How does the body distinguish "self" from "non-self"?

The immune system is the body's defense mechanism that protects against infections, diseases, and other potentially harmful foreign substances. It does this by recognizing and eliminating pathogens, cancerous cells, and damaged cells, while leaving the body's healthy cells unharmed.

The immune system is equipped with specialized cells, molecules, and mechanisms to differentiate between the body's own cells (self) and foreign substances or cells (non-self). This ability to distinguish between self and non-self is crucial for the immune system to work effectively and without causing harm to the body.

Major histocompatibility complex (MHC) molecules play a critical role in self and non-self recognition. MHC molecules are proteins present on the surface of almost all cells in the body. They are responsible for presenting small protein fragments from inside the cell to the immune system. These protein fragments can be either self (from normal cellular processes) or non-self (from pathogens). There are two classes of MHC molecules: 1. MHC class I: Found on the surface of all nucleated cells and display protein fragments from inside the cell to cytotoxic T-cells. 2. MHC class II: Found on the surface of specialized immune cells, such as macrophages, dendritic cells, and B cells, and display protein fragments acquired from extracellular substances (such as pathogens) to helper T-cells.

T-cells, which are a type of white blood cell, play a central role in the recognition of self and non-self. Each T-cell has a unique T-cell receptor (TCR) on its surface. TCRs are able to recognize and bind to specific protein fragments (antigens) on MHC molecules. During the development of T-cells in the thymus, a process called thymic selection ensures that T-cells with TCRs that have high affinity for self-antigens are eliminated. This helps to prevent an immune response against the body's own cells. When TCRs on cytotoxic T-cells and helper T-cells interact with MHC Class I and MHC Class II molecules, respectively, they are able to differentiate between self and non-self based on the antigens presented by the MHC molecules. If the TCR identifies the antigen as non-self (foreign), it will initiate an immune response to eliminate the threat.

B-cells, another type of white blood cell, also play a role in the recognition of self and non-self. B-cells produce unique B-cell receptors (BCRs) on their surface that can recognize and bind to specific antigens. Like T-cells, B-cells undergo selection in the bone marrow to eliminate those with BCRs that strongly bind to self-antigens. When BCRs on B-cells bind to non-self antigens, the B-cell is activated and starts producing antibodies that can specifically recognize and neutralize the non-self substance, further contributing to the immune response. In conclusion, the immune system can distinguish self from non-self through the actions of MHC molecules, TCRs, and BCRs, as well as through the selection processes that eliminate immune cells with a high affinity for self-antigens.