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Antigen presenting cells (APCs) are the critical link between your innate and adaptive immune systems—and understanding how they function is essential for exam success. You're being tested on 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 your immune system mounts an attack, tolerates a substance, or forms long-lasting memory.
When you study 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.
These cells are the professional APCs—their primary job is capturing antigens in peripheral tissues and delivering them to lymphoid organs where T cell activation occurs. They express high levels of MHC molecules and co-stimulatory signals, making them uniquely efficient at priming naïve T cells.
Compare: Dendritic cells vs. Langerhans cells—both are professional APCs that migrate to lymph nodes and activate naïve T cells, but Langerhans cells are tissue-specific (skin/mucosa) while conventional dendritic cells are distributed throughout most tissues. If an FRQ asks about cutaneous immunity or barrier defense, Langerhans cells are your go-to example.
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, antibody production for B cells.
Compare: Macrophages vs. B cells as APCs—macrophages are nonspecific phagocytes that present whatever they engulf, while B cells selectively capture antigens matching their BCR. This means B cells present antigens at much lower concentrations but only their specific antigen, making them essential for T-dependent antibody responses.
These cells occupy a unique niche in the immune system—they support B cell responses but function completely differently from other APCs.
Compare: Conventional dendritic cells vs. follicular dendritic cells—despite similar names, they serve completely different functions. Conventional DCs activate T cells via MHC-peptide complexes, while FDCs support B cell maturation by displaying unprocessed antigens. Don't confuse them on exams—FDCs are stromal cells, not bone marrow-derived APCs.
| Concept | Best Examples |
|---|---|
| Naïve T cell activation | Dendritic cells, Langerhans cells |
| MHC class II presentation | Dendritic cells, Macrophages, B cells |
| Cross-presentation (MHC class I) | Dendritic cells |
| T-dependent B cell activation | B cells (as APCs to T helper cells) |
| Germinal center support | Follicular dendritic cells |
| Barrier/tissue immunity | Langerhans cells, Macrophages |
| Cytokine-mediated T cell polarization | Macrophages, Dendritic cells |
| Antigen-specific capture | B cells (via BCR) |
Which two APC types are capable of activating naïve T cells, and what shared features make this possible?
A patient has defective cross-presentation. Which APC function is impaired, and what type of immune response would be compromised?
Compare and contrast how macrophages and B cells capture antigens for presentation—what does this difference mean for the specificity of their APC function?
Why are follicular dendritic cells considered distinct from other APCs, even though they display antigens to lymphocytes?
An FRQ asks you to trace the pathway from skin infection to T cell activation. Which APCs are involved, and what is the sequence of events?