Antibodies, also called immunoglobulins, are proteins produced by specialized immune cells called B cells. When an invading pathogen such as a bacterium or virus, enters the body, it activates the immune system, which is the body's defense against infection. Antibodies produced by B cells circulate the bloodstream, bind to pathogens and mark them for destruction by other immune cells. As such, while antibodies themselves don't actually destroy invaders, they are nevertheless important tools in the arsenal of the immune system, helping system cells to recognize invading species for rapid eradication of infection. There are several different classes of antibodies in the blood.
IgA
Antibody names are abbreviated with the letters "Ig," for "immunoglobulin," followed by a letter specific to an antibody type. Different antibody types appear in different parts of the body, and serve slightly different functions, explains Dr. Lauralee Sherwood in her book, "Human Physiology." IgA, for instance, is the antibody commonly found in the blood vessels of the mucous membranes. Since the lining of the mouth, urethra and genital area, and lungs is exposed to outside material, these areas are very susceptible to infection. IgA localized in the mucous membranes helps prevent pathogens from entering the body in the first place.
IgD and IgM
Both IgD and IgM are much less common in the bloodstream as free antibodies than other antibody types. Instead, they're mainly found attached to B cells that circulate in the blood. IgD appears to activate other immune cells, such as the mast cells involved in allergic reactions, explains Dr. Sherwood. IgM helps to activate immune reaction cascades that aid in other immune cell functions. It also is a major antibody to proteins on the exterior of red blood cells, which is the reason that individuals with, for instance, type A blood can't receive transfusions with type B blood, and vice versa.
IgE
Dr. Gary Thibodeau, in his book "Anatomy and Physiology," notes that IgE plays an important role in allergic reactions. It triggers mast cells and basophils to release the molecule histamine, which causes the classic allergy symptoms of runny nose, sneezing, itchy eyes, and membrane inflammation. IgE also appears to protect the body against parasites, including infection by worms.
IgG
The vast majority of free antibody in the bloodstream is IgG. These antibodies protect against a large variety of pathogens, including viruses and bacteria. Dr. Thibodeau also notes that they're the only type of antibody capable of crossing the placenta from pregnant mother to child, meaning that a child's early immunity is based entirely upon IgG from its mother.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
