How does the immune system differentiate between self and non-self cells?

The immune system differentiates between self and non-self cells primarily by recognizing antigen patterns on pathogens. This process is fundamental to immune function, as it allows the body to distinguish between its own cells, which generally carry self-antigens, and foreign invaders such as bacteria and viruses, which possess distinct non-self antigens.

Antigens are molecules or molecular structures that are recognized by immune cells, specifically by antibodies and receptors on immune cells known as lymphocytes. These antigens are unique patterns presented on the surfaces of pathogens, such as proteins or polysaccharides, which the immune system can identify as foreign. When the immune system encounters these non-self antigens, it triggers an immune response to eliminate the threat. This ability to recognize and respond to non-self antigens helps to prevent infections while also maintaining tolerance to self-antigens, which is crucial to avoid autoimmune reactions.

Other factors such as counting cell surface markers, measuring cell size, or observing cell mobility, while relevant in different contexts of immunology, do not play the primary role in the immune system's ability to discriminate between self and non-self. The recognition of antigen patterns is the core mechanism driving this essential function.