Morphology of delayed-type hypersensitivity reactions in man. II. Ultrastructural alterations affecting the microvasculature and the tissue mast cells.

Profound alterations in the dermal microvasculature and the closely related fixed tissue mast cells were observed in allergic contact dermatitis reactions to dinitrochlorobenzene and urushiol in man. The vascular changes were of two types. The superficial capillary venule, a distinct anatomical and functional vessel system in the dermal papillae, developed interendothelial cell gaps, maximal at 3 days after skin test, which very likely were responsible for locally increased vascular permeability and consequent dermal edema. The products of degranulating tissue mast cells and infiltrating basophils may have been partly responsible for these changes. By contrast, vessels of the superficial venular plexus, oriented parallel to the cutaneous surface at the junction of the papillary and reticular dermis, were the site of diapedesis and perivascular cuffing by lymphocytes and other inflammatory cells. The superficial venular plexus vessels exhibited striking endothelial cell and pericyte hypertrophy, occasional necrosis of endothelial cells, endothelial cell mitoses, and an extensive and progressive laying down of new and abnormally disposed basal lamina. These changes, somewhat slower to develop than those affecting the superficial capillary venule, were present in all biopsies taken at 3 days or later and were accompanied by mast cell mitoses and the appearance of immature dermal mast cells. The basal lamina-alterations persisted and evolved for at least several weeks. The mechanisms initiating these vessel and mast cell changes have not been identified, but circumstantial evidence suggests that lymphocytes or their products or both may have been responsible.