Which of the following statements about immune response are CORRECT?
[P] T cells are activated by antigen-presenting cells
[Q] Foreign peptides are not presented to helper T cells by Class II MHC proteins
[R] Dendritic cells are referred to as professional antigen-presenting cells
P and R only
P and Q only
Q and R only
P, Q and R
ANSWER
The answer is (A): P and R only.
Official answer: ASource: Official answer key, GATE 2020 BT (IIT Delhi)
Statement P: a naive T cell is activated only when its T-cell receptor recognises a peptide displayed on an MHC molecule of an antigen-presenting cell (APC), together with co-stimulation (CD28 on the T cell binding B7 on the APC). So P is correct.
Statement Q: helper T cells carry CD4, which binds the Class II MHC molecule. Foreign (exogenous) peptides taken up and processed by APCs are loaded onto Class II MHC and presented to CD4 helper T cells. Q says they are NOT presented in this way, so Q is false.
Statement R: dendritic cells, macrophages and B cells are the professional antigen-presenting cells. Dendritic cells are the most potent of these because they express high levels of Class II MHC and co-stimulatory molecules and can activate naive T cells. So R is correct.
P and R are correct and Q is wrong, so the answer is option (A). Options (B), (C) and (D) each include Q, so they are wrong.
KEY IDEA
Class II MHC presents exogenous peptides to CD4+ helper T cells. Professional APCs (dendritic cells, macrophages, B cells) activate T cells, and dendritic cells are the most potent.
WATCH OUT FOR
Reading Q as true by mixing up the two MHC classes. Class I presents endogenous peptides to CD8+ cytotoxic T cells, while Class II presents exogenous peptides to CD4+ helper T cells. Because Q is negated, it is false.
From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 7 Adaptive immunity questions, with notes
