Dendritic cell

Dendritic cells (DCs) are bone-marrow-derived cells present in small numbers in almost all tissues, especially at barriers — the epidermis (Langerhans cells), dermis, airways and gut lamina propria — and in the T-cell zones of lymphoid organs. Immature DCs in tissues are specialised for sampling: they continuously take up antigen by phagocytosis, macropinocytosis and receptor-mediated endocytosis and sense danger through Toll-like and other pattern-recognition receptors.

When they detect PAMPs or inflammatory cytokines, DCs mature: they stop capturing antigen, process it into peptides loaded on MHC class II (and MHC class I), strongly upregulate the co-stimulatory molecules B7 (CD80/CD86) and CD40, express the chemokine receptor CCR7, and migrate via afferent lymphatics to the paracortex of the draining lymph node within about 1–2 days. There they present antigen to the rare naive T cells that recognise it, delivering signal 1, signal 2 and polarising cytokines (signal 3, e.g. IL-12 for Th1). DCs are the only cells that efficiently prime naive T cells. They can also cross-present exogenous antigen (e.g. from virus-infected or tumour cells they have engulfed) on MHC class I to naive CD8 T cells.

Subsets include conventional DCs — cDC1 (specialised in cross-presentation and CD8/Th1 responses) and cDC2 (CD4 T-cell responses, Th2/Th17) — plasmacytoid DCs, which recognise viral nucleic acids through TLR7 and TLR9 and secrete enormous amounts of type I interferon, monocyte-derived inflammatory DCs, and epidermal Langerhans cells. Follicular dendritic cells of B-cell follicles are unrelated stromal cells that hold intact antigen for B cells and do not present peptide–MHC.