Important Properties of Antibodies

The immune system in mammals is capable of producing a seemingly unlimited number of antibodies that can react with the millions of antigens we potentially encounter in our lifetimes. This genetic diversity is a key to the effective protection of our bodies against pathogens that are constantly evolving new antigen molecules.

Does the cell have million of genes to create all this variability? This was one early theory, but given the small number of genes in the human (about 30,000), it is clear that this cannot be the case. Antibody diversity was difficult to understand until researchers realized that eukaryotic genes are formed by remarkable recombination events.

The essence of the current theory of antibody diversity is that the genes responsible for the synthesis of a particular antibody are not contiguous units, but are assembled from clusters of gene fragments present in regions of the DNA. One section of these gene fragments codes for the constant region of the antibody, while several other sections, present in multiple copies, code for the variable regions. Somatic recombination allows shuffling of these gene segments into numerous gene combinations that encode for the varieties of light chains and heavy chains that make up complete antibodies. This shuffling begins in the B-cell germ line and continues as new B-cells are formed and differentiate. Each mature B-cell therefore has a unique combination of these regions and makes a unique antibody.

Antibody Structure – 

Antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to neutralize pathogens such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the pathogen, called an antigen, via the F_ab’s variable region. Each tip of the “Y” of an antibody contains a paratope (analogous to a lock) that is specific for one particular epitope (similarly, analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attach by other parts of the immune system, or can neutralize its target directly (for example, by inhibiting a part of a microbe that is essential for its invasion and survival). Depending on the antigen, the binding may impede the biological process causing the disease or may activate macrophages to destroy the foreign substance. The ability of an antibody to communicate with the other components of the immune system is mediated via its F_c region (located at the base of the “Y”), which contains a conserved glycosylation site involved in these interactions. The production of antibodies is the main function of the humoral immune system.

Antibodies are secreted by B-cells of the adaptive immune system, mostly by differentiated B-cells called plasma cells. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell to be free in the blood plasma, and a membrane-bound form that is attached to the surface of a B-cell and is referred to as the B-cell receptor (BCR). The BCR is found only on the surface of B –cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called plasma cells or memory B-cells that will survive in the body and remember that same antigen so the B-cells can respond faster upon future exposure. In most cases, interaction of the B-cell with a T-helper cell is necessary to produce full activation of the B-cell and, therefore, antibody generation following antigen binding. Soluble antibodies are released into the blood and tissue fluids, as well as secretion to continue to survey for invading microorganisms.