Avoid the jargonistic use of the plural antibodies when the reference is to a single antibody species. An immunoglobulin molecule produced by B-lymphoid cells that combine specifically with an immunogen or antigen. Antibodies may be present naturally, their specificity is determined through gene rearrangement or somatic replacement or may be synthesized in response to stimulus provided by the introduction of an antigen; antibodies are found in the blood and body fluids, although the basic structure of the molecule consists of two light and two heavy chains, antibodies may also be found as dimers, trimers, or pentamers. After binding antigen, some antibodies may fix, complement, bind to surface receptors on immune cells, and in some cases may neutralize microorganisms.immunoglobulin. SYN: immune protein, protective protein, sensitizer (2).