Cutaneous dendritic cells in health and disease

Specialized, bone marrow-derived antigenpresenting cells (dendritic cells, DCs) are critical in the initiation and propagation of skin-related immune reactions in both health (host defense) and disease (inflammatory skin conditions). These DC bridge the innate and adaptive arms of the immune system. Dendritic cell subsets are difficult to define and characterize from a functional perspective because of their heterogeneity and plasticity. All DC can act as antigen-presenting cells (APCs), and thus have the capacity to uptake and process polypeptides into antigens that are readily recognizable by the immune system. This group of cells expresses major histocompatibility complex (MHC) antigens (class I and II) and low levels of co-stimulatory molecules that are regulated. Cytokine production by DCs is also a critical feature that allows these APC to activate and educate T lymphocytes during cell-cell interactions. Dendritic cells can exist in tissues in an immature state, and can be activated into a mature state by microbial products (via Toll-like receptors) that results in migration from a tissue such as the skin to the local lymph node. Dendritic cells are present in normal and inflamed skin (Langerhans' cells in the epidermis, and dermal dendritic cells in the dermis) and may be involved in the pathogenesis of important diseases such as psoriasis and atopic dermatitis.