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In a normally-functioning immune system, the body develops immunological tolerance to its own proteins, cells, and tissues. However, a healthy immune system is not completely incapable of recognizing self antigens, and low-level autoimmunity is believed to be an important part of immunological health. At low levels, this may provide a defense against cancer, by allowing immune cells to recognize and destroy cells which are growing abnormally.
This low-level autoimmunity is not harmful, and may even be beneficial. However, under certain conditions, the immune system can become capable of mounting a full-scale attack on certain tissues. In the case of type 1 diabetes, for example, the targets of the immune system are the insulin-producing cells beta cells in the pancreas.
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Causes of Harmful Autoimmunity
The healthy immune system has a number of mechanisms which prevent high levels of self-reactivity. During early phases of T cell and B cell development, cells which react strongly to self antigens are eliminated. Any which manage to complete the developmental process can be controlled by regulatory T cells which are thought to actively suppress autoimmune responses that threaten to disrupt the system.
If one of these mechanisms fails, however, an autoimmune disorder may result. When the immune system becomes autoreactive, some immune cells become activated in the presence of self antigen. They recognize certain proteins expressed on the surface of one or more cell types, and the immune system becomes primed to destroy any cells which express those proteins.
The exact reason why this occurs is currently unknown – as in the case of allergies, a combination of genetic and environmental factors is most likely at fault.
An individual’s sex, for example, is known to play a role. With few exceptions, women are more likely to develop autoimmune diseases than men – but the reason for this is no more than speculation. It has been suggested that women may be predisposed to develop autoimmunity following pregnancy, during which small exchanges of cells between mother and child may result in the disruption of the mother’s immunological tolerance to certain self antigens.
An environmental factor which has been proposed to explain the increased incidence of allergies as well as the development of autoimmunity is the so-called hygiene hypothesis. This attributes these immune disorders to the modern world’s increased focus on hygiene, which has lead to a reduction in the number of infections in both children and adults. According to the hygiene hypothesis, this can cause the immune system to become dysfunctional.
Many researchers consider this theory somewhat spurious. Even so, it has been noted that in regions where several infectious diseases are endemic, the incidence rate of autoimmunity of any kind is very low, and vice versa, lending some credence to the hygiene hypothesis.