Several types of irregularly shaped dendritic cells have been identified, including dendritic cells and interdigitating cells in lymphoid tissues, epidermal Langerhans cells, follicular or germinal center dendritic cells, and veiled cells in lymph. All these cells exhibit phenotypic differences from monocytes and macrophages that include cytology, surface markers, endocytic activity, and capacity to adhere to tissue culture surfaces. Phenotypic differences have provided the means for separating dendritic cells from macrophages and other cell types. The precise lineage of these dendritic cells is not established. Functional studies, in which positive and negative selection procedures are used, indicated that dendritic cells are specialized stimulator cells for immune responses.