Antibody
From AIDS Wiki
An antibody (or immunoglobulin (Ig)) is a protein used by the immune system to identify and neutralize foreign objects and pathogens such as bacteria and viruses. Antibodies are the major component of the humoral immune response.
Biochemical structure and classification
Antibodies consist of four polypeptide chains, two heavy and two light, joined by disulfide bonds. (See the figure at right.)
There are five major classes, or isotypes, of immunoglobulins in humans:
- IgA, found mostly in mucosal areas, and secreted in breast milk;
- IgD, of unknown function;
- IgE, associated with allergies;
- IgG, the most important human Ig isotype: it comprises 75% of all serum antibodies, binds to a wide variety of antigens, and is the only isotype capable of passing the human placenta;
- IgM, the largest of antibodies in size, effective at activating the complement system
Immunological role
Antibodies are secreted by B cells in blood and bodily fluids and bind to specific epitopes, the sites on antigens recognized by antibodies. Since distinct antigens can present identical epitopes, all antibodies, including monoclonal antibodies (those derived from a single immune cell), can potentially cross-react with other antigens. Antibodies contribute to immunity via direct neutralization of antigens, opsonization (coating the antigen so that it can be digested by other immune cells), and the triggering of other immune responses (such as the complement system) which help the antibodies destroy pathogens.
External links
- Wikipedia article
- Mike's Immunoglobulin Structure/Function Page at University of Cambridge
- Antibodies as the PDB molecule of the month Discussion of the structure of antibodies at Protein Data Bank
- Microbiology and Immunology On-line Textbook at University of South Carolina
- How Lymphocytes Produce Antibody from Cells Alive!
