Dendritic cell-initiated T lymphocyte activation

The dendritic cell (DC) is a unique cell type that is clearly distinguishable from the macrophage with which it has often been confused (for reviews see references 1–4). For example, it shares none of the macrophage-restricted markers such as F4/80, Mac 1 and the Fc receptor identified by 2.4G2 (5). The DC is also physiologically distinct: it is unable to phagocytose particles, has a weak pinocytic capacity and lacks several lysosomal markers. DC constitutively express Ia antigens. Following the demonstration that DC are potent stimulators of the allogeneic mixed leukocyte reaction (MLR), the possibility arose that it was the DC rather than the macrophage that was essential for the induction of immune responses (6).