Dendritic Cell Function: Antigen Presentation Explained

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Dendritic cell function is to sample antigen in peripheral tissue, process it into peptide fragments, and present those fragments to naive T cells in lymph nodes to start an adaptive immune response. Dendritic cells are the only antigen-presenting cells that can reliably activate a T cell that has never met that antigen before, which is why they are called professional antigen-presenting cells.

This article walks through the full sequence: how a dendritic cell captures antigen, how it cuts that antigen into peptides, how those peptides get loaded onto MHC molecules, how the cell migrates to a lymph node, and how it primes a T cell. It also separates the major dendritic cell subsets, because a conventional dendritic cell in skin and a follicular dendritic cell in a lymph node follicle do very different jobs.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What Are Dendritic Cells?

Dendritic cells are bone marrow-derived leukocytes named for the branching, tree-like processes they extend when they mature. Ralph Steinman described them in the 1970s, and the British Society for Immunology describes them as sentinel cells that bridge innate recognition and adaptive immunity [1].

They sit in nearly every tissue that meets the outside world: skin, airway, gut lamina propria, urogenital mucosa, and the interstitium of solid organs. In those locations they act as scouts. They capture material from pathogens, dying cells, allergens, and commensal organisms, then carry a report of what they found to the draining lymph node.

Three features define a dendritic cell as a professional antigen-presenting cell:

• It expresses MHC class II constitutively, even before it meets antigen.
• It can process exogenous antigen into peptides for MHC loading.
• It upregulates costimulatory molecules on maturation, which lets it deliver signal 2 to a T cell.

Macrophages and B cells also present antigen, but macrophages are largely focused on killing and clearance, and B cells present antigen they bind through their B-cell receptor. Dendritic cells are built to migrate and to prime.

The Stepwise Path of Antigen Presentation

Antigen presentation is not a single event. It is a relay with five stages, and each stage has its own failure modes.

Step 1: Antigen Uptake by Phagocytosis or Endocytosis

Immature dendritic cells are specialized for capture. They use several routes depending on the antigen.

Phagocytosis takes in particles larger than roughly 0.5 micrometers: bacteria, fungal elements, apoptotic bodies, and cell debris. Receptors such as mannose receptors, scavenger receptors, and Fc receptors pull the particle into a phagosome.

Receptor-mediated endocytosis takes in soluble antigen. The mannose receptor CD206 is one well-characterized route. Infliximab aggregates, used as a model of therapeutic antibody immunogenicity, are taken up by dendritic cells through the mannose-sensitive receptor CD206, and aggregation increased both uptake and the frequency of responsive T cells in an autologous dendritic cell and T cell coculture assay [2].

Macropinocytosis is a nonspecific route that samples bulk extracellular fluid. It matters for soluble proteins that have no dedicated receptor.

The timing of uptake relative to activation matters for what happens next. An oligosaccharide adjuvant that activates dendritic cells rapidly through TLR2 and then triggers CD14-mediated antigen internalization about 6 hours later produced deferred uptake that improved cross-presentation, because antigen trafficked into endosomal compartments where NOX2 recruitment alkalinized the compartment [3]. In plain terms, when the cell is activated before it swallows antigen, the antigen tends to be handled for cross-presentation rather than destroyed.

Step 2: Processing in Endosomes and the Proteasome

Once inside, antigen is degraded into peptides. The route depends on which MHC pathway the peptide will feed.

For MHC class II, the antigen stays in the endocytic pathway. The phagosome or endosome acidifies and fuses with lysosomes. Acid proteases, principally cathepsins, cut the protein into peptides of roughly 13 to 25 amino acids. Meanwhile, new MHC class II molecules are assembled in the endoplasmic reticulum with the invariant chain, which blocks the peptide groove. The invariant chain is trimmed until only a small fragment, CLIP, remains in the groove. A chaperone called HLA-DM then exchanges CLIP for a high-affinity peptide from the endosome.

For MHC class I, the antigen must reach the cytosol. Cytosolic proteins are tagged with ubiquitin and fed into the proteasome, a barrel-shaped protease complex. The proteasome generates peptides of about 8 to 10 amino acids, which are transported into the endoplasmic reticulum by the transporter associated with antigen processing, or TAP. There they load onto MHC class I.

Step 3: Loading onto MHC Class II or Class I

The two loading pathways produce two different messages.

MHC class II loading reports on material from outside the cell. It presents to CD4 helper T cells. This is the default pathway for a dendritic cell that has phagocytosed a bacterium.

MHC class I loading reports on material from inside the cell, which is how a virus-infected cell flags itself for CD8 cytotoxic T cells. Dendritic cells add a special trick on top of this: cross-presentation.

Step 4: Cross-Presentation for CD8 T-Cell Activation

Cross-presentation is the process by which dendritic cells communicate to CD8 T cells the detection of exogenous foreign substances known as antigens, priming naive CD8 T cells to eliminate antigen-expressing pathogens and tumors [4]. In other words, the dendritic cell takes material that came from outside, routes it into the MHC class I pathway, and activates a cytotoxic T cell.

Pathogens and tumors evade CD8 T-cell immunity by reducing antigen uptake into the endocytic pathway or by limiting endosomal escape of antigen into the cytosol [4]. Both steps are required for efficient cross-presentation, which is why the pathway is a bottleneck.

Conventional type 1 dendritic cells, or cDC1s, are the main cross-presenting subset. In a tauopathy mouse model, animals lacking cDC1s or lacking antigen cross-presentation were protected from neurodegeneration, with reduced brain CD8 T cell infiltration and glial activation, and brain-derived antigens were presented in secondary lymphoid tissues [5]. That study is a neurodegeneration model rather than a veterinary disease, but it demonstrates the principle cleanly: cDC1-dependent cross-presentation in a lymph node is what puts effector CD8 T cells into tissue.

Step 5: Migration to the Lymph Node and T-Cell Priming

Capture happens in tissue. Priming happens in lymph node. The dendritic cell has to travel between the two.

During maturation, the cell switches its chemokine receptor profile. It downregulates receptors that kept it in peripheral tissue and upregulates CCR7, which responds to CCL19 and CCL21 produced in the T-cell zone of the draining lymph node. The cell enters a lymphatic vessel and arrives in the node within hours to a day or two.

In the node, the dendritic cell presents three signals to the T cell:

• Signal 1 is peptide on MHC. It gives specificity.
• Signal 2 is costimulation through CD80 and CD86 engaging CD28 on the T cell. Without it, the T cell receives signal 1 alone and becomes anergic or regulatory instead of effector.
• Signal 3 is cytokine instruction, such as interleukin-12 for a Th1 response or interleukin-4 for a Th2 response.

The sequence has been traced directly in a mouse transfusion model. Stored red blood cells are taken up by splenic dendritic cells, which activate and migrate into T-cell zones, where they drive MHC class II-dependent T-cell activation and eventually alloantibody production [6]. That is the whole arc in one system: uptake, activation, migration, MHC class II presentation, T-cell help, antibody output.

flowchart TD
    [Antigen in tissue] --> [Uptake by phagocytosis or endocytosis]
    [Uptake by phagocytosis or endocytosis] --> [Endosomal processing]
    [Uptake by phagocytosis or endocytosis] --> [Proteasomal processing]
    [Endosomal processing] --> [MHC class II loading]
    [Proteasomal processing] --> [MHC class I loading]
    [Endosomal processing] --> [Cross presentation]
    [Cross presentation] --> [MHC class I loading]
    [MHC class II loading] --> [Maturation and CCR7 upregulation]
    [MHC class I loading] --> [Maturation and CCR7 upregulation]
    [Maturation and CCR7 upregulation] --> [Migration to draining lymph node]
    [Migration to draining lymph node] --> [T cell priming]
    [T cell priming] --> [Effector and memory T cells]

Immature Versus Mature Dendritic Cells

The maturation state of a dendritic cell determines whether it activates or restrains the immune response. This is the single most important concept in dendritic cell biology.

Immature dendritic cells are tolerogenic. They express relatively low levels of MHC class II and costimulatory molecules, and they are efficient at capturing antigen. When an immature dendritic cell presents a self-peptide to a T cell without costimulation, the T cell is deleted, anergized, or converted into a regulatory T cell. This is how peripheral tolerance to self-antigens and harmless environmental antigens is maintained.

Mature dendritic cells are immunogenic. On maturation, they upregulate CD80, CD86, and MHC class II, and they downregulate antigen capture. The trigger is pattern recognition: toll-like receptors, NOD-like receptors, and cytokine signals such as tumor necrosis factor and interleukin-1.

The tolerogenic program is not passive. It can be actively induced. In influenza infection, extracellular lactate enters dendritic cells through monocarboxylate transporter-dependent transport and induces a tolerogenic-like state marked by impaired maturation, reduced costimulation, and diminished CD8 T-cell priming capacity [7]. The mechanism ran through H3K18 lactylation and NRF2 pathway activation, which reinforced tolerogenic programs and limited mitochondrial reactive oxygen species-dependent XBP1 splicing [7]. This is a metabolic switch that flips a dendritic cell from priming to suppressing.

Conventional, Plasmacytoid, and Follicular Dendritic Cells

Dendritic cells are not one cell type. They are a family with different origins, locations, and jobs. Two of the three groups below are true dendritic cells. The third, follicular dendritic cells, is not.

Conventional Dendritic Cells

Conventional dendritic cells, also called myeloid dendritic cells, come from a myeloid precursor. They are the main antigen-presenting and T-cell-priming population.

cDC1 cells depend on the transcription factor BATF3 and are the primary cross-presenting subset. They activate CD8 T cells and drive Th1 responses. In skin, the equivalent migratory population is marked by CD207 (langerin). In atopic dermatitis lesions, CD207-positive migratory dendritic cells expanded and showed enhanced migratory and antigen-presenting capabilities, with CD1B upregulated and uniquely enriched in that subset [8].

cDC2 cells are the main MHC class II presenters to CD4 T cells and drive Th2 and Th17 responses. Hepatitis B surface antigen disrupted type 2 conventional dendritic cell-mediated natural killer cell activation through TLR7 and TLR8-dependent pathways [9]. That finding links cDC2 cells to innate immune crosstalk, not just T-cell priming.

Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells, or pDCs, look like plasma cells under the microscope. They circulate in blood and accumulate in lymphoid tissue and at sites of inflammation. Their defining function is rapid production of type I interferon in response to viral nucleic acids sensed through TLR7 and TLR9.

Plasmacytoid dendritic cells link innate and adaptive immunity, and they are implicated in autoimmune disease. In Sjogren's syndrome, a pDC-related gene module was identified and used to build a diagnostic model, and cell-cell communication analysis showed active macrophage migration inhibitory factor signaling between pDCs and monocyte subsets [10]. That is a human autoimmune study, but the biology of pDC-monocyte crosstalk is conserved.

Plasmacytoid dendritic cells are also vulnerable in systemic illness. In sepsis, experimental studies show tissue-specific pDC vulnerability or rescue, alongside splenic dendritic cell loss, disturbed bone marrow generation, and impaired antigen-specific T-cell priming [11].

Follicular Dendritic Cells

Follicular dendritic cells are the exception in this family. They are not bone marrow-derived hematopoietic cells in the same lineage sense, they do not express MHC class II for T-cell priming, and they do not process antigen. They are stromal cells in the germinal center of lymph node follicles.

Their function is to hold intact antigen on their surface for long periods so that B cells can sample it. They display antigen-antibody complexes and complement-coated antigen through complement receptors and Fc receptors. This retained antigen drives affinity maturation and the survival of high-affinity B-cell clones. Follicular dendritic cells support B-cell memory and antibody responses. They are not antigen-presenting cells in the T-cell priming sense, and grouping them with conventional dendritic cells is a common student error.

Table of Dendritic Cell Subsets

What Happens When Dendritic Cell Function Fails

Dendritic cell dysfunction shows up as failed T-cell priming, and the mechanisms are increasingly well mapped.

In sepsis, human studies show reduced circulating dendritic cell abundance, altered phenotype, and impaired ex vivo responsiveness, while experimental studies show splenic dendritic cell loss, disturbed bone marrow generation, and impaired antigen-specific T-cell priming [11]. The authors frame this as four potentially coexisting contributions: depletion, dysfunction of residual cells, redistribution, and impaired replacement [11]. A low blood count alone cannot distinguish among them [11].

In the tumor microenvironment, hypoxia, acidosis, nutrient scarcity, and immunoregulatory metabolite accumulation reprogram myeloid antigen-presenting cell metabolism, impair antigen processing and presentation, weaken costimulatory signaling and cytokine production, and compromise antitumor T-cell responses [12]. Lipid metabolic reprogramming in endocrine-resistant hormone receptor-positive breast cancer can affect tumor immune visibility through MHC class I-associated pathways, and lipid accumulation in the microenvironment may contribute to dendritic cell dysfunction and shift tumor-associated macrophages toward immunosuppressive phenotypes [13].

The practical point for veterinary medicine is that any condition producing chronic inflammation, metabolic stress, or sustained antigen exposure can push dendritic cells toward the tolerogenic end of the spectrum. That includes chronic viral infections, chronic enteropathies, and some chronic dermatoses.

Clinical Relevance, Limitations and Common Mistakes

Dendritic cell biology matters clinically because it explains why some immune responses succeed and others fail.

Vaccination depends on it. A vaccine that delivers antigen without a maturation signal may induce tolerance instead of protection. This is why adjuvants exist. Adjuvants provide the pattern recognition signal that matures the dendritic cell, upregulates CD80 and CD86, and licenses it to prime rather than tolerize. The oligosaccharide adjuvant work shows that the timing of activation relative to antigen uptake also matters, because deferred uptake improved cross-presentation [3].

Cancer immunotherapy depends on it. Dendritic cell-based vaccines are built on the premise that loading dendritic cells with tumor antigen and maturing them will generate antigen-specific T cells. In an in vitro study, expanded dendritic cells loaded with placental gp96 induced strong T-cell proliferation, activation, and cytotoxicity against MCF-7, U87MG, and SH-SY5Y cell lines, with 67 to 71 percent cytotoxicity versus 23 to 26 percent for recombinant gp96, and the effect was MHC-restricted [14]. That is an in vitro result and not a clinical protocol, but it demonstrates the antigen-loading principle.

Chronic infection depends on it. In chronic hepatitis B, dendritic cell and natural killer cell function is compromised, and hepatitis B surface antigen disrupts cDC2-mediated natural killer cell activation through TLR7 and TLR8-dependent pathways [9]. The pathogen has effectively disarmed the sentinel.

Common mistakes in reading dendritic cell biology:

• Treating all dendritic cells as one population. Subset identity determines function, and cDC1, cDC2, pDC, and follicular dendritic cells are not interchangeable.
• Assuming maturation always means activation. Maturation is a change in phenotype. Whether that phenotype primes or tolerizes depends on the signals that drove it.
• Confusing follicular dendritic cells with conventional dendritic cells. Follicular dendritic cells retain intact antigen for B cells and do not prime T cells.
• Forgetting that immature dendritic cells are tolerogenic by default. Most self-tolerance depends on this.
• Assuming a low circulating dendritic cell count means dendritic cell failure. Redistribution and impaired replacement produce the same blood picture [11].

Limitations: much of the mechanistic detail in this field comes from mouse models and in vitro human systems. Species differences in dendritic cell markers are substantial, and veterinary markers are less well standardized than human ones. Individual patient assessment requires a veterinarian.

Frequently Asked Questions

What are dendritic cells in simple terms?

Dendritic cells are immune sentinel cells that capture material from pathogens and damaged tissue, carry it to lymph nodes, and show it to T cells to start a targeted immune response.

What is the main function of dendritic cells?

The main function is antigen presentation to naive T cells, which links innate detection of a threat to the adaptive immune response.

How do dendritic cells take up antigen?

They use phagocytosis for particles, receptor-mediated endocytosis for soluble antigen, and macropinocytosis for bulk fluid sampling.

What is the difference between MHC class I and MHC class II presentation?

MHC class I presents peptides from inside the cell to CD8 cytotoxic T cells, and MHC class II presents peptides from outside the cell to CD4 helper T cells.

What is cross-presentation?

Cross-presentation is when a dendritic cell routes exogenous antigen into the MHC class I pathway to activate CD8 T cells against pathogens and tumors [4].

Why are immature dendritic cells tolerogenic?

They present antigen with low costimulation, which deletes, anergizes, or converts responding T cells into regulatory T cells instead of activating them.

What is the difference between conventional and plasmacytoid dendritic cells?

Conventional dendritic cells process and present antigen to prime T cells, while plasmacytoid dendritic cells mainly produce type I interferon in response to viral nucleic acids.

Are follicular dendritic cells real dendritic cells?

No. Follicular dendritic cells are stromal cells in lymph node follicles that retain intact antigen for B cells and do not prime T cells.

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Sources

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2. Mechanisms of therapeutic antibodies immunogenicity: infliximab oligomeric aggregates increase T-cell response and uptake by dendritic cells involving mannose-sensitive receptor CD206.
3. Oligosaccharide adjuvant-deferred antigen endocytosis promotes cross-presentation.
4. Improved cross-presentation enabled by fusion-dependent mini-proteins that efficiently reach the dendritic cell cytosol.
5. Priming of CD8(+) T cells by peripheral dendritic cells exacerbates tau-mediated neurodegeneration.
6. Defining the Role of Dendritic Cells in Red Blood Cell Alloimmunization.
7. Local lactate-driven H3K18 lactylation impairs anti-influenza immunity through NRF2-dependent dendritic cell dysfunction.
8. Identifying dendritic cell heterogeneity and potential risk genes in atopic dermatitis: integrative scRNA-seq, bulk RNA-seq analyses and experimental validation.
9. Hepatitis B Surface Antigen Directly Alters Dendritic Cell-Mediated Natural Killer Cell Responses.
10. Integrated analysis of single-cell and bulk RNA-sequence data reveal plasma dendritic cell-related diagnostic model for Sjögren's syndrome based on machine learning.
11. Dendritic-cell depletion and dysfunction in sepsis: a compartment- and evidence-aware synthesis.
12. Metabolic reprogramming of myeloid antigen-presenting cells in the tumor microenvironment: mechanisms of antigen presentation failure and therapeutic opportunities.
13. Immune evasion driven by lipid metabolic reprogramming in endocrine-resistant HR(+) breast cancer: antigen presentation defects and T-cell dysfunction.
14. Expanded dendritic cells loaded with placental gp96 to generate antigen-specific T cells for anti-tumor immunity: an in vitro study.