Antigen-binding sites in antibodies are formed by the variable regions of light (L) and heavy (H) chains. Delineation of antigen interactions with the individual subunits of antibodies is of interest for several reasons, including:

1.	Free L chains are present within B-lymphocytes and in the circulation of patients with certain types of B-lymphocyte tumors (1,2);
2.	Derivation of high-affinity antibodies in vitro from randomly combined libraries of L and H chains is dependent on pairing of subunits that display appropriate interactions with each other and the antigen (3); and
3.	Pairing of L and H chains from different antibodies may permit the development of new functions in the hybrid. For example, pairing of a catalytic L chain with H chains with a defined antigen-binding specificity could generate a catalyst with specificity dictated by the H chain.