Ia antigen on the surface of a subfraction of T cells that bear Fc receptors for IgG.

Human peripheral blood T cells and T cell subpopulations were studied for the presence of Ia-like antigen, by using indirect immunofluorescence with heterologous rabbit anti-Ia antisera. In 13 normal subjects the percentage of T cells staining with the anti-Ia reagent was 2 to 5% and appeared limited to the T gamma subfraction. After concanavalin A (Con A) or phytohemagglutinin (PHA) activation, the proportion of Ia(+) T cells increased due to an increase in the percentage of T gamma cells bearing Ia-like antigen as well as an increase in the proportion of T gamma cells in the T cell fraction.