Review of agonist and antagonist messenger and receptor binding. We discussed antigens as chemical substances which are recognized as "foreign", targeted by antibodies, bind to specific receptors, may originate within or outside the cell and elicit an antigen-antibody reaction. Antibodies were described as chemicals produced which are specific to a specific antigen, produced by specific cells and neutralize a pathogen or antigen. Based on the above, the antigen antibody reaction was outlined, and the allergic responses listed.