Virus antigens and recognition

Viral proteins, both structural and nonstructural, are foreign to the host and many induce the activity of immune cells. The ability to stimulate an immune response is referred to as antigenicity or immunogenicity and the stimulating molecule (the antigen or immunogen) is usually a protein, although carbohydrates and lipids can also act as antigens. The smallest part of an antigen is referred to as an epitope. For many viruses the structural proteins are highly immunogenic and the main triggers of the immune response, particularly envelope proteins that are inserted into the plasma membrane of infected cells. Antigen recognition is the trigger for activation of immune lymphocytes and is determined by binding of the unique molecular structure of the antigen with a protein receptor in the plasma membrane of lymphocytes.