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Crude extracts of Plasmodium falciparum schizont‐infected erythrocytes (PfSE) induce polyclonal activation of peripheral blood T lymphocytes from naive (malaria unexposed) humans. We demonstrate that the active component of PfSE is membrane bound, soluble in sodium dodecyl sulphate (SDS) and partially heat stable, but distinct from the tumour necrosis factor (TNF)‐inducing, exoantigen‐like activity of schizont extracts. Malaria pigment induces little or no T‐cell activation. The responding cells are predominately CD4+, CD45RO+, T‐cell receptor (TCR)αβ+. Contrary to previous reports, expansion of the TCRγδ+ subset was observed in cells from only one of eight donors. Proliferating cells secrete interferon‐γ (IFN‐γ) and release large amounts of soluble interleukin‐2R (sIL‐2R) into the culture supernatant but produce no detectable interleukin‐4 (IL‐4), a phenotype typical of the T‐helper (Th)1 subset of CD4+ T cells. We propose that these activated T cells may initiate the inflammatory response to malaria infection in non‐immunes and may contribute to the pathology of the disease.

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