Dendritic cells present antigens by capturing foreign material, breaking it into small peptide fragments, loading those fragments onto specialized display molecules (called MHC), and carrying them to the cell surface where passing T cells can inspect them. This process has two main versions depending on which type of T cell needs to be activated, and dendritic cells are uniquely capable of using both.
Capture and Storage in Immature Dendritic Cells
Dendritic cells begin their life cycle as immature sentinels stationed throughout your body’s barrier tissues: skin, gut lining, airways, and other surfaces exposed to the outside world. In this immature state, they are optimized for one job: grabbing anything that looks foreign. They engulf bacteria, viruses, dead cells, and soluble proteins through several uptake methods, pulling material into internal compartments called phagosomes or endosomes.
What makes immature dendritic cells unusual is their ability to stockpile captured material without immediately processing it. Internalized antigens can be held intracellularly for at least 60 hours, waiting for the right activation signal. This delay matters because it lets dendritic cells accumulate a broad sample of whatever is present in the tissue before committing to an immune response.
The Maturation Switch
The transition from passive collector to active presenter is triggered when dendritic cells detect danger signals, known as pathogen-associated molecular patterns. These are molecular signatures unique to microbes, such as bacterial cell wall components or viral genetic material. Dendritic cells recognize them through surface sensors called pattern recognition receptors, including the well-known Toll-like receptors.
Once these receptors fire, a cascade of internal signaling raises intracellular calcium levels, which activate transcription factors that reprogram the cell. The result is a dramatic shift in identity. Immature dendritic cells express low levels of the surface molecules needed for T-cell activation. Mature dendritic cells ramp up production of MHC class II molecules, costimulatory molecules (CD80, CD86, and CD83), the homing receptor CCR7, and cytokines like IL-12 and TNF. At the same time, their ability to capture new material drops. They’ve switched from collection mode to presentation mode.
Presenting to Helper T Cells: The MHC Class II Pathway
The pathway for activating CD4+ helper T cells relies on MHC class II molecules, and the loading process is more intricate than it might seem. Newly made MHC class II molecules don’t travel to the cell surface empty. Instead, a protein called the invariant chain occupies their binding groove during assembly, preventing them from picking up random peptides inside the cell prematurely. This invariant chain also directs the MHC class II molecules into the same acidic endosomal and lysosomal compartments where captured antigens are being broken down.
Inside these compartments, enzymes called proteases chew up both the captured antigen and the invariant chain itself. But a small fragment of the invariant chain, called CLIP, stays wedged in the MHC groove. Removing it requires a specialized molecule called HLA-DM, which pries CLIP out and stabilizes the now-empty MHC class II molecule in a state ready to accept a new peptide. HLA-DM does more than just remove CLIP. It acts as a quality-control editor, testing incoming peptides and favoring those that bind tightly. Low-affinity peptides get kicked out; high-affinity peptides stick. This editing ensures the cell surface displays the most stable, informative peptide fragments rather than a random assortment.
Once a strong peptide locks in, the loaded MHC class II complex travels to the cell surface. In immature dendritic cells, MHC class II molecules tend to accumulate in internal compartments. Maturation signals redirect this traffic, sending peptide-loaded complexes efficiently to the plasma membrane where T cells can encounter them.
Presenting to Killer T Cells: The MHC Class I Pathway
MHC class I molecules normally display fragments of proteins made inside the cell itself, which is how killer CD8+ T cells detect virus-infected cells or cancerous cells. The standard version of this pathway works like this: proteins in the cell’s cytoplasm are chopped into short peptides by a protein-recycling machine called the proteasome. Those peptides are then shuttled into the endoplasmic reticulum (the cell’s protein-assembly factory) by a transporter called TAP. Inside, the peptides are loaded onto MHC class I molecules with the help of several chaperone proteins, and the finished complex is shipped to the cell surface.
For most cells, this means MHC class I only shows what’s happening internally. Dendritic cells break this rule through a process called cross-presentation.
Cross-Presentation: Breaking the Rules
Cross-presentation is the ability to take material captured from outside the cell and load it onto MHC class I molecules. This is critical for fighting viruses and tumors, because dendritic cells themselves may never be infected. They need to present viral or tumor fragments picked up from dead infected cells to CD8+ killer T cells, which then go hunt down the actual infected or cancerous cells.
There are three recognized routes for cross-presentation, all starting with antigen sitting inside a phagosome:
- Vacuolar pathway: The antigen is broken down by enzymes called cathepsins right inside the phagosome, and MHC class I molecules in the same compartment pick up the resulting peptides. This is the simplest route and resembles MHC class II loading, just with a different display molecule.
- Phagosome-to-cytosol pathway: The antigen escapes from the phagosome into the cytoplasm, possibly through membrane rupture or active transport. Once in the cytoplasm, it’s processed by the proteasome and loaded onto MHC class I in the endoplasmic reticulum via TAP, following the same steps as the standard internal pathway.
- Phagosome-to-cytosol-to-phagosome pathway: This hybrid route starts the same way, with antigen escaping to the cytoplasm and being cut by the proteasome. But instead of going to the endoplasmic reticulum, the resulting peptides are pumped back into the phagosome through TAP transporters located on the phagosome membrane. There, they’re trimmed to final size and loaded onto MHC class I molecules that have been delivered to the phagosome from the cell surface or the endoplasmic reticulum.
The existence of multiple cross-presentation pathways gives dendritic cells flexibility. Different types of antigens and different activation signals may favor one route over another.
Migration to Lymph Nodes
Antigen presentation would be useless if dendritic cells stayed in the tissues where they picked up their cargo. The T cells they need to activate are concentrated in lymph nodes, not scattered throughout the skin or gut lining. Maturation solves this problem by turning on CCR7, a surface receptor that responds to two chemical signals (CCL19 and CCL21) produced along the route to lymph nodes.
CCR7 signaling triggers changes in the cell’s internal skeleton, reorganizing structural proteins to create a front-back polarity that allows efficient crawling. Dendritic cells follow increasing concentrations of CCL21 into nearby lymphatic vessels, travel to the lymph node, pass through the outer region, and home in on the T-cell zone deep inside. There, they settle and begin scanning passing T cells.
The Three Signals That Activate a T Cell
Arriving in the lymph node with antigen on display is necessary but not sufficient. Full T-cell activation requires three distinct signals delivered simultaneously at the contact point between the dendritic cell and the T cell.
Signal 1 is the antigen itself, presented in the MHC groove and recognized by the T-cell receptor. This tells the T cell what threat to respond to. Signal 2 comes from costimulatory molecules. CD80 and CD86 on the dendritic cell engage CD28 on the T cell, while CD40 on the dendritic cell binds CD154 on the T cell. Without these costimulatory interactions, a T cell that recognizes its antigen will become unresponsive rather than activated, a safeguard against false alarms. Signal 3 is delivered by cytokines secreted by the dendritic cell, such as IL-12, which shape what kind of immune response the T cell will mount.
The Physical Interface Between Cells
When a dendritic cell and a T cell make contact, they don’t just bump into each other randomly. Within minutes, adhesion molecules on both cells reorganize into a structured interface. Molecules like LFA-1, ICAM-1, and ICAM-3 rush to the contact zone within about four minutes, forming a ring around the edges. Inside that ring, the T-cell receptor, CD28, and signaling proteins cluster at the center. This organized structure, sometimes called the immunological synapse, concentrates the three activation signals into a tight space, ensuring efficient communication between the two cells.
The synapse is not static. It forms, dissolves, and reforms as the dendritic cell scans many T cells in sequence. A single dendritic cell in the lymph node can interact with hundreds or thousands of T cells, searching for the rare cells whose receptors match the displayed antigen. When a match occurs, the synapse stabilizes, signals are delivered, and the T cell begins dividing to mount a full immune response.
Specialization Among Dendritic Cell Types
Not all dendritic cells are equal at every form of presentation. The immune system contains several subtypes, each tuned for different tasks. One major group, called cDC1 cells, excels at cross-presentation and is particularly important for activating CD8+ killer T cells against viruses and tumors. Another group, cDC2 cells, is more focused on presenting antigens to CD4+ helper T cells via MHC class II. A third type, plasmacytoid dendritic cells, is best known for producing large amounts of antiviral interferons, though they can also present antigens to CD4+ T cells under certain conditions. Their cross-presentation ability remains debated, as some early experiments may have been complicated by contamination with other cell types.
This division of labor means the immune system doesn’t rely on a single all-purpose presenter. Different infections and threats engage different dendritic cell subtypes, tailoring the downstream T-cell response to the specific danger.