The human immune system is a complex, yet effective system that continually protects your body from harmful microorganisms. Exposure to a foreign substance (antigen) activates the immune system, which produces antibodies to attack and destroy the invading antigen. In response to an antigen, B cells, a type of white blood cell responsible for immunity, are activated and release these antibody proteins. Antibodies are also called immunoglobulins because they are immune proteins found in the globulin part of the plasma in the blood. There are different general classes of immunoglobulins that all have the same general structure--two heavy and two light polypeptide chains in a "Y" shape containing a variable region that differs between each individual antibody, and a constant region that determines the class and function of the antibody.
IgG
IgG, or immunoglobulin gamma, is the most prevalent class of antibody, making up 75 percent to 85 percent of the body's total antibodies. IgG antibodies are found in the blood and tissues and protect against bacteria, fungi, viruses and toxins. They are the only class of antibody that can cross the placenta from mother to fetus to provide immune protection to the newborn. IgG antibodies are responsible for the Rh factor in blood typing and are considered "opsonins," which are substances that prepare the foreign antigen for phagocytosis, the engulfing of the antigen by an immunity cell. IgG antibodies are more commonly produced after a second exposure to an antigen that has already been encountered, so they are faster and more effective at destroying the foreign invader.
IgM
IgM, or immunoglobulin Mu, is the third most common antibody, making up 5 percent to 10 percent of the total antibodies in the body. IgM antibodies are often the first to be produced in response to an encounter with a new antigen. It activates an immune cascade called the "complement" system, which is a biochemical response that assists the antibodies in destroying the antigen. IgM antibodies are responsible for transfusion reactions in the ABO blood system.
IgA
IgA, immunoglobulin Alpha, is the second most prevalent antibody, making up 15 percent of the total antibodies in the body. IgA antibodies are secreted through saliva and tears, and parts of the body lined with mucous membranes, including nose, eyes, lungs and digestive tract, to protect against invasion of microorganisms through body surfaces. IgA antibodies are also found in colostrum and breast milk for newborn immune protection.
IgE
IgE, or immunoglobulin Epsilon, is the least common antibody, only making up 0.002 percent of the antibodies in the body. IgE antibodies are involved in allergic reactions, and when an allergen encounters white blood cells with IgE antibodies, they release substances such as histamines that stimulate the inflammatory response and trigger allergic symptoms.
IgD
IgD, or immunoglobulin Delta antibodies, make up 0.2 percent of the antibodies in the body. IgD antibodies are present on immature B cells to help them mature and to act as an antigen receptor. Their function in the bloodstream, however, is not very well understood.
References
- University of South Carolina School of Medicine: Immunology
- Merck: Acquired Immunity
- "Anatomy and Physiology"; Rod Seeley, Trent Stephens and Philip Tate; 1998.
