Which of the following cells begins the defensive response against a specific antigen?

Question

BlackTom AI · Accepted Answer

Helper T cells (CD4+ T lymphocytes) recognize antigenic peptides presented on MHC class II molecules and serve as the primary initiators and coordinators of the antigen-specific (adaptive) immune response.

1. Relevant knowledge points and concepts
- Innate vs adaptive immunity: innate responses are nonspecific; adaptive responses are antigen-specific and involve lymphocytes.
- Antigen-presenting cells (APCs: dendritic cells, macrophages, B cells) process pathogens and display peptide fragments on MHC II.
- Helper T cells (CD4+) use their T-cell receptor (TCR) to recognize peptide–MHC II complexes, become activated, proliferate (clonal expansion), and secrete cytokines.
- Cytokines from helper T cells activate B cells (antibody production), cytotoxic T cells (cell killing), and macrophages; they also promote memory cell formation.

2. Step-by-step reasoning
- A pathogen or its antigen is taken up and processed by an APC.
- The APC displays antigenic peptides on MHC II and migrates to lymphoid tissue.
- Naive CD4+ T cells survey peptide–MHC II complexes; specific TCR binding plus costimulatory signals activates the helper T cell.
- Activated helper T cells undergo clonal expansion and release cytokines.
- Those cytokines direct B cells to class-switch and produce high-affinity antibodies, stimulate CD8+ T cells, and enhance macrophage microbicidal activity—thus launching the antigen-specific defense.

3. Calculations
- None required for this conceptual question.

4. Why the correct answer is correct
- Helper T cells are the first adaptive immune cells to respond specifically to presented antigen and are required to orchestrate and amplify the specific immune response, making them the correct initiators of defense against a particular antigen.

5. Why other options are incorrect (brief)
- B cells produce antibodies but often require helper T cell signals for full activation in T-dependent responses.
- Cytotoxic (CD8+) T cells kill infected cells but typically require helper T cell–mediated priming.
- APCs (dendritic cells/macrophages) present antigen but do not themselves enact the full adaptive, antigen-specific program; they trigger helper T cells.
- Natural killer cells are innate and not antigen-specific.

6. Conclusion
This explanation confirms that Helper T cells are the correct answer.

Which of the following cells begins the defensive response against a specific antigen?Single choice

Similar Questions

Question at position 22 Superantigens differ from regular peptide antigens because theycan result in toxic shock.bind outside the peptide-binding groove.interact with the V β domain of the TCR.All of the answers are correct.activate a larger proportion of T cells.

Question at position 59 What happens when a T cell recognizes a processed foreign antigen in association with a self-MHC molecule?It undergoes clonal expansionIt triggers B cell activation directlyIt immediately produces antibodiesIt is eliminated by apoptosis

Question at position 33 The activation of T cells by antigen-presenting cells is a key step in the adaptive immune response.TrueFalse

Choose the correct outcome that matches these two scenarios depicted:

A benefit of T cells requiring co-stimulation is:

More Practical Tools for Students Powered by AI Study Helper

Homework AI Solver

Stylized AI Paper Writer

Plagiarism Checker Assistant

Citation AI Academic Writing Tool

In-Class Translation Assistant

AI Note Generator

AI Quiz Answers

Past Exam Questions from University Test Bank

Smart Practice Assistant

Adaptive Practice

Making Your Study Simpler