Recognition of an antigen by immune cells takes place through their epitopes or determinant sites.
Immune response follows with the interaction of antigens and lymphocytes. Generally antigens express more than one epitope on their surface and hide some epitopes inside. Exposure of hidden epitopes takes place only when the antigen is processed by antigen presenting cell.
The number of antigenic determinants or epitopes per antigen molecule is refereed as the “valence of the antigen”.
The functional valence number (surface epitopes) is different from the non functioning (hidden epitopes) valence. For example the bovine serum albumin valence is 18 but functional epitopes or functional valence is only 6.