Self vs Non-self
The immune system has the capacity to distinguish between the body’s own cells (‘self’) and foreign materials (‘non-self’). When foreign materials are detected, the immune system initiates a response to eliminate these intruders, protecting the body from potential harm.
All nucleated cells possess unique and distinctive surface molecules known as major histocompatibility complex (MHC) molecules, specifically MHC class I. These molecules function as identification tags that signal to the immune system that the cells are self. Normally, the immune system exhibits self-tolerance, meaning it does not react to cells bearing these genetically determined markers.
Foreign substances that trigger an immune response are known as antigens. Antigens are recognised by lymphocytes, which prompts the adaptive immune response, where lymphocytes produce antibodies.
Antigenic determinants include surface markers on pathogens; self-markers on cells from different organisms (which is why transplanted organs can be rejected); and proteins from food, which may trigger an immune response unless broken down into smaller components by the digestive system.