lecular association similar to an enzyme-substrate interaction, with an important distinction: it does not lead to an irreversible chemical alteration in either the antibody or the antigen. The association between an anti Fluorescent Antibody Staining Reveals Intracellular body and an antigen involves various noncovalent interac- tions between the antigenic determinant, or epitope, of the ntigen and the variable-region(vH/Vi) domain of the an- a Strength of Antigen-Antibody Interactions tibody molecule, particularly the hypervariable regions