Cell-Mediated Immunity: Production of Cells of Immune System

This immunity system has the ability to distinguish the body cells from foreign cells. Any foreign molecule is called as antigen (Derived from the terms; Antibody & generation).

The antibodies are produced in response to the antigens. The antigens are mostly proteins or polysaccharides found on the cell walls of the invading germs.

Even the pollen, egg white, shell fish, some fruits, chicken, feathers, vegetables, blood cells of others, tissue, organ transplants, drugs, chemicals can act as antigens.

(1) Production of cells of immune system (Fig. 6(B).1):

The lymphocytes of WBC are the major cells of the immune system. The lymphocytes are of two types’ β-cells and T-cells. These β-cells and T-cells are produced from the stem cells of liver in the embryo and from bone marrow cells of adults. Some of the lymphocytes migrate into the thymus and differentiate into T-cells or T-Lymphocytes, while the lymphocytes, present in the bone- marrow remain as β-cells.

The lymphocytes migrate to lymphoid tissues like spleen, lymph nodes and tonsils, where they undergo maturation. Matured lymphocytes circulate along with body fluids and remain in lymph nodes. The T-cells are responsible for cellular immunity where as β-cells, take part in humoral immunity. Both of them require antigens to activate them into action, but they react differently.

Antibody:

It is a specialized protein, capable of combining with the specific antigens which stimulates its production.

(i) Structure:

All antibodies are proteins with two long or heavy chains and two short or light chains. The specific antigen binding site differs from one antibody to the other.

(ii) Immunoglobins:

Antibodies belong to the family of proteins known as immunoglobulin’s. These are of five classes. (I_g-immunoglobulin)

(a) I_gG. Gamma globulins are most abundant of plasma antibodies.

(b) I_gM. It provides specific immunity against bacteria and viruses.

(c) I_gE. It responses for allergen.

(d) IgA. These are secreted in the gut, respiratory and genito-urinal tract. These are mostly present in the milk.

(e) I_gD. Its function is uncertain.

(iii) Production of Antibody (Fig. 6(B).2):

When a foreign antigen reaches the lymphoid tissues, it triggers off antibody synthesis. Some p-lymphocytes are stimulated to divide and produce a clone of plasma cells. These plasma cells produce antibodies at the rate of about 2000 molecules per second. The ‘capacity’ to produce a specific antibody is required by the p-cells during its development. Some of the cell produced by β-cells differentiates into memory cells. It rapidly recognises the antigen, if it reappears again.

(2) Cell-mediate immune system:

It is carried out by highly specialized cells and works against pathogens (including fungi and protozoans). This system also reacts against the transplant of organs. It is also believed that it protects the body from its own cells if they become cancerous.

How T-cells respond to Antigens (Fig. 6(B).3):

T-cells also respond to antigen by producing a clone of T-cells. Each T-cell recognizes a specific antigen. So the body contains separate T-cells for every antigen that enters into the body. Normally T-cells remain alive for 4 to 5 years or even more. The clone of T-cells for a particular antigen is similar, but they perform different functions. According to their function. There are three types of T-cells.

1. Cytotoxic T-cells:

The cytotoxic T-cells are sensitised which combine with antigens present on the cell surfaces of foreign pathogens. It releases some chemicals which break down the plasma membrane, killing the pathogens. Recently another population of killer cells are discovered as Natural Killer (NK) cells. They kill the pathogens directly. It can bind to different antigens unlike cytotoxic T-cells. They mostly destroy tumor cells and virus inflected host cells.

2. Helper T-cells:

The activated T-cells stimulate (3-cells to produce more antibodies).

3. Suppressor T-cells:

These cells suppress the entire immune system keeping it away from attacking the own body cells. Some of these cells become memory cells.