T Cells and Cell-Mediated Immunity

Q1 — Sample Band 6 response (7 marks), annotated

In cell-mediated immunity, cytotoxic T cells (CD8+) recognise and destroy infected or abnormal host cells by binding their T cell receptor (TCR) to a specific antigen peptide displayed on an MHC class I molecule on the target cell’s surface. MHC class I is expressed on all nucleated body cells and continuously displays fragments of internally produced proteins — in a healthy cell, only self-peptides appear; in an infected or cancerous cell, foreign or aberrant peptides flag the cell for CTL destruction. This recognition is the central mechanism of immune surveillance against intracellular threats. [1 — MHC class I role in CTL recognition explained]

By downregulating MHC class I, a tumour cell becomes “invisible” to cytotoxic T cells: even though the cell is producing tumour-specific peptides, there is no MHC class I molecule on which to display them. The CTL TCR has nothing to bind, so no activation signal is generated and no killing occurs. This is precisely why checkpoint inhibitor therapy (which removes brakes on CTL activity) is less effective in MHC-class-I-low tumours: you can “release the brakes” on CTLs, but if the CTL still has nothing to recognise, it cannot kill. [1 — explains precisely why CTL cannot kill without MHC I, including the checkpoint-therapy connection from the stimulus]

However, a separate innate-adjacent mechanism — Natural Killer (NK) cells — operates on an opposite logic: NK cells kill cells that are “missing self.” Normal healthy cells express MHC class I, and this inhibits NK cell activation; if MHC class I is absent or reduced, the inhibitory signal is gone and NK cells receive a net activating signal that triggers release of perforin and granzymes, killing the target cell. Thus the same MHC class I loss that hides a tumour from CTLs actually flags it to NK cells. [1 — identifies NK “missing self” as the alternative mechanism; explains why MHC I absence activates it]

The tumour therefore faces an evolutionary trade-off: retaining MHC class I makes it visible to CTLs (highly specific, adaptive killing); losing MHC class I evades CTLs but exposes it to NK cells (less specific, innate killing). In the short term, evading the adaptive CTL response is the stronger selective advantage because the adaptive response is far more powerful and specific when activated — a single antigen-specific CTL clone can kill thousands of identical target cells and be sustained over weeks. NK responses, while valuable, are less clonally amplified and do not produce the same long-lived memory. Clinical data from Merkel cell carcinoma cases support this: some patients with MHC-class-I-low tumours do clear them (via NK activity), but response rates remain substantially lower overall than in MHC-class-I-normal tumours, confirming CTL evasion is the dominant short-term selective advantage. [1 — evaluates the evolutionary trade-off; CTL vs NK pressure] [1 — uses stimulus detail (Merkel cell carcinoma / checkpoint inhibitor data) in the evaluation]

Justified conclusion: Downregulating MHC class I is an effective short-term immune evasion strategy because it neutralises the most powerful arm of specific adaptive immunity (CTL killing). The NK “missing self” response is a real but less potent counter-pressure, which is why tumours frequently select for MHC class I loss and why checkpoint inhibitors targeting CTLs fail in a subset of these patients. [1 — explicit evaluative conclusion tied to both mechanisms] [1 — command-verb compliance: analysis + evaluation + justified conclusion]

Marking criteria:

• 1 mark — Explains the normal role of MHC class I in CTL recognition: all nucleated cells express it; viral/tumour peptides appear on it; CTL TCR binds the peptide–MHC I complex to trigger killing.
• 1 mark — Explains precisely why reduced MHC class I prevents CTL killing: no MHC I = no TCR binding signal = no CTL activation, even if tumour peptides are present. Connects this to failed checkpoint inhibitor response.
• 1 mark — Identifies NK cells as the alternative mechanism and explains the “missing self” logic: absence of MHC class I removes the inhibitory signal on NK cells, triggering killing via perforin/granzymes.
• 1 mark — Evaluates the trade-off: CTL response is more powerful/specific/sustained, so evading it provides greater short-term advantage; NK response is real but less amplified and less targeted.
• 1 mark — Uses at least one specific detail from the stimulus (Merkel cell carcinoma, checkpoint inhibitor failure, or named cancer type) to support the evaluation.
• 1 mark — Reaches a justified conclusion that correctly weighs both selective pressures and explains which is dominant and why.
• 1 mark — Response is structured as a genuine analysis + evaluation (not simply a description); uses HSC-appropriate scientific terminology throughout (MHC class I, TCR, perforin, granzymes, apoptosis, NK cells, clonal expansion).

Q2 — Sample Band 6 response (8 marks), annotated

The claim contains three distinct sub-claims, each requiring evaluation.

Claim 1: “T helper cells are unnecessary — the innate system handles most infections.” This is false. While the innate system provides the first-line response (phagocytosis, NK cells, inflammation), it does not eliminate all pathogens and does not generate immunological memory. The adaptive immune system — coordinated by T helper cells — is essential for clearing complex intracellular infections and for long-term immunity. The statement that T helper cells are “unnecessary” is refuted by the clinical reality of AIDS: patients with near-zero CD4+ counts cannot clear pathogens (like Pneumocystis jirovecii) that a healthy immune system eliminates routinely, demonstrating that T helper cells are indispensable, not redundant. [1 — refutes Claim 1 with mechanism and clinical evidence]

Claim 2: “Cytotoxic T cells can activate themselves using MHC class I signals alone.” This is incorrect. Cytotoxic T cells require two signals for full activation: (1) TCR binding to the antigen peptide displayed on MHC class I, and (2) a co-stimulatory cytokine signal from T helper cells, principally IL-2. TCR binding alone produces an incomplete, anergic response. Without IL-2, CTLs cannot undergo the clonal expansion needed to produce a sufficiently large population of killers to clear an infection. This “two-signal rule” is the reason CTL responses fail in HIV patients even though CTLs and MHC class I still exist. [1 — refutes Claim 2 with the two-signal rule and IL-2 requirement]

Claim 3: “B cells produce antibodies without additional co-stimulation.” Partially correct: B cells can produce some antibodies without T helper signals (T-independent antigens, generating short-lived IgM responses). However, for a full, high-affinity IgG antibody response with class switching and memory B cell formation, B cells require a co-stimulatory signal from T helper cells (CD40L binding CD40 on the B cell, plus IL-4 and IL-21). Without this, only weak, short-lived IgM responses are generated — insufficient for long-term protection. The claim is therefore misleading. [1 — evaluates Claim 3 as partially correct; distinguishes T-independent IgM from T-dependent IgG]

Claim 4: “HIV destroying T helper cells would only affect one narrow pathway.” This is false. T helper cells are the central coordinator of the adaptive immune response. Their loss simultaneously impairs: B cell activation and antibody class switching (humoral immunity); CTL clonal expansion (cell-mediated immunity); macrophage activation (bridges to innate immunity); and recruitment of NK cells, neutrophils, and eosinophils. HIV’s ability to collapse multiple immune pathways by destroying a single cell type is precisely what makes it so devastating — it is not a narrow single-pathway effect. [1 — refutes Claim 4; correctly identifies T helper cells as multi-arm coordinators]

Overall evaluation: The claim is largely false; only Claim 3 has a partial basis in T-independent antibody production. The overall claim fundamentally misrepresents T helper cells as peripheral players when they are the central coordinators without whom neither humoral nor cell-mediated adaptive immunity functions properly. [1 — overall evaluative judgement]

HIV application: In untreated HIV infection, progressive CD4+ T helper cell loss below 200 cells/µL collapses both arms simultaneously — B cells cannot produce high-affinity IgG (inadequate antibody response) and CTLs cannot expand (inadequate cell-killing response) — resulting in susceptibility to opportunistic pathogens such as Pneumocystis jirovecii, Toxoplasma gondii, and Cryptococcal meningitis. [1 — applies consequences specifically to HIV + both arms + named pathogens]

Accurate reformulation: “T helper cells (CD4+) are the central coordinators of adaptive immunity. They are activated by antigen-presenting cells displaying antigen on MHC class II, and they provide essential cytokine signals (including IL-2 and CD40L co-stimulation) required for full B cell activation and IgG antibody production, for cytotoxic T cell clonal expansion, and for macrophage activation. Their loss — as occurs in untreated HIV infection — simultaneously impairs both humoral and cell-mediated immunity, leaving the host unable to mount effective adaptive responses to most pathogens.” [1 — accurate reformulation using precise lesson terminology] [1 — command-verb compliance: systematic evaluation with judgement and reformulation]

Marking criteria:

• 1 mark — Refutes Claim 1 (“unnecessary”) with reference to clinical consequences of T helper loss (AIDS / opportunistic infections).
• 1 mark — Refutes Claim 2 (“self-activation via MHC I alone”) with the two-signal rule: CTLs require both MHC I recognition AND IL-2 from T helper cells.
• 1 mark — Evaluates Claim 3 as partially correct: B cells can produce T-independent IgM but require T helper co-stimulation for IgG class switching and memory.
• 1 mark — Refutes Claim 4 (“narrow pathway”) by correctly listing the multiple downstream targets of T helper signals (CTLs, B cells, macrophages, innate cells).
• 1 mark — Provides an overall evaluative judgement (e.g. “largely false, only partial basis in T-independent antibody production”).
• 1 mark — Applies consequences specifically to HIV infection, naming impairment of both humoral and cell-mediated immunity and at least one opportunistic pathogen.
• 1 mark — Provides a biologically accurate reformulation of the claim in precise lesson terminology.
• 1 mark — Response demonstrates systematic evaluation (not simple description); uses HSC-appropriate terminology throughout (MHC class II, CD4+, IL-2, clonal expansion, class switching, co-stimulation).