Antigen presentation by macrophages Human macrophages are professional antigen-presenting cells. Through which molecules do they present processed peptide antigens to T cells?

Introduction / Context:
Antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells process antigens and present peptide fragments to T cells. The molecular platform for presentation in humans is HLA (the human MHC). Understanding these interactions underlies vaccine design and immunotherapy.

Given Data / Assumptions:

• Macrophages present peptides derived from phagocytosed or endocytosed antigens.
• CD4 T cells recognize peptides bound to class II HLA; CD8 recognize peptides bound to class I HLA.
• Accessory molecules and cytokines support activation but are not the presenting platform.

Concept / Approach:

HLA class II molecules (HLA-DP, -DQ, -DR) present exogenous peptides to CD4 T helper cells after processing in endosomal compartments. HLA class I (HLA-A, -B, -C) presents endogenous peptides to CD8 cytotoxic T cells. Macrophages express both classes (especially after activation) and prime/activate T cells in coordination with costimulatory signals (e.g., CD80/CD86) and cytokines.

Step-by-Step Solution:

Verification / Alternative check:

Flow cytometry shows upregulation of HLA-DR on activated macrophages; functional assays demonstrate T-cell activation dependent on peptide–HLA complexes.

Why Other Options Are Wrong:

• Actin: cytoskeletal protein; not involved in antigen presentation on the cell surface.
• Interleukin: soluble mediators; they modulate responses but do not present peptides.
• CD8: coreceptor on T cells; recognizes HLA class I, not a presenter.
• TLRs: pattern-recognition receptors; initiate innate signaling, not peptide display.

Common Pitfalls:

• Confusing costimulation and cytokines with the peptide–MHC presentation mechanism.

Final Answer:

HLA (MHC molecules)