What is the primary function of antigen-presenting cells (APCs) in the immune response?

• A. Produce antibodies
• B. Present antigens to T cells
• C. Phagocytose bacteria
• D. Secrete histamine

Answer: (B)

Antigen-presenting cells serve as the link between recognizing a threat and starting the targeted immune response by taking up a pathogen, processing its proteins, and displaying peptide fragments on MHC class II molecules to CD4+ helper T cells. This presentation is the signal that activates helper T cells, which then coordinate the rest of the adaptive response—helping B cells make antibodies and aiding cytotoxic T cells in killing infected cells. Dendritic cells are especially potent at this, with macrophages and B cells also acting as APCs in different contexts.

Producing antibodies is the job of plasma cells derived from B cells, not the APCs themselves. While APCs can perform phagocytosis, the defining, primary role in initiating adaptive immunity is presenting antigens to T cells. Secreting histamine is a function of mast cells and basophils, not APCs.