Q.No. 28 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

• (A) P and R only
• (B) P and Q only
• (C) Q and R only
• (D) P, Q and R

  Correct Answer: (A) P and R only

  T cells require activation by antigen-presenting cells (APCs) displaying foreign peptides on MHC molecules, while dendritic cells serve as professional APCs due to their high efficiency in this process. Statement Q is incorrect because foreign peptides are indeed presented to helper T cells via Class II MHC proteins.

  Statement Analysis

  P: T cells are activated by antigen-presenting cells
  This holds true as APCs process and present antigens on MHC molecules to T cell receptors, delivering signals for activation alongside costimulatory molecules.​

  Q: Foreign peptides are not presented to helper T cells by Class II MHC proteins
  This is false; Class II MHC proteins specifically bind and display foreign peptides to CD4+ helper T cells, enabling their role in coordinating immune responses.​

  R: Dendritic cells are referred to as professional antigen-presenting cells
  Dendritic cells qualify as professional APCs for their constitutive expression of MHC Class II, costimulatory molecules, and potent ability to prime naive T cells.​

  In immune response mechanisms critical for exams like IIT JAM Biotechnology, understanding T cells activation by antigen-presenting cells forms the foundation of adaptive immunity. This MCQ evaluates knowledge of antigen-presenting cells (APCs), MHC Class II presentation to helper T cells, and dendritic cells as professional APCs.

  Key Immune Response Concepts

  • T Cells Activation Process: APCs like dendritic cells, macrophages, and B cells process pathogens, loading peptide fragments onto MHC molecules for T cell receptor recognition, followed by costimulatory signals.​

  • MHC Class II Role: Contrasting Class I (for cytotoxic T cells), Class II MHC displays extracellular pathogen peptides to CD4+ helper T cells, driving cytokine release and B cell activation.​

  • Professional APCs Defined: Dendritic cells lead due to high MHC II and B7 expression, efficiently activating naive T cells; macrophages and B cells activate under specific conditions.​

  Why Option (A) Prevails

  P and R align with established immunology: APCs universally activate T cells, and dendritic cells exemplify professional status. Q misrepresents MHC II function, a common exam trap testing pathway specificity.​

  This breakdown equips competitive exam aspirants with precise recall for immune response questions on T cells activated by antigen-presenting cells.