an immunoglobulin protein secreted by activated plasma cells, which evolve from B-cells. Antibodies are present throughout the blood and tissues; they are produced in response to stimulation by foreign antigens as part of the body's defense against disease. Specific antibodies bind to and act upon specific antigens; the antigen/antibody reaction forms the basis of humoral (TH2) immunity. Neutralizing antibodies destroy or inactivate infectious agents,
while enhancing antibodies promote infection. There are 5 classes of immunoglobulins: IgA, IgD, IgE, IgG and IgM. See also immunoglobulin A-M.