Morphology of delayed-type hypersensitivity reactions in man.

The new morphologic findings reviewed here substantially alter prevalent conceptions of delayed hypersensitivity as a simple cutaneous infiltration of lymphocytes and macrophages. By assigning an integral role of basophils, mast cells, the microvasculature, and the clotting system, the findings have far-reaching implications for an understanding of these clinically important reactions. Morphologic observations, of course, represent only a first step, a foundation on which subsequent immunologic, physiologic, and biochemical experiments can build. Much further work will be required to interrelate these new findings and to integrate them with older observations into a coherent sequence of events which can explain the pathogenesis of cell-mediated reactions. A preliminary attempt in this direction, based on present, rather incomplete information, is presented in Figure 8 as a basis for further investigation.