The major cells of the immune system
. Lymphocytes that are critical for immune reactions are of two types namely B-cells and T-cells. Both cells develop from stem cells
located in the liver of the foetus and in bone marrow
cells of adults.
The lymphocytes which are differentiated in the bone marrow are B-cells. The lymphocytes that migrate to thymus
under its influence are called T-cells. The young lymphocytes migrate to lymphoid tissues
such as spleen
, lymph nodes and tonsils where they undergo final maturation. Matured lymphocytes circulate in the body fluids. T-cells are responsible for cellular immunity
and B-cells produce antibodies about 20 trillion per day.
Both components require antigens
to trigger them into action but they respond differently. Antigens
An antigen is a substance when introduce into an individual, stimulates the production of an antibody
with which it reacts. Antigens are large molecules of proteins
or polysaccharides. Some of the antigens are the parts of microorganisms
others include pollen, egg white, certain fruits, vegetables, chicken, feathers etc.Antibodies
Antibodies are protein molecules called immunoglobulin (Ig
). They are produced by lymphocytes. The antibodies inactivate antigens. An antibody consists of four amino acid
chains bounded together by disulphide bonds. Of the four chains two are long, heavy chains and two are short, light chains. All of them are arranged in the shape of the letter ‘Y’. The tail portion of antibody having two heavy chains is called constant fragment (Fc). On the tip of each short arm, an antigen- binding fragment (Fab) is present which specifically hold antigen.
Based upon the five types of heavy chains, the immunoglobulin's are classified into five major types. Light chains are similar in all types of Immunoglobulin's.
TYPES OF IMMUNOGLOBULIN'S
lgG is the most important long acting antibody representing about 80% of the antibodies. The second important antibody is IgM. IgA is called secretory antibody, found in tears, saliva and colostrum, (the first milk secreted by mother). IgD serves as a receptor site at the surface of B cells to secrete other antibodies. IgE plays an important role in allergic reactions by sensitizing cells to certain antigens.