ABSTRACT
The production of antibodies is the sole function of the B-cell arm of the immune system. Antibodies are present in the blood, lymph, and extracellular fluids, where they bind to extracellular bacteria and virus particles. Antibodies are also present at mucosal surfaces, where they control the populations of commensal microorganisms, as well as pathogenic microorganisms and the larger multicellular parasites. Antibodies are not inherently toxic or destructive to pathogens; their principal role is to bind to the pathogen, and the best antibodies are those that bind tightly and do not let go. Antibodies also act as molecular adaptors that bind to pathogens using their variable regions (V regions) and to complement components and receptors on effector cells using their constant regions (C regions). The differences between the classes and subclasses of antibody enable antibody-coated pathogens to be delivered to different types of effector cell. Coating a bacterium with IgG and complement promotes its uptake and destruction by a phagocyte, whereas coating a parasite with IgE activates mast cells to induce violent reactions that expel the parasites from the body. Some antibodies have a direct effect on the course of infection by covering up sites on a pathogen’s surface that are necessary for growth or replication. For example, antibodies against the hemagglutinin and neuraminidase glycoproteins of influenza virus can prevent infection because the virus uses these glycoproteins to bind to human cells and infect them. Such antibodies are said to neutralize the pathogen. In the development of a vaccine against an infectious agent or its toxic products, the gold standard that a company aims for is the induction of a neutralizing antibody.
