Autoimmune Diseases

Usually, the body's immune system recognizes its self-antigens and deletes clones of cells that would respond and attack its own tissues. A growing number of diseases are suspected of being caused by an autoimmune process, however, meaning that the immune system of the body is responding to the tissues of the body as if they are foreign. Some of these diseases are listed in table 18.4. Susceptibility to many of them is influenced by the major histocompatibility makeup of the patient, and so, not surprisingly, they often occur in the same family. The table notes some of the diseases in which an association with MHC genes (such as DR genes) has been found. ■ negative selection, p. 413

Autoimmune diseases may result from reaction to antigens that are similar though not identical to antigens of self. Some bacterial and viral agents try to evade destruction by the immune system by developing amino acid sequences that are similar to self-antigens. As a result, the immune system is unable to discriminate between the agent and self. The immune system may then destroy the substances of self as well as those of the bacteria or virus. It has been found that the likeness of amino acid sequences need not be exact for this self-destruction; even a 50% likeness may lead to an autoimmune response. Autoimmune responses may also occur after tissue injury in which self-antigens are released from the injured organ, as in the case of a heart attack. The autoantibodies formed react with heart tissue and evidence suggests that they can cause further damage.

The Spectrum of Autoimmune Reactions

Autoimmune reactions occur over a spectrum ranging from organ-specific to widespread responses not limited to any one tissue. Examples of organ-specific autoimmune reactions are several kinds of thyroid disease, in which only the thyroid is affected. Widespread responses include lupus erythematosus and rheumatoid arthritis. Lupus is a disease in which autoantibodies are made against nuclear constituents of all body cells. In rheumatoid arthritis, an immune response is made against the collagen protein of supporting connective tissues. In these widespread diseases, many organs are affected. In both organ-specific and widespread autoimmune diseases, the damage may be caused either by antibodies, immune cells, or both.

Myasthenia gravis, characterized by muscle weakness, is an example of an autoantibody-mediated disease. The disease is caused by the production of antibodies to the acetylcholine receptor proteins that are present on muscle membranes where the nerve contacts the muscle. Normally, transmission of the impulses from the nerve to the muscle takes place when acetylcholine is released from the end of the nerve and crosses the gap to the muscle fiber, causing muscle contraction. Immunofluorescent tests have shown autoantibodies that bind to the acetylcholine receptors, blocking access of acetylcholine to the receptors. These antibodies along with complement cause many of the receptors to be taken into the muscle cells and degraded, so that fewer receptors are present on the muscle membranes. Further evidence that IgG antibodies are involved comes from babies born to mothers with myasthenia gravis. The

1 Nester-Anderson-Roberts: 1 III. Microorganisms and 1 18. Immunologic Disorders 1 Microbiology, A Human Humans Perspective, Fourth Edition

© The McGraw-Hill Companies, 2003

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  • Lonnie
    What is the causative agen for autoimmne?
    1 year ago

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