Antigen presentation – identifying non-APCs Which of the following is NOT considered a professional antigen-presenting cell (APC) capable of priming naïve T cells under physiologic conditions?

Introduction / Context:
Professional antigen-presenting cells (APCs) process antigens and present peptide–MHC to T cells with necessary costimulation to initiate adaptive immunity. Knowing which cells are true professional APCs is crucial for understanding vaccine responses and immune tolerance.

Given Data / Assumptions:

• Professional APCs express high levels of MHC and costimulatory molecules (e.g., CD80/CD86).
• Candidates listed include B cells, dendritic cells, PMNs, and macrophages.
• Task is to identify the non-APC among them.

Concept / Approach:

Dendritic cells, macrophages, and B cells are classic professional APCs. They can activate naïve T cells (especially dendritic cells) and present antigens effectively. Polymorphonuclear leukocytes (neutrophils/PMNs) are innate effector phagocytes; while they can internalize pathogens, they generally lack the sustained MHC II and costimulatory expression required for priming naïve T cells.

Step-by-Step Solution:

Verification / Alternative check:

Experimental T-cell priming assays show robust activation by dendritic cells and, to lesser degrees, macrophages and B cells; PMNs do not reliably prime naïve T cells despite limited antigen presentation capacities in certain contexts.

Why Other Options Are Wrong:

• B cell: can present antigen via BCR-mediated uptake to CD4 T cells.
• Dendritic cell: most potent professional APC.
• Macrophage: presents to effector/memory T cells and provides cytokines.
• “All of the above”: incorrect because several listed are APCs.

Common Pitfalls:

• Equating phagocytosis with professional antigen presentation; costimulation is required for priming.

Final Answer:

Polymorphonuclear leukocyte (PMN)