Immune system is like any other system which leads to failure of some or all the parts. This failure can lead to disastrous consequences. When the immune system fails to recognize its own cells and begins to destroy them the result is autoimmunity. When the system errs by failing to protect the host from the disease causing agents the result is immunodeficiency. It was Paul Ehrlich who coined autoimmunity and termed this condition as 'horror autotoxicus.'

Initially it was believed that failure to eliminate lymphocytes during the development of bone marrow and thymus led to autoimmune diseases. Later in the 1970's it was seen that not all self-reactive lymphocytes were depleted during T-cell and B- cell maturation instead sometimes these cells re-circulate and mature. These do not always lead to autoimmune responses in humans, so they must be regulated by clonal suppression. Breakdown of this regulation can lead to activation of T cell and B cell clones generating secondary responses against self antigens. These can cause serious damage to tissues and organs. Many autoimmune diseases are characterized by tissue destruction mediated by T cells for example Rheumatoid Arthritis (RA) where the T cells attack the joints leading to an inflammatory response.

Other probable causes of autoimmune diseases could be genetic, environmental, infectious factors. Recent reports have shown association between 2-nonynoic acid, a cosmetic substance and primary biliary cirrhosis (PBC). This is nine times more likely to occur in women. Other triggers of autoimmune diseases could be smoking, bacterial and retroviral proteins, hormone replacement therapy and use of nail paint. These are just some probable causes for autoimmune diseases. Many of these are still being researched.

Autoimmune diseases can be classified into two categories:
1) Organ specific diseases.
2) Systemic autoimmune diseases.
Several animal models are used to understand autoimmune diseases and recently there have been many therapies for treating these outrageous yet miraculous diseases.

1) Organ Specific Autoimmune diseases
Here the immune responses are directed to a target antigen unique to a particular organ or gland. The cells of the organ may be directly affected by humoral or cell mediated effector mechanisms or sometimes the antibodies over stimulate or block the normal functions of target organ.
Some autoimmune diseases involve direct cellular damage and thus happen when lymphocytes or antibodies bind to the cell membrane antigens causing cell lysis. Gradually the organ declines due to cellular destruction.

Some of the common organ specific autoimmune diseases are Hashimoto's Thyroiditis which is characterized by infiltration of thyroid gland by lymphocytes, macrophages and plasma cells. Autoimmune Hemolytic Anemia is where the person makes auto antibodies to RBC antigens triggering complement mediated lysis and phagocytosis of RBC's
Some auto immune diseases are mediated by auto-antibodies which act as antagonists. These bind to hormone receptors and stimulate inappropriate activity. This leads to overproduction of mediators or increase in cell growth. These auto-antibodies may act as blocking receptors too. This in turn causes impaired secretion of mediators and atrophy of affected organ. Examples of these are Graves' disease and Myasthenia Gravis.

2) Systemic Autoimmune Diseases
Here the response is directed towards a broad range of target antigens and involves a number of organs and tissues. These affect the immune system on the whole, which in turn activates the T cells and B cells. Tissue damage is widespread here. The best example of this type is Systemic Lupus Erythematosus (SLE). It appears mostly in females. Affected individuals produce auto-antibodies to a vast array of tissue antigens such as DNA, histones, RBC's, platelets, leukocytes and clotting factors, interaction of these with specific antigens produce various symptoms.
Multiple Sclerosis (MS) is another common systemic autoimmune disease. People who have MS produce autoreactive T cells that participate in the formation of inflammatory lesions of nerve fibres. The Cerebrospinal Fluid (CSF) of MS patients has T lymphocytes activated which penetrate the brain tissue and cause lesions which in turn destroys the myelin sheath. Breakdown of the myelin sheath leads to many neurological disorders.

Autoimmunity is still a wonder scientists are trying to understand. A lot of time and money is currently invested in research of the causes of autoimmune diseases, diagnosis and treatment. Until then one can just hope not to have these deadly 'horror autotoxicus.'

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