cells, resulting in the production of secreted antibodies. The cell-mediated immune response involves the activation and clonal selection of cytotoxic T cells.


The avian immune system operates on the same general principles as the mammalian immune system. Antigenic stimulation initiates an immune response that involves cellular cooperation most notably between B lymphocytes, T lymphocytes, cytokines and chemokines. The bird’s immune system mainly consists of lymphatic vessels and lymphoid tissues.


Primary lymphoid organs:  Thymus, Bursa of Fabricius and bone marrow are primary avian lymphoid organs. Secondary lymphoid organs:  Spleen, cecal tonsils, payer’s patches, mucosal associated lymphoid tissues (MALT), lymph nodes, germinal centers and diffuse lymphoid tissues are secondary lymphoid organs. The thymus, where T cells develop is located in the neck of birds.

The Bursa of Fabricius is an organ that is unique to birds and is the only site for B cell differentiation and maturation. Located in the rump of birds, this organ is full of stem cells and very active in young birds but atrophies after six months. Bronchial associated lymphoid tissue (BALT) and gut associated lymphoid tissue (GALT) are found along the bronchus and intestines, respectively. In the avian respiratory system heterophils are an important part of bird immunity.

Within the head, there is head associated lymphoid tissues (HALT) that contain the Harderian gland, lacrimal gland and other structures in the larynx or nasopharynx. The Harderian gland is located behind the eyeballs and is the major component of HALT.

It contains a large number of plasma cells and is the main secretory body of antibodies. Alongside these primary and secondary lymphoid organs, there is also the lymphatic circulatory system of vessels and capillaries that communicate with the blood supply and transport the lymph fluid throughout the bird’s body.


The immune response development has different stages

Antigen overcome non-specific natural barriers → recognized as foreign to the organism → Macrophages and heterophils engulf the antigen → attract T lymphocytes and inflammatory cells → Antigen is processed and presented to immune cells → Cytokines (chemical messengers secreted by T helper) attract B lymphocytes to the site → B cells proliferate and produce specific antibodies → antigen is eventually eliminated → birds have Memory cells which allow a quicker response to that particular antigen in case of another exposure to it


  • Immunosuppression can be due to infectious agents, improper feeding balances (deficiencies), lack of biosecurity, managemental failures (stress) or to a combination of them.
  • Immunosuppression may affect both health and performance. Increased mortality, uneven growth, decreased body weight, higher feed conversion and increased medication costs are common findings in immunosuppressed birds.
  • Performance results must be correlated with the examination of the main lymphoid tissues to establish a complete and practical evaluation of the immune system.
  • Damaged lymphoid tissues and cells: including bursa of fabricius, thymus, spleen and bone marrow etc.
  1. Viral infections: Infectious bursal disease virus (Gumboro disease), Chicken infectious anemia virus, Marek’s disease virus, Reovirus, Reticuloendotheliosis virus, Avian leukosis virus (responsible for myeloid leukosis), Newcastle disease virus, Infectious bronchitis virus and Avian pneumovirus etc.
  2. Bacterial infections: g. E.coli and Mycoplasma infections mainly effect the cell mediated immunity in birds.
  3. Parasitic infections: e.g. Coccidiosis

(3) Non infectious causes:

Nutritional imbalances: Diet effect on immunity as low protein diet and diet deficient in selenium cause immune suppression. The diet low in B complex, vit C and vit E cause atrophy of bursa of fabricius, thymus and spleen. The consumption of lead, mercury and cadmium lead to immunosuppression.

b. Mycotoxicity: Aflatoxin B1, Ochratoxins, Fumonisins and T2 toxins cause depression in antibody producing cells in lymphoid organs and lead to decrease in size of bursa and thymus.

c. Ammonium or dust: They act by damaging the respiratory system. The target concentration of ammonia should be less than 10ppm.

d. Stress: Cold or heat stress, lack of access to drinking water, density, poor ventilation and noise cause stress. Long term stress is responsible for the release of steroids that are immunosuppressive as they decrease the synthesis of lymphoid cells.

Antibiotics: e.g. Chlortetracycline and oxytetracycline have adverse effect on the immune system development.


If the chicken is exposed a second time to the same antigen the response is quicker and a much higher level of immune cell and antibody production occurs (memory). This is the basis for vaccination

Types of Vaccines

There are different types of vaccines include (1) Live and live-attenuated vaccines (2) Killed (or inactivated) vaccines (3) Sub-unit vaccines (4) Recombinant vaccines (5) DNA vaccines (6) Conjugate vaccines


  • First try to identify the causative agent through a sound diagnosis.
  • In addition to post mortem examination, the diagnosis will usually require histopathology analysis g. bursa, spleen, liver, sciatic nerve, thymus and brain.
  • Evaluate the flock’s performance.
  • Check the day old chick quality with a special focus towards vertically transmitted infection.
  • Revise management practices, starting with biosecurity, good sanitation and litter management.
  • Check the diet composition and quality.
  • To minimize the spreading risk of any disease to susceptible birds, always travel from youngest to oldest age birds.
  • Implement a relevant vaccination program. This includes to choose the right product strengths, to vaccinate at the right time, to deliver every vaccine using the most appropriate administration route.
  • Adequate vaccination of the breeder flocks is necessary as gumboro disease, chicken anemia, reovirus, salmonellosis, mycoplasmosis, marek’s disease and adenoviruses vaccination in breeders will provide protection to the progeny (‘passive immunity’) for the critical first weeks of age.
  • Antibody surveillance program to check the immune status of birds periodically
  • A continuous control of the immunosuppression causes is importance to protect the integrity and function of the immune system. This will give better flock health, performances and response to any vaccination.

Zain Ul Abadeen (Ph.D. Scholar),

Department of Pathology, Faculty of Veterinary Science,

University of Agriculture Faisalabad, Pakistan