You’re cruising through a Q-bank and a deceptively simple stem shows up: “MHC I vs MHC II.” Easy… until every answer choice looks almost right. The trick isn’t memorizing one-liners—it’s knowing what each option is trying to test, and why it’s wrong in this patient.
Tag: Immunology > Innate & Adaptive Immunity
The Clinical Vignette (Q-bank style)
A 22-year-old man presents with a 5-day history of fever, malaise, and sore throat. Exam shows posterior cervical lymphadenopathy and mild splenomegaly. Labs reveal atypical lymphocytes. A heterophile antibody test is positive. His symptoms are caused by infection of B cells with Epstein–Barr virus (EBV). The immune response responsible for controlling this infection most directly involves which of the following?
A. CD8+ T cells recognizing viral peptides presented on MHC I
B. CD4+ T cells recognizing viral peptides presented on MHC II
C. Natural killer (NK) cells recognizing decreased MHC II expression
D. B cells producing IgM against viral capsid antigen after MHC I presentation
E. Neutrophils killing infected B cells via complement receptor (CR1)-mediated phagocytosis
Step-by-Step: What the Stem is Actually Asking
This is infectious mononucleosis (EBV): fever, posterior cervical LAD, splenomegaly, atypical lymphocytes, +heterophile.
Key immunology translation:
- EBV infects B cells but becomes an intracellular pathogen inside them.
- Control of intracellular viral infection is primarily via CD8+ cytotoxic T cells.
- CD8+ T cells recognize antigen on MHC I, which is expressed on all nucleated cells (including infected B cells).
Correct Answer: A. CD8+ T cells recognizing viral peptides presented on MHC I
Why it’s right (high-yield chain)
- Viruses → intracellular → MHC I → CD8+
- In EBV mono, the “atypical lymphocytes” on smear are reactive CD8+ T cells (Downey cells) responding to infected B cells.
- MHC I presentation pathway (board-favorite):
- Endogenous proteins → proteasome → peptides transported by TAP into RER → loaded onto MHC I → cell surface → recognized by CD8+ T cells.
Rapid High-Yield Map: MHC I vs MHC II
| Feature | MHC I | MHC II |
|---|---|---|
| Where expressed | All nucleated cells | APCs (dendritic cells, macrophages, B cells) |
| What it presents | Endogenous (intracellular: viral, tumor) | Exogenous (extracellular: bacteria, toxins) |
| Recognized by | CD8+ cytotoxic T cells | CD4+ helper T cells |
| Loading location | RER (via TAP) | Endosomes/lysosomes (via invariant chain/CLIP; HLA-DM swaps) |
| Clinical buzz | Viral killing, tumor surveillance | Helper activation, antibody class switching, macrophage activation |
Now the Real Value: Why Each Distractor is Wrong
B. CD4+ T cells recognizing viral peptides presented on MHC II
Tempting because EBV infects B cells, which are professional APCs and do express MHC II. But the stem asks what response most directly controls the infection.
Why it’s wrong here:
- CD4+ T cells are critical for orchestrating immunity (e.g., helping B cells class-switch, activating macrophages), but direct killing of infected cells is the job of CD8+ T cells.
- EBV is an intracellular infection; primary clearance depends on recognizing infected cells via MHC I.
When CD4+ / MHC II would be the best answer:
- Extracellular pathogens (many bacteria, parasites)
- Situations emphasizing macrophage activation (Th1) or class switching (Tfh/Th2), not direct cytotoxic clearance.
C. Natural killer (NK) cells recognizing decreased MHC II expression
This option is designed to see if you know the “missing self” concept—but it misstates which MHC matters.
Why it’s wrong:
- NK cells respond to decreased MHC I, not MHC II.
- Many viruses downregulate MHC I to evade CD8+ T cells; NK cells counter this by killing cells that are “missing” MHC I inhibitory signaling.
High-yield NK cell rules:
- NK cells kill when:
- Low MHC I (loss of inhibitory signal), and/or
- Stress ligands up (activating signal), and/or
- Antibody-coated targets via CD16 (ADCC)
- NK cells are part of innate immunity, important early—especially before robust T-cell responses mature.
D. B cells producing IgM against viral capsid antigen after MHC I presentation
This distractor mixes a real fact (IgM in acute infection) with the wrong antigen presentation logic.
Why it’s wrong:
- B cells don’t “receive” antigen via MHC I to make antibody. MHC is for presenting antigen to T cells, not for B cells to detect antigen.
- Antibody responses to protein antigens typically need CD4+ T-cell help:
- B cell binds antigen via BCR → internalizes it → presents peptides on MHC II to CD4+ Tfh → class switching + affinity maturation.
High-yield EBV antibody pearls:
- Heterophile antibodies (Monospot) are usually IgM that agglutinate animal RBCs.
- Anti–viral capsid antigen (VCA) IgM suggests acute infection; VCA IgG persists.
E. Neutrophils killing infected B cells via complement receptor (CR1)-mediated phagocytosis
This option tests whether you can separate extracellular opsonization from intracellular viral control.
Why it’s wrong:
- Neutrophils excel at phagocytosing extracellular bacteria/fungi, especially when opsonized (IgG, C3b).
- Complement receptor CR1 binds C3b (opsonin), supporting phagocytosis—but infected host cells aren’t typically cleared by neutrophil phagocytosis in a targeted way.
When complement/neutrophils are high-yield:
- Encapsulated bacteria (opsonization is crucial)
- Acute bacterial infections with neutrophilia
- Complement deficiencies (e.g., C5–C9 and Neisseria)
The “Answer Choice Decoder”: What This Question Was Testing
If you see virus-infected cells (or tumor cells), your reflex should be:
- Intracellular antigen → MHC I
- MHC I → CD8+ cytotoxic T cells
- CD8+ → perforin/granzyme or Fas–FasL killing
- If MHC I is downregulated → NK cells step in
If you see extracellular bacteria/toxins, your reflex should be:
- Exogenous antigen → MHC II
- MHC II → CD4+ helper T cells
- Helper functions:
- Th1: macrophage activation (intracellular bacteria)
- Th2: eosinophils/IgE (helminths, allergy)
- Th17: neutrophil recruitment (extracellular bacteria/fungi)
- Tfh: B-cell help (germinal centers, class switching)
Ultra–High-Yield Boards Facts (Quick Hits)
- MHC I: “1 type of T cell (CD8) sees it” (memory hook), but more importantly: endogenous → CD8.
- MHC II: on APCs only (dendritic cells are best at priming naïve T cells).
- TAP deficiency → impaired MHC I expression → recurrent viral infections; low CD8 counts.
- Bare lymphocyte syndrome type II (MHC II deficiency) → impaired CD4 activation → severe immunodeficiency.
- EBV infects B cells via CD21 (CR2); mono shows reactive CD8+ T cells.
Takeaway: How to Lock This Down in 10 Seconds on Test Day
Ask yourself two questions:
-
Where is the pathogen living?
- Inside cells → MHC I → CD8
- Outside cells / phagocytosed → MHC II → CD4
-
What effector mechanism clears it?
- Kill infected cells → CD8
- Coordinate macrophages/antibodies → CD4
Once you do that, the distractors stop looking “kinda right”—they become obviously mismatched to the stem.