Hypersensitivity to mosquito bites is characterized by severe systemic as well as local symptoms, and associated with chronic active EBV infection and NK cell lymphocytosis. In this HEN disease, we investigated the response of PBMC to MSG extracts. PBMC were taken from three defined cases of HEN disease, three borderline cases, five individuals with simple exaggerated reactions to mosquito bites without systemic symptoms (simple responders), and eight healthy donors. PBMC, or purified CD4+, CD8+ or CD56+ cells, were cultured with MSG extracts prepared from each of five mosquito species to examine their proliferation and cytokine secretion. The patients with HEN disease had high stimulation indices with variations in responses to the extracts from Aedes albopictus, Aedes aegypti, Anopheles sinensis and Culex pipiens pallens. However, a non-Japan-habitant species Anopheles stephensi did not stimulate the patients' PBMC. Some borderline or simple responders showed moderate proliferation, and healthy donors had no reactive PBMC. In HEN disease, both CD56+ NK cells (producing IFN-gamma) and CD4+ Th0 cells (producing IL-4 and IFN-gamma) were increased in the blood. CD4+ cells, but not CD56+ NK cells or CD8+ cells, propagated in response to MSG extracts. However, this response of CD4+ cells and their IL-4 production were strongly enhanced by coexisting CD56+ cells. We suggest that the CD4+ T cell serving as the primary responder to MSG antigen and the NK cell functioning as the enhancer are both pathogenic in the development of HMB.