Key Functions of Antigen Presenting Cells

Why This Matters

Antigen presenting cells (APCs) are the critical link between the innate and adaptive immune systems. Understanding how they function is essential for grasping concepts like MHC-mediated antigen presentation, T cell activation requirements, and the coordination between humoral and cell-mediated immunity. APCs don't just passively display antigens; they actively shape whether the immune system mounts an attack, tolerates a substance, or forms long-lasting memory.

When studying APCs, focus on where each cell type operates, which MHC class it uses, and which lymphocytes it activates. These details determine whether you're looking at a cytotoxic response, a helper response, or antibody production. Don't just memorize cell names. Know what immunological principle each APC demonstrates and how they work together to create a coordinated defense.

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Professional Antigen Presenters: The Dendritic Cell Family

Professional APCs exist primarily to capture antigens in peripheral tissues and deliver them to secondary lymphoid organs where T cell activation occurs. They express high levels of both MHC class I and class II molecules along with co-stimulatory molecules (CD80/CD86), making them uniquely efficient at priming naïve T cells.

Dendritic Cells

• Primary activators of naïve T cells. Dendritic cells (DCs) are the only APCs that reliably initiate primary adaptive immune responses. Other APCs can restimulate previously activated T cells, but DCs are required to get things started.
• Cross-presentation capability allows them to load exogenous (extracellular) antigens onto MHC class I molecules. Normally, MHC class I only presents endogenous (intracellular) peptides. Cross-presentation breaks this rule, enabling CD8+ cytotoxic T lymphocyte (CTL) responses against viruses and tumors that don't directly infect DCs.
• Migration from tissues to lymph nodes is triggered by pathogen encounter. Immature DCs in peripheral tissues are highly phagocytic. Once they capture antigen and receive danger signals (via pattern recognition receptors like TLRs), they mature, upregulate CCR7, and migrate via afferent lymphatics to the T cell zones of draining lymph nodes. During this transit, they process antigen and upregulate MHC and co-stimulatory molecules.

Langerhans Cells

• Skin-resident dendritic cells that form an immunological sentinel network within the epidermis, positioned at a key barrier surface.
• Express Langerin (CD207), a C-type lectin receptor that forms characteristic Birbeck granules on electron microscopy. This marker distinguishes them from other dendritic cell subsets in the skin (such as dermal DCs).
• Balance immunity and tolerance by determining whether skin-derived antigens trigger immune responses or are tolerated. In the absence of danger signals, Langerhans cells that migrate to lymph nodes tend to present self-antigens in a tolerogenic manner, helping prevent autoimmune or allergic reactions at the skin surface.

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Dual-Function APCs: Innate Meets Adaptive

These cells serve as APCs while also performing other critical immune functions. Their antigen presentation typically occurs in the context of their primary roles: phagocytosis for macrophages and antigen-specific capture for B cells. Both express MHC class II constitutively or upon activation, but neither is as effective as dendritic cells at activating naïve T cells. They primarily restimulate already-primed effector or memory T cells.

Macrophages

• Phagocytic APCs that engulf pathogens, dead cells, and debris, then present processed peptide fragments via MHC class II to CD4+ T helper cells. This interaction goes both ways: the T helper cell, in turn, activates the macrophage (via CD40L and IFN-γ), enhancing its microbicidal capacity.
• Cytokine production polarizes T cell responses. For example, macrophage-derived IL-12 drives Th1 differentiation (promoting cell-mediated immunity), while IL-10 promotes regulatory T cell responses and dampens inflammation. The cytokine profile a macrophage produces depends on the signals it receives.
• Functional plasticity allows macrophages to shift between pro-inflammatory (classically activated / M1) and tissue-repair (alternatively activated / M2) phenotypes depending on microenvironment signals. M1 macrophages are driven by IFN-γ and TLR ligands; M2 macrophages are driven by IL-4 and IL-13. This is a spectrum rather than a strict binary.

B Cells

• Antigen-specific APCs that use their B cell receptor (BCR) to capture and internalize specific antigens with very high efficiency. Because the BCR binds a particular epitope, B cells can concentrate and present antigens present at extremely low concentrations, something macrophages and DCs cannot do as effectively.
• Cognate T-B interaction is central to T-dependent antibody responses. The B cell internalizes antigen via its BCR, processes it, and presents peptide fragments on MHC class II to a CD4+ T follicular helper (Tfh) cell that recognizes the same antigen complex. This interaction (requiring CD40-CD40L co-stimulation) is required for germinal center entry, somatic hypermutation, affinity maturation, and class-switch recombination.
• Dual output: activated B cells differentiate into antibody-secreting plasma cells for immediate defense and memory B cells for rapid recall responses upon re-exposure.

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Specialized B Cell Support: Follicular Dendritic Cells

These cells occupy a unique niche in the immune system. They support B cell responses but function completely differently from classical APCs.

Follicular Dendritic Cells

• Do NOT express MHC class II and are not derived from bone marrow hematopoietic precursors. They are stromal (mesenchymal) cells found in B cell follicles of secondary lymphoid organs. Despite the name, they are unrelated to conventional dendritic cells.
• Display intact, unprocessed antigens on their surface in the form of immune complexes (antigen-antibody or antigen-complement complexes), retained via complement receptors (CR1/CD35, CR2/CD21) and Fc receptors. B cells in the germinal center sample these native antigens through their BCRs, testing whether their receptor (after somatic hypermutation) still binds effectively.
• Essential for germinal center reactions. FDCs provide the long-lived antigen depots that drive affinity maturation: B cells that have undergone somatic hypermutation compete for limited antigen on FDC surfaces. Only B cells with the highest-affinity BCRs successfully capture antigen, receive survival signals from Tfh cells, and avoid apoptosis. This selection process ensures that the antibody response improves over time.

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Quick Reference Table

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Self-Check Questions

1. Which two APC types are capable of activating naïve T cells, and what shared features make this possible?

2. A patient has defective cross-presentation. Which APC function is impaired, and what type of immune response would be compromised?

3. Compare and contrast how macrophages and B cells capture antigens for presentation. What does this difference mean for the specificity of their APC function?

4. Why are follicular dendritic cells considered distinct from other APCs, even though they display antigens to lymphocytes?

5. Trace the pathway from a skin infection to T cell activation. Which APCs are involved, and what is the sequence of events?