How does the immune system differentiate between self and foreign substances?

The immune system plays a critical role in protecting our body from harmful pathogens and foreign substances, while also distinguishing them from self. To do this, the immune system has developed a complex set of mechanisms that allow it to differentiate between self and non-self.

One of the main ways the immune system does this is by using specialized cells called antigen-presenting cells (APCs) to display small pieces of foreign substances, called antigens, on their surface. These antigens are then recognized by other immune cells, such as T cells and B cells, which can produce antibodies that specifically target the foreign substance.

However, in order to avoid attacking self tissues, the immune system must also be able to recognize and tolerate self antigens. This is achieved through a process called central tolerance, which occurs during development of immune cells in the bone marrow and thymus. During this process, immune cells that recognize self antigens are eliminated or suppressed, while those that can recognize foreign antigens are selected and allowed to mature.

In addition, the immune system also uses a set of regulatory cells, such as regulatory T cells, to suppress immune responses against self antigens and prevent autoimmune diseases.