Antigen binding is an immune process in which an antibody binds to an antigen. An antibody is an immune molecule intended to recognize and defeat threats to the body, such as harmful infections; an antigen is a "non-self" molecule that is recognized by the immune system and, generally speaking, attacked by antibodies. Antigens bind to immune receptors throughout the body, generally on the surface of cells, and provoke an immune response. Antigen binding properties may also be used in a laboratory setting, particularly in the area of immunohistochemistry. In immunohistochemistry, specific antibodies are used to determine the prevalence of various antigens in a tissue sample.
Each antibody is primarily defined by a small region at its tip, referred to as its antigen binding site. The binding of antigens and antibodies tends to be highly specific; a given antibody is likely to bind to only a single type of antigen. The antigen binding site is the part of the antibody that determines the particular antigens to which it can bind. Most antibodies are structurally very similar in all areas except the binding site. This antibody specificity means that there are millions of different antibodies, each of which targets a specific antigen.
Antigen binding can have a few different purposes in natural immune system processes. In some cases, an antibody binds to an antigen in order to indicate to other parts of the immune system that it is a threat. The antibody does not neutralize the antigen on its own; it only serves as a marker for other immune mechanisms. In other cases, the antibody does actually neutralize the antigen by binding to the particular part of the antigen that makes it harmful. Antigen binding, then, is an essential immune system process without which many pathogens could not be stopped effectively.
The specificity of antigen binding also is essential to laboratory experiments that utilize antibody-antigen interactions, as the specificities of known antibodies can be used to identify unknown antigens. Antibodies are exposed to a tissue sample and fluorescent markers on the antibodies are often used to indicate whether or not binding occurs. High fluorescence represents a high level of antigen binding, while low fluorescence represents a low level of binding. Certain antigens in a tissue sample can serve as indicators for various diseases. Such processes in immunochemistry are often aimed at developing effective chemical diagnostic methods for diagnosing a variety of diseases.