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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 the 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 that matches the shape of the antigen to which it is specific. This specificity of structure that the antigen receptor has is what enables each cell to match only one type of antigen.
One kind of immune cells, lymphocytes, are capable of producing a type of protein called an antibody. Each antibody is also specific to 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 to the relevant antigen only. 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 the antibodies. When 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 to 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 quickly and in large numbers, but they do not live long. Memory cells, on the other hand, remain in the body for weeks and sometimes months. If the same antigen is encountered again, they develop into plasma cells and begin 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 then release hormone-like molecules that stimulate the B cells to produce and secrete antibodies. When T killer recognize a specific antigen, they attach to the surface of the infected cells and secrete toxic substances to kill the cells, as well as the antigens.