Keratinocytes make up the bulk of the cells in the epidermis. They are responsible for preventing the entry of toxic substances into the body. The cells lock together to form tight barriers and also release a waxy substance to help the skin remain waterproof. Keratinocytes start at the stratum basale in the epidermis and eventually are pushed upward through the layers of the epidermis. They die along the way and are sloughed off when they reach the topmost layer.
Melanocytes are located in the basal layer of the epidermis and are in far less numbers than the keratinocytes. The melanocytes produce melanin, which determines the color of skin and hair. These cells transfer the pigment into the keratinocytes and that is what gives skin its color. The melanin is transferred via melanosomes, which are granules that contain the pigment. They move along cytoplasmic processes that connect to the keratinocytes.
Langerhans cells are cells that are involved in helping the immune system detect and neutralize threats. The cells recognize foreign materials and pass them along to lymphocytes to be dealt with by the immune system at large. This is just one of the ways the skin acts as part of the immune system. The Langerhans cells also have long dendrites or fingerlike projections to reach between the keratinocytes and search out foreign materials.
Merkel cells have some connection with the nervous system, and it is believed they have something to do with the perception of light touch. However, their complete function is something of a mystery to scientists. These cells are smaller than the other cells of the epidermis, and they seem to have a relation to some forms of skin cancer.