The structural and functional damage produced by the action of antibodies against the normal components of the body can be termed; autoimmunity. It implies ’injury to self’. Several diseased conditions produce autoantibodies inside our body.
Certain autoantibodies such as, antiidiotypic antibodies are essential for the normal functioning of the immune system.
Autoimmune diseases are produced along with the following characteristics;
1. Increased level of immunoglobulin
2. Demonstratable autoantibodies
3. Deposition of immunoglobulins at the site of infection
4. Lymphocytes along with plasma cells get accumulated around the lesions.
5. Immunosuppressive therapy proves to be beneficial
6. More than one type of autoimmune lesions appears in an individual
7. Higher incidence in females
8. It’s a chronic condition which cannot be reversed
Mechanism:
Cells or tissues may undergo changes in their antigenic structure due to the influence of physical, chemical and biological agents. They are termed as ‘neoantigens’ and they elicit immune response. They sensitize the cells towards self antigens
Physical agents,
1. Irradiation
2. Light
3. Cold
Chemical agents,
1. Drugs
2. Allergens
Biological,
1. Infectious bacteria
2. Viruses
3. Intracellular pathogens
4. Bacterial enzymes
5. Neuraminidases produced by Myxovirus

Cross reacting foreign antigens also induce immune responses leading to cell damage. Some of the self antigens resemble closely the foreign antigens which forms the basis of ‘cross reacting antigen’ theory. Example, repeated Streptococcal infections can induce damage to the heart.

When heterologous organ specific antigens are injected, it may induce an immune response, damaging the particular organ or tissue in the host. Such an example is the neurological damage caused by the antirabies immunization (phenol denatured infected sheep brain cells) in humans.
Types of autoimmune diseases:
1. Autoimmune hemolytic anemia; autoantibodies against erythrocytes are produced. Red cells become coated with antibodies and are prematurely destroyed in the spleen and liver.
2. Thrombocytopenia; autoantibodies against platelets.
3. Hashimoto’s disease; antibodies that react with thyroglobulin. Enlarged thyroid gland in females.
4. Grave’s disease; also called thyrotoxicosis with antibodies against thyroglobulin.
5. Addison’s disease; presence of circulating antibodies against the cells of zona glomerulosa (surface cells of the adrenal gland) resulting in infiltration of lymphocytes  in the gland.
6. Pernicious anemia; two types of antibodies are produced. One type against the cells of gastric mucosa. The other type of antibodies prevents the absorption vitamin B12.
7. Myasthenia Gravis; antibodies are produced against the neurotransmitter acetylcholine.
8. Systemic lupus erythematosus; autoantibodies are produced against cell nuclei, cell components, thyroid, immunoglobulins and other organ specific antigens.
9. Rheumatoid arthritis; circulating autoantibodies against the Rheumatoid Factor (RF).
10. Sjogren’s syndrome; antinuclear antibodies is seen in the serum.
Pathogenesis of autoimmune diseases:
Diseases are considered to be autoimmune as most of them are associated with cellular or humoral responses against self antigens. Autoantibodies in the body may be due to tissue injury and thereby eliciting immune response

Antibodies can also cause damage by the cytolytic or cytotoxic and toxic complex reactions.  Another type of autoimmune tissue damage may be caused by sensitized T lymphocytes. Some humoral and cellular immune responses work in unison in case of certain diseases resulting in autoimmune diseases.