Question

Which of the following is the most potent activator of T cells?

• A. B cells
• B. Follicular dendritic cells
• C. Mature dendritic cells
• D. Macrophages

Hint:

Think of the professional APC that uniquely primes naive T cells with both peptide-MHC and B7 co-stimulation.

Answer 1

The Correct Option is C

Step 1: Identify the concept. T-cell activation requires a professional antigen-presenting cell (APC) that displays processed peptide on MHC molecules together with high levels of co-stimulatory signals.
Step 2: Compare the candidates. B cells, follicular dendritic cells (which trap antigen for B cells in germinal centres), and macrophages can all present antigen, but they express lower co-stimulatory molecule density.
Step 3: Recognise the key player. Mature (activated) dendritic cells express the highest levels of MHC class II and the co-stimulatory molecules CD80 (B7-1) and CD86 (B7-2), which engage CD28 on the naive T cell.
Step 4: Conclude. Because mature dendritic cells uniquely provide both robust signal 1 (peptide-MHC) and signal 2 (co-stimulation), they are the most potent activators of naive T cells, making them the principal initiators of the primary immune response.
Answer: Mature dendritic cells (option C).