Human blood lymphocytes cultured for 3 days with concanavalin A (Con A), phytohemagglutinin or pokeweed mitogen, in mixed lymphocyte culture with added interleukin 2 and stimulated by a lymphoblastoid cell line were found to activate and bind C3 molecules when exposed to human serum. The split products of C3 were detected in the supernatants and on the surface of the activated cells. The surface-attached C3 fragment on the Con A blast was identified as C3b by immune adherence i.e. binding of CR1 carrying human erythrocytes. In the Con A-stimulated population the majority of cells that activated and bound C3 were CD3 and Fc gamma receptor (CD16)-positive but complement receptor-negative blasts. In this cell subset both CD4 and CD8-positive cells were detected but their frequency suggested that a proportion of them carried both markers.
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