Cells that can act as antigen-presenting cells for exogenous antigens include which of the following?

• A. All nucleated cells
• B. Endothelial cells
• C. B lymphocytes
• D. T lymphocytes

Answer: (C)

Antigen presentation to CD4+ T helper cells relies on MHC class II molecules and occurs when cells specializing in capturing extracellular (exogenous) antigens process them in endosomes and display peptide fragments to T cells. B lymphocytes fit this role because they use their surface immunoglobulin as a specific receptor to bind and internalize antigen. After internalization, the antigen is processed and the resulting peptides are loaded onto MHC II and presented on the B cell surface to CD4+ T cells. This interaction not only activates the helper T cells but also provides signals that promote B cell activation and antibody production, forming a key link between innate-like capture of antigen and the adaptive humoral response.

All nucleated cells primarily present endogenous antigens on MHC I to CD8+ T cells, so they are not the specialized route for exogenous antigen presentation. Endothelial cells can present antigens under certain inflammatory conditions but are not the main professional APCs for exogenous antigens. T lymphocytes do not present antigen to other T cells; they respond to antigen presented by APCs.

So the best answer is that B lymphocytes can act as antigen-presenting cells for exogenous antigens.