Release of platelet factor 4 into the blood after cold challenge of patients with cold urticaria.

Five individuals with idiopathic cold urticaria but not normal volunteers released platelet factor 4 (PF4) detected by radioimmunoassay into the circulation after cold challenge. In three patients, a biphasic rise in PF4 was noted with increases at 1 and 10 to 20 min after immersion, whereas in two others only the later rise was detected. Peak levels of PF4 were detected in all five patients 20 min after cold immersion, whereas peak levels of other mediators such as histamine and eosinophil and neutrophil chemotactic activity occurred earlier at 10, 3 to 10, and 5 to 10 min, respectively. The identification of PF4 in the circulation of patients with cold urticaria after cold challenge provides further evidence for the activation of platelets in mast cell-dependent disorders and suggests new potential mechanisms for the expression of cold urticaria.