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 unique molecular markers on their surfaces, known as antigens. These antigens consist of various proteins and carbohydrates specific to each type of cell. Self-cells, which are part of the body’s own tissues, display a unique set of markers identified as "self" due to the presence of specific molecules like Major Histocompatibility Complex (MHC) proteins.

When immune cells encounter antigens from non-self cells, such as pathogens or cancer cells, they utilize specific receptors to detect these foreign markers. This ability to distinguish between self and non-self is crucial for the immune system to react appropriately and protect the body from infections and diseases while avoiding attacks on its own cells.

While antibodies play a role in identifying and neutralizing pathogens after the immune system has recognized them, and natural barriers such as skin provide a first line of defense, these factors alone do not account for the nuanced recognition process. Additionally, hormones are involved in the regulation of immune responses but are not directly responsible for distinguishing self from non-self. Therefore, the recognition of unique molecular markers on cells is the foundational mechanism behind this essential function of the immune system.