Antigens are present on the surface of toxins, invading bacteria, viruses, pollen or other foreign substances that enter the body. The immune system in the body reacts to the presence of these antigens in order to protect the body in several ways, including the production of antibodies. Antibodies are proteins that are produced which have specific shapes and reactive sites which bind specifically to antigens. The antigen-antibody reaction is part of the cell-mediated system immune response. Monocytes and macrophages can pick up the antigens and present it to B cell and T cell lymphocytes. The T-cell lymphocytes produce lymphokines that bring more T-cells and B-cells to the area where antigens are present.http://www.nlm.nih.gov/medlineplus/ency/article/002224.htm <ref>http://www.lasalle.edu/~frizzell/immunweb.htm http://www.nlm.nih.gov/medlineplus/ency/article/002223.htm

An understanding of antigen-antibody reactions was discovered in the work of Michael Heidelberger and Elvin Kabat in their work on quantitative precipitin reactions. From this work, there were subsequent advances in the understanding of antibody struction in the late 1950s by Rodney Porter and Gerard Edelman. http://www.columbia.edu/itc/hs/medical/pathophys/immunology/readings/Concise_History_of_Immunology.doc

Most antigens are either proteins, polysaccharides or lipopolysaccharides (polysaccharides with a fat molecule attached) which are present on the surfaces of foreign particles. When the antigen is first encountered, B cells recognize with their surface immunoglobulins and produce plasma cells that make antibody against it. Immunoglobulin of the same antigen specificity is secreted as antibody by the plasma cells. It takes from 48-72 hours for this process to occur before there are significant detectable amounts of the antibody in the blood. There are two types of plasma cells produced, the second of which is a memory cell, which guards against subsequent encounters of the antigen. In T cells, antigen recognition occurs via membrane-bound proteins. These proteins serve only to activate T cell function.http://www.lasalle.edu/~frizzell/immunweb.htm http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=imm&part=A322 http://pathmicro.med.sc.edu/mayer/antigens2000.htm