How does the immune system avoid attacking self-antigens?

The immune system avoids attacking self-antigens through a process called central and peripheral tolerance.

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders. It does this by recognising and responding to antigens, which are substances that are foreign to the body. However, the immune system also has to be able to distinguish between foreign antigens and self-antigens, which are substances produced by the body's own cells. This is achieved through a process known as central and peripheral tolerance.

Central tolerance occurs in the thymus and bone marrow, where immune cells are produced. During their development, these cells are exposed to self-antigens. If they react strongly to these self-antigens, they undergo apoptosis, or programmed cell death. This process effectively eliminates self-reactive cells before they can enter the circulation.

Peripheral tolerance occurs outside the thymus and bone marrow, in the rest of the body. Here, self-reactive cells that escaped central tolerance are kept in check by various mechanisms. These include anergy (where the self-reactive cells become unresponsive), suppression by regulatory T cells, and again, apoptosis.

In addition to these mechanisms, the immune system also has a 'safety net' in place. Some areas of the body, known as immune-privileged sites, are able to tolerate the introduction of antigens without eliciting an immune response. These sites include the eyes, brain, and testes.

However, sometimes these tolerance mechanisms fail, leading to autoimmune diseases. These are conditions where the immune system mistakenly attacks the body's own cells, treating them as if they were foreign invaders. Examples of autoimmune diseases include type 1 diabetes, rheumatoid arthritis, and multiple sclerosis.

In conclusion, the immune system avoids attacking self-antigens through a combination of central and peripheral tolerance mechanisms, as well as the existence of immune-privileged sites. However, when these mechanisms fail, it can lead to autoimmune diseases.