Antibodies

Typical immunoglobulin consist of 4 polypeptides: 2 short ones called light chains and 2 long ones called heavy chains. They form Y shape. Tips of the fork are where antigens are bound. It’s chemical structure gives it shape that fit the one of the antigen. Each polypeptide have constant and variable parts. The constant part is characteristic for each of the 5 classes of human immunoglobulins (IgM, IgG, IgA, IgD, or IgE). They have different functions and are present in different parts of the body. IgG antibodies (most common), can be found in blood and tissue fluids. IgA antibodies are present in the nose’s, air passages’, alimentary canal’s discharge, saliva and tears. IgE antibodies take part in allergic reactions. The variable part gives antibody it’s specificity and that it can combine only with certain antigen. Antibodies are narrow specialists. They bind with one antigen and don’t see other even similar molecules.

Reproductive cells and cells of the early embryo don’t contain antibodies’ genes. They are put together later when lymphocytes originate. Light chain gene origins as a result of DNA recombination in of the chromosomes. One of the 300 versions of variable part fragment (V region) is randomly spliced with one of the 5 versions of the second fragment of variable part (J region) and with fragment that codes for constant part (C region). As a result of rearrangement enhancing sequence is placed near to the promotor and gene is activated. It’s product is immunoglobulin’s light chain. Different lymphocytes put different fragments together and consequently there are 15 000 genes coding for the light chain. These are even more of them because V and J regions are often inaccurately spliced. Light chain can join many heavy chains. It is assumed that lymphocytes can produce 100 000 000 000 genes coding for antibodies.

The mechanism of rearrangement explains how such a huge number of genes can be reached. If they would be all part of genome they would take an disproportionately big share. Besides organism doesn’t know all antigens it will have to protect against. Antibodies originate without contact with antigens. Producing vast number of different antibodies increases probability of efficient defense against every foreign molecule.