What term describes an antigen that the immune system of the host is tolerant to?

• A. Foreign antigen
• B. Allergen
• C. Autoimmune antigen
• D. Self antigen

Answer: (D)

The immune system learns not to react against components that come from the body, so it remains tolerant to those elements. An antigen that the host is tolerant to is a self antigen—molecules from the body's own tissues that are usually ignored by the immune system due to central and peripheral tolerance. Foreign antigens are non-self and typically provoke an immune response; allergens are a subset of non-self substances that can trigger allergic reactions; autoimmune antigens are self antigens that, when tolerance breaks down, become targets of an autoimmune attack. Therefore, the term describing an antigen the immune system is tolerant to is self antigen.