High magnification electron micrograph of Langerhans' cells showing Birbeck granules. Langerhans' cells are of dendritic appearance, strongly MHC Class II positive and weakly phagocytic. They are found in the basal layers of the epidermis where they serve as accessory cells, responsible for antigen processing and antigen presentation. Having been exposed to antigen, they migrate to the lymph nodes. They are derived from bone marrow and are immature dendritic cells. Part of the immune surveillance system, their location means that they are readily exposed to antigens that penetrate the dermal barrier.