Antibodies are blood circulating glycoproteins termed immunoglobulins that are secreted by plasma cells (differentiated B lymphocytes) in response to immune stimulation. They constitute what is known as the humoral immune response, which is highly specific. Alternatively, B lymphocytes may differentiate into "memory cells" which display antibody molecules on their cell surface contributing to active immunity for future exposure to an antigen. There are five major classes of antibodies that differ upon their location in the body, structure, and function: IgA, IgD, IgM, IgG, IgE. During a typical bacterial or viral infection, the major classes that operate are IgM and IgG.
Antibodies recognize and bind tightly to small molecules (peptide or carbohydrate) termed antigens. Antigens may be free floating in the blood, or bound to the plasma membrane of a pathogen or presented at the surface by another immune cell. An antibody may directly neutralize a pathogen, mark a microbe for attack, or be used to bind to antigens presented by other cells for further immune stimulation.