Immunity in Animals
Immunity is the ability of the body to resist the tissue damage by destroying the disease organisms or neutralizing their toxins.
Types of immunity
There are two types of immunity present in animals- Innate immunity and Acquired immunity.
Innate immunity
This type of immunity is present from birth. This type of immunity is non specific and act on many organisms. It prevents first entry of microorganisms into the tissues or if the organisms has gained entry, eliminates them prior to the occurrence of the disease.
- This type of immunity is not effective on subsequent exposure to the same organisms. Non specific elimination of microorganisms is done by two methods:
- Phagocytosis: It is ingestion and killing of microorganisms by specialised cells such as neutrophils, monocytes, macrophages etc.
- Opsonization: It is the process of coating the microorganisms with some of the proteins found in the plasma to make them more phagocytosable.
Acquired immunity
The development of immunity against specific organisms, the bacteria, virus and foreign tissues or toxins is referred to as acquired immunity.
This type of immunity is also known as adaptive or specific immunity. It is caused by a special immune system that forms antibodies or activated lymphocytes that attack and destroy the microorganisms.
- Types of Acquired immunity is-
- Humoral immunity or B cell immunity
- Cell mediated immunity or T cell immunity
In mammals, shortly before or after birth the stem cells or the committed lymphoid progenitor cells migrate either to thymus, liver and bone marrow (in mammals) or to bursa of Fabricius (in birds) where they are processed and differentiated.
The `T’ lymphocytes are processed in the thymus and `B’ lymphocytes are processed in bursa, liver and bone marrow. These lymphocytes then get into the circulation and localised in spleen and lymph nodes as clones of `T’‚ and `B’ lymphocytes.
Thymus plays a key role in the development of cellular immunity shortly before birth or few months after birth. The T-lymphocytes provide cellular immunity‚ and the B-lymphocytes provide humoral immunity‚ by producing specific antibodies. There are different types of B and T cells, each of which responds specifically to one particular antigen.
All the different B and T cells capable of forming a specific antibody or activated T cells against a specific antigen which are called “clone of lymphocytes”.
The invading organisms provide proteins, large polysaccharides and large lipoprotein complexes, which function as an antigen. The antigen stimulates specific clone of lymphocytes.
Macrophages in lymphoid tissues stimulated by antigen releases interleukin I, and it promotes growth of specific lymphocytes. The helper T cells release interleukin II, which stimulate B cells to produce specific antibodies. Those B-lymphocytes specific for an antigen enlarge and form lymphoblasts, which differentiates to plasmablast and finally to plasma cells. The plasma cells produce antibodies very rapidly.
When these sensitised plasma cells are activated by an antigen, specific antibodies, the gamma globulins- IgG (75%), IgM, IgA, IgD and IgE (during allergy) are produced by them as primary response. However these antibodies have weak potency and short life.
Some of the lymphoblasts remain dormant and function as memory T and B cells, which on subsequent stimulation by the same antigen (booster) produce more potent secondary response. On exposure to a specific antigen, the T cells of the specific clone proliferate and release large numbers of activated T cells, which neutralises the antigen. The memory T cells are retained in the lymphoid tissues.
Humoral immunity is developed as specific antibody (gamma globulin) by the “B”-lymphocytes, rarely persists more than few months or utmost few years. On the other hand, the specific clones of “T” lymphocytes on stimulation by the very same antigens are converted into three major groups of “T” lymphocytes.
Cytotoxic or killer “T” cells: These are the direct attack cells killing the micro-organisms. They have the lysosomal enzymes, the peroxidases, phosphatases and oxygenases and produce H2O2, which is lethal to the invading organisms.
Helper `T’ cells: These cells are very sensitive even to very small quantities of the specific antigen, release interleukin-2 (lymphokines)‚ and activate the other two types of “T” cells, the cytotoxic and suppressor “T” cells and also the “B” lymphocytes. Helper “T” cells are not capable of secreting antibodies by themselves. It promotes the production of lymphokines.
Lymphokinesact as a macrophage migration inhibition factor, stop the chemotactically attracted macrophages and neutrophils in the infected area to effect more efficient phagocytosis, and thus limit the spread of the infection.
Suppressor or regulatory “T” cells: Regulate the activities of both the helper and killer “T” cells. The activated “T” lymphocytes provide the cell-mediated immunity, which is far more persistent for many years than humoral immunity.
