Antigen-presenting cells are capable of participating in the stimulation of T cells by antigen presentation. Antigen-presenting cells are considered essential for the induction and expansion of the immune reaction because their interaction with antigen is the first step in immune induction.We have studied the distribution of class II expressing cells in developing, healthy and carious human teeth to clarify when human pulp acquires an immunologic defense potential. Antigen-expressing cells were identified immunohistochemically with HLA-DR monoclonal antibodies (for dendritic cells) and CD68 monoclonal antibodies (for macrophages).In the pulp of unerupted developing teeth, HLA-DR-positive cells were distributed mainly in and around the odontoblast layer. A few CD68 positive cells were located more coronary around the blood vessels. In erupted teeth, HLA-DR-positive cells were located, for the most part just beneath the odontoblast layer. CD68 positive cells were also located coronary mainly around the blood vessels. Superficial caries lesions caused aggregation of HLA-DR-positive cells and macrophages in the dental pulp corresponding to the lesion.Our results showed that human teeth are already equipped with an immunological defense potential before the eruption. In the initial stage of caries infection, an immune response mediated by class II expressing cells is initiated in human dental pulp.