Antigen presenting cell-T cell interaction in the synovium. Dendritic cells (DCs) in the synovium present an external or local antigen to T cells. T cell receptor interaction and signaling via CD28-CD80/86 are essential for initial T cell activation leading to upregulation of CD40L on the T cells. Activated T cells are then capable of inducing further differentiation of DCs, as well as T cell help to rheumatoid factor B cells that have taken up antigen-Ig complexes. CD40-CD40L interaction between DCs, B cells and T cells may play a critical role in repeated activation of memory T cells in the synovium and, thus, maintenance of the inflammatory reactions.