Morphology of the cellular infiltrate in delayed pressure urticaria

In seven patients with delayed pressure urticaria, the dermal inflammatory infiltrate was analyzed in early (4-6 hours) and late (20-24 hours) wheals by light and electron microscopy and by histomorphometric and immunohistologic methods. Results were compared with findings in normal skin of patients and in wheals induced with concanavalin A and compound 48/80 injections in patients and five healthy volunteers. Pressure wheals were characterized by a mild mononuclear perivascular infiltrate and by patchy dermal infiltrates of eosinophils. Neutrophils were almost completely absent. The number of dermal mast cells was markedly increased in early wheals, whereas their number was decreased in late wheals. Numerous mast cells were degranulated in late wheals, as shown by electron microscopy. On immunohistologic analysis, the perivascular infiltrate consisted primarily of lymphocytes with the helper phenotype. Epidermal OKT6 Ia-positive Langerhans cells were of normal number. The local increase of mast cells, eosinophils, and helper T lymphocytes suggests that pressure wheals result from a cellular immune response to as-yet-unknown antigens that are generated at pressure sites.