A substance formed in the body by B cells. They reacted very specifically with bacteria, viruses, allergens or other substances that initiated their production. Depending on their subtype, they can cause hypersensitivity (IgE) or resistance to infection (IgA, IgD, IgE, IgG, IgM). The Ig stands for immunoglobulin and the A, D, E, G, and M are the categories of antibodies. Antibodies have several functions. An antibody is specific to an antigen.