The basis of all immunoassays is the specificity of an antibody molecule for a particular chemical structure.

This figure depicts the binding site of an antibody, which has a structure complementary to the target analyte. The noncovalent binding of the target analyte to the antibody's binding site occurs because of the lock and key fit between these complementary structures.

Molecules which differ slightly from the target structure may also bind to the antibody, though less tightly than the target analyte. Molecules which differ greatly from the target structure will not bind to the antibody.

Key concept: lock and key fit between analyte molecule and antibody binding site.