What type of antigens does the immune system recognize as "foreign"?

The immune system identifies non-self antigens as "foreign" because these are molecules that originate from outside the body and are distinct from the body's own cells and tissues. Non-self antigens can include proteins, polysaccharides, or other substances that are present on the surface of pathogens like bacteria, viruses, or fungi. When these non-self antigens are detected, the immune system activates to defend against potential threats, such as infection or disease. This recognition is a critical function of the immune system, allowing it to differentiate between the body's own components and those that could harm it.

Self-antigens are the components that belong to the individual's own body, and the immune system typically recognizes these as "self" to prevent autoimmune responses. Pathogenic antigens specifically refer to those associated with pathogens; however, they are a subset of non-self antigens. Autoantigens are self-antigens that can trigger an immune response in autoimmune diseases, but they are not recognized as foreign by a healthy immune system. Thus, non-self antigens is the most accurate description of what the immune system perceives as foreign entities.