Failure of the immune system to "tolerate" self-tissues may result in pathologic processes known as autoimmune diseases. At the clinical level, autoimmunity is involved in a variety of apparently unrelated diseases, such as systemic lupus erythe-matosus (SLE), insulin-dependent diabetes mellitus, myasthenia gravis, rheumatoid arthritis, multiple sclerosis, and hemolytic anemias. There are at least 40 diseases known or considered to be autoimmune, affecting about 5% of the general population. Their distribution by sex and age is not uniform. As a rule, autoimmune diseases predominate in females and have a bimodal age distribution. A first peak of incidence is around puberty, and the second peak is in the forties and fifties.

There are several different ways to classify autoimmune diseases. Because several autoimmune diseases are strongly linked with MHC antigens, one of the most recently proposed classifications, given in Table 1.1, groups autoimmune diseases

Table 1.1. Classification of autoimmune diseases

I. Major histocompatibility complex class II associated

A. Organ specific

(autoantibody directed against a single organ or closely related organs)

B. Systemic

(systemic lupus erythematosus - variety of autoantibodies to DNA, cytoplasmic antigens, etc)

II. Major histocompatibility complex class I associated

A. HLA-B27-related spondyloarthropathies (ankylosing spondylitis, Reiter's syndrome, etc.)

B. Psoriasis vulgaris

(which is associated with HLA-B13, HLA-B16, and HLA-B17)

according to their association with class I or class II MHC markers. Although both sexes may be afflicted by autoimmune diseases, there is a female preponderance for the class Il-associated diseases and a definite increase in the prevalence of class I-associated diseases among males.

The effector mechanisms in the development of autoimmune pathology and disease involve autoantibodies, immune complexes, and autoreactive T cells. These mechanisms are exemplified in SLE, and because LE is the study object of this book, we present them in that context in the following pages (Theofilopoulos 2002).

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