A misunderstanding of the term "immunity" has arisen because the general public usually interprets immunity to mean the ability to avoid a disease, but the medical scientist knows that infection does occur, although in a mild form that precludes serious or life-threatening consequences. Antigens are the proteins in viruses and bacteria that trigger an immune response, and the result of a satisfactory immune response is immunity -- protection from repeated disease caused by a specific type of virus or bacteria ( 1).
The immune system has evolved to deal with enormous numbers and varieties of every conceivable foreign substance, that is, antigen. A consequence of virus entry and replication in an organism -- the host -- is the manufacture of viral antigens that, in most cases, elicit an immune response by that host. Success of this system defines an organism's capacity for survival. In addition, the immune system must discriminate between foreign antigens, such as viral proteins, that are nonself and those antigens that are self, one's own proteins (i.e., hormones such as insulin and cell proteins that make up muscle).