Encyclopedia of Health and Medicine

immunosuppressive medications 275
ABLE IMMUNODEFICIENCY (CVID); COMPLEMENT CASCADE;
IMMUNE DISORDERS; LIVING WITH IMMUNE DISORDERS;
PARTIAL COMBINED IMMUNODEFICIENCY (PCID); SEVERE
COMBINED IMMUNODEFICIENCY (SCID); T-CELL LYMPHO-
CYTE.
immunoglobulin A protein structure the IMMUNE
SYSTEM produces. Immunoglobulins are the foundation
molecules for the formation of antibodies.
Immunoglobulins circulate in the BLOOD. The
immunoglobulin’s class designation reflects its
molecular structure, which in turn dictates the
action of the immunoglobulin. The five major
classes of immunoglobulin provide different kinds
of antibodies:
• Immunoglobulin A (IgA) is the main
immunoglobulin in the body’s secretions (tears,
saliva, and mucus) and in colostrum, the first
discharge from the mother’s breasts after childbirth.
It is the second most abundant
immunoglobulin in the blood circulation. IgA
boosts the IMMUNE RESPONSE capacity of the various
MUCOSA-ASSOCIATED LYMPHOID TISSUE (MALT)
structures. IgA blood levels decrease in lymphoblastic
leukemias and increase in certain
AUTOIMMUNE DISORDERS, notably RHEUMATOID
ARTHRITIS and SYSTEMIC LUPUS ERYTHEMATOSUS (SLE).
• Immunoglobulin D (IgD) resides on the surface
of the cell membrane of B-cell lymphocytes. Its
primary role is to bind with antigens. IgD blood
levels increase with chronic infections and certain
myelomas.
• Immunoglobulin E (IgE) produces the antibodies
responsible for HYPERSENSITIVITY REACTION as
well as primary INFECTION-fighting antibodies. It
also is the immune response’s main defense
against parasitic infection. IgE is the least abundant
of the immunoglobulins in the blood circulation.
Blood levels of IgE rise with
hypersensitivity reactions.
• Immunoglobulin G (IgG) is the most abundant
and versatile of the immunoglobulins. It makes
up 75 percent of the immunoglobulin in the
blood circulation. IgG binds with many types of
leukocytes and activates the COMPLEMENT CAS-
CADE. IgG is the only immunoglobulin that can
cross the placental barrier between mother and
fetus. IgG blood levels increase with infection
and rheumatoid arthritis and decreases with
lymphoblastic LEUKEMIA.
• Immunoglobulin M (IgM) is the third most
abundant class of immunoglobulin in the blood
circulation. The first contact with an ANTIGEN
causes a B-CELL LYMPHOCYTE to produce IgM. IgM
antibodies help collect cellular debris for more
efficient PHAGOCYTOSIS. Blood levels of IgM
increase with infectious mononucleosis,
MALARIA, SLE, and rheumatoid arthritis.
Immunoglobulins collected from donated blood
and PLASMA are blended to produce GAMMAGLOBU-
LIN, a therapeutic form that boosts the nonspecific
immune response.
For further discussion of immunoglobulins
within the context of the structures and functions
of the immune system, please see the overview
section “The Immune System and Allergies.”
See also ANTIBODY; ANTIBODY-MEDIATED IMMUNITY;
LEUKOCYTE; LYMPHOCYTE; MONONUCLEOSIS, INFECTIOUS;
VACCINE.
immunosenescence A decline in immune function
and IMMUNE RESPONSE that occurs with aging.
Researchers believe immunosenescence accounts
for the increase in cancer and infections such as
INFLUENZA and PNEUMONIA in people of old age. The
decline occurs in both CELL-MEDIATED IMMUNITY
(sometimes called cytotoxic immunity), in which
T-cell lymphocytes attack and kill foreign antigens,
and humoral immunity, in which B-cell lymphocytes
generate the antibodies that circulate in the
BLOOD. Though immunosenescence appears a normal
physiologic process in that it happens to
everyone as they grow older, researchers question
whether it is an intrinsic function under genetic
regulation or an extrinsic reaction to environmental
factors, ranging from EATING HABITS to toxic
exposure.
See also AGING, EFFECTS ON IMMUNE RESPONSE;
ANTIBODY; ANTIGEN; APOPTOSIS; B-CELL LYMPHOCYTE;
CELL STRUCTURE AND FUNCTION; LYMPHOCYTE; T-CELL
LYMPHOCYTE.
immunosuppressive medications Drugs that
limit or suppress some aspect of the IMMUNE
RESPONSE. Immunosuppressive medications such as
cyclosporine work by many different mechanisms