Antigens stimulate an immune response by immune system cells with specific antigen receptors. Antibodies, produced by B lymphocytes, bind to specific antigens. Memory cells can produce antibodies upon re-exposure. T helper cells stimulate antibody production, while T killers kill infected cells and antigens.
Any molecule that is recognized by the body as foreign, or not belonging to the body, is called an antigen. When an antigen enters the body, it stimulates an immune response by the cells of the immune system. These cells recognize different antigens through the use of receptors on the surface of their cell membranes. Each cell has a specific antigen receptor, so the cell will only be activated by the specific antigen.
The antigen receptor is a polypeptide chain or a chain of amino acids. Its structure corresponds to the form of the antigen to which it is specific. This structural specificity that the antigen receptor has is what allows each cell to correspond to only one type of antigen.
One type of immune cell, lymphocytes, are capable of producing a type of protein called an antibody. Each antibody is also specific for a particular antigen. Only when that antigen is found in the body will the production of the antibody be stimulated. Antibodies do not have antigen receptors on their cell membranes, but instead have a very specific three-dimensional shape that allows them to bind only to the relevant antigen. This area is referred to as the antigen binding site.
There are two types of lymphocytes involved in the production and secretion of antibodies, B lymphocytes and T lymphocytes. B lymphocytes, or B cells, are the lymphocytes that produce and secrete antibodies. As B cells mature, a small number of antibodies are produced, but not released from the cell. Instead, part of the antibody forms a protein antigen receptor on the surface of the cell membrane. Depending on whether an antigen has been recognized or not, a different response will result.
If an antigen is encountered for the first time, the B cells with the antigen receptor that is specific for the antigen begin to produce antibodies, which is a slow process and can take days and sometimes weeks for enough to be produced. Some of these activated B cells become plasma cells and some become memory cells. Plasma cells are able to produce and secrete antibodies rapidly and in large numbers, but they do not live long. Memory cells, on the other hand, stay in the body for weeks and sometimes months. If the same antigen is encountered again, they develop into plasma cells and start producing antibodies.
As with B cells, there are two types of T lymphocytes involved in the immune response. T helper cells are activated by a specific antigen. When this happens, they release hormone-like molecules that stimulate B cells to produce and secrete antibodies. When T killers recognize a specific antigen, they attach themselves to the surface of infected cells and secrete toxic substances to kill the cells, as well as the antigens.
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