# Antigen --- An **antigen** (or ag') is a toxin or other foreign substance that triggers an [[immune system|immune response]]. For example, our **Self-antigens** are molecules that immune cells can bind to, but they recognize as part of our own body and do not provoke an immune response. In humans the "self marker" that presents self-antigens are our [[MHC molecule]] molecules. As [[red blood cells]] lack these markers, they can be safely transferred from one person to the next (provided the [[blood|blood types]] match). ## Exogenous and Endogenous Antigens **Exogenous antigens** are molecules that are made by *foreign* cells that stimulate our immune system. This can be things like bacteria parts or virus parts, both of which have many membrane/envelope/capsid molecules, or fungi or parasites. Exogenous antigens are presented on [[MHC molecule]] molecules by those white blood cells that have them, to [[helper T-cells]]. **Endogenous antigens** are molecules that are made by *our own* cells that stimulate our immune system. These can be cancerous cells or molecules made by cells with a [[virus|viral]] infection. Endogenous antigens are presented by [[MHC molecule]] molecules to [[cytotoxic T-cells]] or [[natural killer cells]]. ## Epitopes **Epitopes** are a small region of an antigen that are more specific than just the antigen in general. this is the site that [[antibodies]] bind to. One antigen can have many different epitopes, and each one binds to only a specific antibody. We have thousands of types of antibodies in our systems. ![[epitope.png]] ___