This study investigates the leukocyte-dependent histamine release (LDHR) mechanism from rabbit platelets, a process that is not dependent on complement. The authors demonstrate that the antibody responsible for LDHR is of the IgE class, and that the basophil is the leukocyte involved in this reaction. They show that basophils release histamine and a platelet-activating factor (PAF) upon sensitization with IgE, which causes platelet aggregation and histamine release. The study also explores the conditions necessary for PAF recovery and preservation, finding that PAF is associated with serum albumin and is stable under certain conditions. The findings suggest that LDHR is an immediate hypersensitivity mechanism that may link allergic reactions with immunologic diseases, particularly in the context of immune complex deposition.This study investigates the leukocyte-dependent histamine release (LDHR) mechanism from rabbit platelets, a process that is not dependent on complement. The authors demonstrate that the antibody responsible for LDHR is of the IgE class, and that the basophil is the leukocyte involved in this reaction. They show that basophils release histamine and a platelet-activating factor (PAF) upon sensitization with IgE, which causes platelet aggregation and histamine release. The study also explores the conditions necessary for PAF recovery and preservation, finding that PAF is associated with serum albumin and is stable under certain conditions. The findings suggest that LDHR is an immediate hypersensitivity mechanism that may link allergic reactions with immunologic diseases, particularly in the context of immune complex deposition.