Abstract
The increasing number of antigens detectable in human lymphoid tissue (particularly since the advent of monoclonal antibodies) makes it necessary to have techniques available for studying the relative distribution patterns of pairs of antigens in tissue sections. Double immunoenzymatic labelling (using peroxidase and alkaline phosphatase) offers a number of advantages over double immunofluorescence, including the fact that the two antigens can be visualised simultaneously (rather than sequentially) and that the labels are permanent. In studying paraffin-embedded human lymphoid tissue an important application of the double immunoenzymatic technique lies in distinguishing Ig-positive cells containing exogenous Ig (which stain for only a single light chain class). In addition double staining of paraffin sections for IgG and IgM has been used to show that "switch" cells containing both these classes of heavy chain are rare in reactive lymphoid tissue. The potential scope of the double immunoenzymatic technique has been extended by showing that the procedure is applicable to cryostat sections (in which antigenic reactivity is better preserved than in paraffin sections) and by adapting it for use with monoclonal antibodies (by preparing "monoclonal PAP" complexes).