Cross‐Reactivity in the HL‐A System

A comprehensive study to elucidate the cross‐reactivity between 25 specificities of the HL‐A system was carried out systematically with 65 monospecific anti‐HL‐A antisera and nearly 150 cell donors. Antisera were absorbed by platelets in microvolumes and the degree of absorption was measured by the inhibition of lymphocytotoxicity. The following conclusions were drawn: (1). Cross‐reactivity occurred only between allelic gene products of a single HL‐A segregant series and not between those of the two different series. (2). Cross‐reactive groups did not form isolated clusters but were interconnected. (3). Cross‐reactivity was found to be one‐way (unidirectional) or two‐way (bidirectional). One‐way cross‐reactivity was observed between 24 pairs of antigen groups and two‐way cross‐reactivity was observed between 8 pairs of antigen groups. (4). Cross‐reactivity is exhibited by certain antisera of a given specificity and not by others of the same specificity. (5). Some cells possessing a cross‐reacting antigen cross‐absorbed a certain anti‐HL‐A antiserum, whereas other cells with the same antigen did not. These results suggest the possibility that cross‐reactivity depends also upon other antigens present on the cells.