Idiopathic thrombocytopenic purpura See thrombocytopenia

immune response A complex system of related defenses against foreign organisms. Immunity is innate and acquired. The innate immune response can act immediately and does not require previous exposure to an organism or the production of antibodies. Innate immunity is provided by phagocytic cells that engulf and digest invading organisms; as well as a particular type of lymphocyte called the natural killer or NK cell. Acquired immunity is targeted against specific parts of organisms called antigens that the immune system recognizes as being foreign. It has two components, humoral and cellular immunity. The complement system is an important part of the immune response. Complement helps the process of ingestion and destruction of foreign organisms that have been coated with antibody as well as recruits inflammatory cells to sites of inflammation.

The cause of many rheumatological conditions, for example RA and SLE, is not known. However, because antibodies (rheumatoid factor and antinuclear antibody (ANA), respectively) against normal components of the body occur, they are considered autoimmune diseases. The presence of autoanti-bodies does not automatically imply the presence of an autoimmune disease, for example low levels of ANA are common in healthy people. Some autoantibodies do not cause damage directly but may do so when they bind to complement and lodge in tissues such as the kidney. others directed against a patient's cells can damage them. For example antiplatelet antibodies can cause thrombocytopenia. Why autoantibodies develop and cause disease in some people is not clear. one theory is that autoreactive T cells that are normally destroyed in the thymus gland escape destruction and trigger an autoimmune response. Another is that the ability of the immune system to tolerate self-antigens is broken down.

Innate immunity Several different types of phagocytic cells are important mediators of innate immunity. Monocytes are a type of white blood cell that circulate in the bloodstream and move into inflamed tissues where they change their structure a little and are called macrophages. These mononu-clear cells are important phagocytic cells, but they also serve another important function. They ingest and break up foreign proteins into smaller pieces. These pieces (antigens) are then carried to the surface of the cell where they may be recognized by T cells. Because of this function they are sometimes called antigen-presenting cells. Neutrophils, also called polymorphonuclear leukocytes or polys make up 80 percent of the circulating white blood cell population and are important phagocytic cells that also contain enzymes that are able to kill and digest ingested cells. NK cells are also called large granular lymphocytes and are T cells that are able to phagocytose invading cells. People with a deficiency of NK cells have an increased risk of viral infection.

Humoral immunity B lymphocytes that produce immunoglobulins mediate humoral immunity. These proteins are also called antibodies and are targeted at antigens on invading cells that the body recognizes as foreign. There are five major subtypes of immunoglobulins, IgG, IgA, IgM, IgE, and IgD, that have different properties. For example, igM is produced rapidly after an antigen is recognized and igA is produced mainly by lymphocytes in the gut and other mucosal surfaces.

When B cells are exposed to an antigen, they proliferate and differentiate into activated B cells that produce antibodies that bind to the antigen and neutralize it. Some B cells become memory cells so that when they come across the same antigen at a later time, they are able to recognize it rapidly, multiply, and produce antibodies.

Cellular immunity T lymphocytes that produce cytokines and other mediators to inhibit or stimulate other immune cells to carry out the immune response largely mediate cellular immunity. T and B lymphocytes look similar under a microscope, but they can be differentiated by markers on their surface. These markers can be detected by antibodies and cells categorized into subtypes designated CD (cluster of differentiation) types. Most T cells are CD4 (helper cells) or CD8 (cytotoxic/suppressor) cells. The importance of cD4 cells in the body's defense against infection is illustrated by the susceptibility of HIV patients with depleted cD4 cell counts to a range of infections.

immunodeficiency A state in which the body's ability to fight infection is decreased. Immunodeficiency can be primary, caused by genetic abnormalities, or secondary, due to drugs or infections that affect the number or function of white blood cells (see immune response). The major complication of immunodeficiency is increased susceptibility to infection. Patients who are unable to make antibodies efficiently (deficient B lymphocyte or humoral immunity) are particularly susceptible to recurrent bacterial infections. Those with a decreased cellular immunity (deficient T lymphocyte or cell-mediated immunity) have a greater chance of getting viral or fungal infections.

Primary immunodeficiencies are a group of disorders caused by genetic defects that affect the function of the immune system. Many of the primary immunodeficiencies are also associated with an increased risk of developing malignancy and autoimmune disease.

A drug, infection, or radiation usually causes secondary immunodeficiency. This suppresses or damages bone marrow and decreases the number of white blood cells or affects their function. Human immunodeficiency virus (HIV) infection is the most common cause of immunodeficiency worldwide. In countries where HIV infection is less common, the common causes are cancer chemotherapy and immunosuppressive treatment to prevent rejection after organ transplantation. Many serious rheumatic diseases such as vasculitis are treated aggressively with drugs such as corticosteroids and cyclophosphamide that suppress the immune system and increase the risk of infection. Several other conditions such as diabetes, decreased kidney or liver function, malnutrition, and severe burns also suppress the immune system.

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