A dendritic cell is a specialized white blood cell of the immune system that captures and processes antigens and presents them to T cells. Dendritic cells are particularly effective antigen-presenting cells for initiating responses in naïve T cells, which have not previously encountered their specific antigen. They connect innate immunity, including detection of microbial signals, with adaptive immunity. Depending on their subtype and the surrounding signals, they can promote protective responses or help maintain immune tolerance. Their name refers to the branching projections visible in many of these cells. (nobelprize.org)
Discovery and defining characteristics
Ralph M. Steinman and Zanvil A. Cohn described dendritic cells in mouse peripheral lymphoid organs in 1973. Subsequent experiments established their unusually strong ability to stimulate primary T-cell responses, distinguishing them functionally from other accessory cells, including macrophages. Steinman received half of the 2011 Nobel Prize in Physiology or Medicine for discovering the dendritic cell and its role in adaptive immunity. (nobelprize.org)
Dendritic cells occur in peripheral tissues and lymphoid organs. Tissue-associated cells can collect antigens and migrate to sites where T cells encounter antigen-presenting cells. Their shape alone does not define their identity: classification also considers developmental origin, surface markers, and function. Modern studies distinguish several populations rather than treating all dendritic cells as a single uniform cell type. (nobelprize.org)
Development and major populations
Conventional dendritic cells arise from hematopoietic progenitors associated with the bone marrow. Their development depends on growth-factor signaling, notably the FLT3 ligand–FLT3 pathway, and on combinations of transcription factors. Two broad conventional populations, cDC1 and cDC2, are recognized in mice and humans, although their identifying markers differ between species and tissues. (pubmed.ncbi.nlm.nih.gov)
Type 1 conventional dendritic cells (cDC1) are especially important for presenting material obtained from other cells to CD8-positive T cells. This supports responses against infected cells and tumors. Type 2 conventional dendritic cells (cDC2) have prominent roles in activating CD4-positive helper T cells and coordinating responses to diverse extracellular antigens. These functional specializations overlap and depend on tissue location and inflammatory conditions. (pubmed.ncbi.nlm.nih.gov)
Plasmacytoid dendritic cells are distinguished by their capacity to rapidly produce large amounts of type I interferons after detecting viral or other nucleic acids. These cytokines induce antiviral programs in surrounding cells and influence other immune populations. Plasmacytoid cells also have antigen-presenting and immunoregulatory functions, but their specialization differs from that of conventional dendritic cells. (pubmed.ncbi.nlm.nih.gov)
Other populations require separate consideration. Monocytes can generate dendritic-like antigen-presenting cells during inflammation or in laboratory cultures. Langerhans cells of the epidermis perform dendritic-cell-like antigen capture and presentation but have a distinctive developmental history. Neither population should be assumed to be developmentally identical to conventional dendritic cells. (pmc.ncbi.nlm.nih.gov)
Antigen processing and T-cell activation
Dendritic cells acquire extracellular material through processes including phagocytosis. Antigenic proteins are broken into peptides, which can be displayed on major histocompatibility complex (MHC) molecules. Peptide–MHC class II complexes are recognized by CD4-positive T cells, whereas peptide–MHC class I complexes are recognized by CD8-positive T cells. Recognition depends on the specificity of the T-cell receptor. (nobelprize.org)
A particularly important pathway is cross-presentation: extracellular or cell-associated antigens are displayed on MHC class I rather than solely entering the class II pathway. This enables dendritic cells to initiate CD8-positive responses against targets even when the presenting cell is not itself infected or malignant. Cross-presentation can also contribute to tolerance; antigen display does not automatically produce an immune attack. (pubmed.ncbi.nlm.nih.gov)
Effective activation of naïve T cells requires more than antigen recognition. Dendritic cells supply costimulatory signals, including interactions involving CD80 and CD86, and cytokines that influence T-cell differentiation. Microbial recognition and inflammatory signals can promote maturation, increasing the cells’ capacity to present antigens and direct T-cell responses. Changes in migration receptors, including CCR7, help position activated cells in lymphoid tissues. (nobelprize.org)
Roles in immune tolerance
Dendritic cells also present material from normal tissues. When antigen presentation occurs without the signals required for a strong effector response, it can contribute to peripheral immune tolerance. Outcomes include T-cell anergy, removal of antigen-reactive cells, or support for regulatory T cells, which restrain immune responses. These processes help prevent inappropriate reactions against the body’s own constituents. (pmc.ncbi.nlm.nih.gov)
Tolerance is not simply the absence of dendritic-cell activity. It can involve active inhibitory signaling, regulatory cytokines, and interactions with other immune cells. The outcome depends on antigen context, the presenting population, and the tissue environment rather than on an absolute division between “immune” and “tolerant” dendritic cells. (nobelprize.org)
Clinical and research relevance
Dendritic-cell biology informs vaccine development and cancer immunotherapy. Experimental strategies include exposing patient-derived antigen-presenting cells to tumor antigens outside the body and then administering the prepared cells. Another approach delivers antigens directly to selected dendritic-cell populations. These methods seek to control antigen presentation together with the signals that determine the resulting immune response. (pubmed.ncbi.nlm.nih.gov)
Sipuleucel-T, used for certain forms of advanced prostate cancer, is an established example of autologous cellular immunotherapy. It contains patient-derived peripheral blood mononuclear cells, including antigen-presenting cells enriched for a dendritic-cell fraction, exposed to a fusion protein containing prostatic acid phosphatase and GM-CSF. It is therefore not a preparation of purified dendritic cells alone. (cancer.gov)