HSC Exam Practice

Sample response. Cell-mediated immunity is the branch of adaptive immunity in which T lymphocytes (specifically cytotoxic T cells) identify and destroy infected host cells, cancer cells, and transplanted cells that display foreign or aberrant antigen on their surface.

Marking notes. 1 mark for identifying T cells / T lymphocytes as the effectors (not antibodies). 1 mark for specifying the targets (infected host cells / abnormal cells) or the distinction from humoral immunity (which targets extracellular pathogens). Accept “a form of adaptive immunity involving T cells targeting cells that display foreign antigen.”

Sample response. MHC class I molecules are found on all nucleated body cells. They display internally produced peptides (including viral peptides in infected cells) to cytotoxic T cells (CD8+); when a CTL’s T cell receptor binds the peptide on MHC class I, it triggers killing of the cell. MHC class II molecules are found only on professional antigen-presenting cells (dendritic cells, macrophages, and B cells). They display fragments of extracellular antigen to T helper cells (CD4+), triggering their activation and cytokine release.

Marking notes. 1 mark — MHC class I location: all nucleated body cells. 1 mark — MHC class I function: presents internal/viral peptides to cytotoxic T cells (CD8+). 1 mark — MHC class II location: professional antigen-presenting cells only. 1 mark — MHC class II function: presents extracellular antigen to T helper cells (CD4+).

Sample response. The cytotoxic T cell (CD8+) recognises a viral peptide displayed on MHC class I on the infected cell surface via its T cell receptor. Once activated (also requiring an IL-2 signal from a T helper cell), the CTL releases perforin, a protein that inserts into the target cell membrane and polymerises to form pores. Granzymes (serine proteases) then enter the target cell through the perforin pores and activate the caspase cascade, triggering apoptosis (programmed cell death) of the infected cell.

Marking notes. 1 mark — recognition: TCR binds viral peptide on MHC class I of the infected cell. 1 mark — perforin forms pores in the target cell membrane. 1 mark — granzymes enter through pores and trigger apoptosis / programmed cell death.

Sample response. T helper cells carry the CD4 surface marker. Two functions of the cytokines they release: (1) IL-2 drives clonal expansion of cytotoxic T cells, producing a large clone army of killers needed to clear an infection. (2) Co-stimulatory cytokines (e.g. IL-4, IL-21) and CD40L provide signals required for full B cell activation, class switching to IgG, and memory B cell formation. Accept also: cytokines activate macrophages / recruit innate immune cells (NK cells, neutrophils).

Marking notes. 1 mark — surface marker CD4 (or CD4+). 1 mark each for any two of: drives CTL clonal expansion (IL-2); B cell activation / antibody class switching; macrophage activation; recruitment of innate cells. Maximum 2 marks for functions.

Sample response. Antibodies (produced by the humoral immune system) operate in body fluids and can bind to viruses in the blood, lymph, or extracellular spaces, preventing them from entering new cells [1]. However, once a virus has entered and is replicating inside a host cell, antibodies cannot reach it — they do not penetrate the plasma membrane of the infected cell [1]. Only cytotoxic T cells, through the recognition of viral peptides on MHC class I on the infected cell surface, can identify and destroy the infected cell, eliminating the viral replication factory [1].

Marking notes. 1 mark — antibodies target extracellular virus / operate in body fluids. 1 mark — antibodies cannot access virus inside host cells / cannot cross the plasma membrane. 1 mark — cytotoxic T cells are required because they recognise MHC class I on infected cells and can destroy the cell and its viral contents.

Sample response. T helper cells (CD4+) are the coordinators of adaptive immunity. They are activated when their T cell receptor binds antigen displayed on MHC class II molecules (found only on professional antigen-presenting cells such as dendritic cells and macrophages). Once activated, they release cytokines — including IL-2, IL-4, and CD40L co-stimulatory signals — that activate B cells for antibody production, drive cytotoxic T cell clonal expansion, and enhance macrophage killing. Crucially, T helper cells do not kill directly. Cytotoxic T cells (CD8+) are the effectors. They are activated when their T cell receptor binds a specific viral or tumour peptide displayed on MHC class I molecules (found on all nucleated body cells), combined with IL-2 from a T helper cell. Once activated and clonally expanded, each cytotoxic T cell kills its target cell by releasing perforin (which forms pores in the target cell membrane) and granzymes (which enter through the pores and trigger apoptosis). The key distinction is coordinator versus effector killer: T helper cells direct the response across multiple pathways; cytotoxic T cells execute the targeted killing of infected or abnormal cells.

Marking notes. 1 mark — T helper: CD4+, MHC class II, coordinator/cytokine-releasing role (does NOT kill). 1 mark — Cytotoxic T: CD8+, MHC class I, killing effector role. 1 mark — Mechanism of CTL killing: perforin + granzymes → apoptosis. 1 mark — T helper role explicitly explained: provides co-stimulatory signal for B cells AND IL-2 for CTL expansion. 1 mark — Explicit comparison statement (coordinator vs effector, or equivalent) that contrasts the two roles as distinct and interdependent.

Sample response (a). Both perforin alone and granzyme B alone are relatively ineffective at inducing apoptosis across the concentration range tested. Perforin alone produces slightly higher apoptosis rates than granzyme B alone at each concentration (e.g. at 4.0 µg/mL: 19% vs 5%), but both remain well below 20%, compared with the combined treatment which reaches 96% at the same concentration. The difference between perforin alone and granzyme B alone is relatively small but consistent across all concentrations.

Sample response (b). The data show that perforin and granzyme B function cooperatively, not independently: each molecule carries out one essential step of a two-step killing mechanism. Perforin inserts into the target cell’s plasma membrane and polymerises to form pores, but by itself it cannot trigger the apoptosis cascade — it only creates the entry point. Granzyme B is the serine protease that activates the caspase cascade and triggers apoptosis, but it cannot enter the target cell on its own because the plasma membrane is impermeable to it. Only when both are present simultaneously can the complete sequence occur: perforin forms the pores and granzyme B enters through them to trigger apoptosis. This explains why the combined treatment produces substantially higher apoptosis rates — it is the only condition in which the full killing sequence can be completed.

Marking notes (a). 1 mark — identifies that both molecules alone are largely ineffective (below 20% even at 4 µg/mL). 1 mark — correctly compares the two, noting perforin alone slightly exceeds granzyme B alone, supported by data values.

Marking notes (b). 1 mark — identifies the cooperative / two-step mechanism. 1 mark — correctly states perforin’s role: forms pores in membrane. 1 mark — correctly states granzyme B’s role: enters through pores and triggers apoptosis / caspase cascade. 1 mark — explains why granzyme B is ineffective alone (cannot cross the plasma membrane without perforin pores).

Sample response. T helper cells (CD4+) are the central coordinators of adaptive immunity because they provide essential activation signals to every major effector arm. In humoral immunity, T helper cells supply the co-stimulatory signal (via CD40L–CD40 interaction and cytokines such as IL-4) required for B cells to undergo full activation, clonal expansion, antibody class switching from IgM to IgG, and memory B cell formation; without this, B cells produce only weak, short-lived IgM responses. In cell-mediated immunity, T helper cells release IL-2, which drives the clonal expansion of cytotoxic T cells needed to generate a population large enough to clear an infection; without IL-2, CTLs cannot mount an effective killing response. HIV targets CD4+ T helper cells specifically because viral envelope protein gp120 binds the CD4 receptor, allowing HIV to enter and destroy these cells. As the CD4+ count falls progressively below 200 cells/µL (the AIDS threshold), the activation signals for both B cells and CTLs are simultaneously withdrawn. Humoral immunity weakens — inadequate high-affinity antibody responses develop against new pathogens. Cell-mediated immunity weakens — virus-infected cells are not cleared. The clinical consequence is susceptibility to opportunistic infections: pathogens such as Pneumocystis jirovecii (causing pneumonia), Toxoplasma gondii (causing encephalitis), and Cryptococcus neoformans (causing meningitis) that a healthy immune system clears routinely become life-threatening because the immune system has lost its coordinator. The patient enters clinical AIDS not because the innate system or the effector lymphocytes have been destroyed, but because the cell that organises both adaptive arms — the T helper cell — has been systematically eliminated.

Marking notes. 1 mark — T helper role in humoral immunity: co-stimulatory signal for B cell activation and IgG class switching; consequence of loss (weak IgM only). 1 mark — T helper role in cell-mediated immunity: IL-2 drives CTL clonal expansion; consequence of loss (inadequate CTL response). 1 mark — HIV mechanism: gp120 binds CD4 receptor; progressive decline in CD4+ count. 1 mark — simultaneous collapse of both arms; links to AIDS threshold (<200 cells/µL). 1 mark — clinical assessment: names at least two specific opportunistic pathogens and explains why they are only dangerous in the immunocompromised context.

Sample response. The student’s claim is partially correct but fundamentally incomplete, because it misrepresents the relationship between cytotoxic T cells and T helper cells as one of independence rather than interdependence. It is true that cytotoxic T cells are the direct effectors of cell-mediated killing: their T cell receptor binds viral peptide displayed on MHC class I on infected host cells, and they release perforin and granzymes to trigger apoptosis, eliminating the infected cell and its viral contents. In this specific respect, CTLs are the executors of the killing response. However, the claim that CTLs are “the most important” cells overlooks a critical dependency: cytotoxic T cells require both TCR binding to MHC class I AND an IL-2 signal from T helper cells for full activation and clonal expansion. Without IL-2, CTLs receive an incomplete activation signal and cannot proliferate into the large clone army needed to clear an infection. In effect, CTLs have the weapon but not the ammunition — the T helper cell provides the activating signal that converts recognition into a full cytotoxic response. Furthermore, cytotoxic T cells address only the cell-mediated arm of adaptive immunity; T helper cells also coordinate the humoral arm (B cell activation and antibody production), macrophage activation, and the recruitment of innate immune cells. The student’s claim implicitly assumes that killing infected cells is the only function of the adaptive immune response, which ignores the equally essential role of antibodies in neutralising extracellular virions and opsonising pathogens. A more accurate assessment is that T helper cells and cytotoxic T cells play complementary, interdependent roles: T helper cells are the coordinators without whom CTLs cannot be fully activated, and CTLs are the specialised killers that T helper cells direct toward intracellular threats. Neither is “most important” in isolation; the adaptive immune response against a viral infection requires both to operate simultaneously.

Marking notes. 1 mark — Concedes the valid element: CTLs are the direct killing effectors (perforin + granzymes → apoptosis of infected cell). 1 mark — Identifies and explains the two-signal requirement: CTLs need BOTH MHC class I recognition AND IL-2 from T helper cells for full activation; without T helpers, CTLs cannot be fully activated. 1 mark — States that CTLs address only the cell-mediated arm; T helper cells also coordinate humoral immunity (B cells / antibodies) and macrophage activation. 1 mark — Evaluative judgement: rejects “most important” framing; frames the relationship as complementary and interdependent, not hierarchical. 1 mark — Uses HSC-appropriate terminology (MHC class I, TCR, IL-2, clonal expansion, perforin, granzymes, apoptosis) throughout. 1 mark — Response demonstrates genuine assessment / evaluation (not description alone); reaches an explicit, justified conclusion that directly addresses the student’s claim.