The immune system relies on barriers, cells and humoral [fluid] factors to protect the body against environmental hazards. The most important humoral immune factors are immunoglobulins, which are proteins that are found in blood and other bodily fluids.
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Immunoglobulins, a collective term for antibodies, are specific immune proteins produced by the body in response to exposure to an antigen. An antigen is any foreign substance that enters the body, usually by ingestion, inhalation, or breaks in the skin, and causes an immune response. Antibodies are specific to a given antigen. The primary purpose of an antibody is to prevent harm to the host [the body] by eliminating the antigen and any threat it might pose. Immunoglobulin concentrations are a very important measure of immune system health.
Immunoglobulin neutralizes invading viruses, bacteria and other toxins by preventing them from attaching to cells and causing infection, and by marking the invader for destruction by specialized white blood cells.