HSC Exam Practice

Sample response. Self-tolerance is the immune system's ability to recognise and refrain from attacking the body's own cells and tissues. It is established primarily through clonal deletion in the thymus, where T lymphocytes that react strongly against self-antigens are identified and destroyed before they can enter circulation.

Marking notes. 1 mark for defining self-tolerance as the immune system's non-reactivity towards self-antigens / own cells. 1 mark for identifying clonal deletion (in the thymus) as the establishing mechanism. "Negative selection" is an accepted synonym for clonal deletion.

Sample response. In an autoimmune disease, the immune target is the body's own tissues (self-antigens); the effector antibody classes are typically IgG or IgM, or cytotoxic T cells attack self-cells directly (e.g. Type 1 diabetes — cytotoxic T cells destroy pancreatic beta cells). In an allergy (Type I hypersensitivity), the immune target is an external, normally harmless substance called an allergen; the antibody class is IgE, which binds to mast cells and triggers degranulation on re-exposure.

Marking notes. 1 mark for identifying autoimmune target as self-antigens / self-tissue. 1 mark for identifying allergy target as a non-self allergen (harmless foreign antigen). 1 mark for correctly naming antibody classes: IgG/IgM (or cytotoxic T cells) for autoimmune; IgE for allergy.

Sample response. Type 1 diabetes: Target tissue — beta cells of the pancreatic islets of Langerhans. Consequence — cytotoxic T cells destroy insulin-producing beta cells → absolute insulin deficiency → chronic hyperglycaemia. Multiple sclerosis: Target tissue — myelin sheath of neurons in the central nervous system. Consequence — demyelination disrupts saltatory nerve conduction → progressive neurological deficits (vision loss, loss of balance, motor impairment). Rheumatoid arthritis: Target tissue — synovial membrane of joints. Consequence — autoantibodies (including rheumatoid factor) cause synovial inflammation, cartilage erosion and joint deformity.

Marking notes. For each chosen disease: 1 mark for correct target tissue; 1 mark for a correct clinical consequence linked to the destruction of that tissue. 4 marks total (2 per disease, 2 diseases required).

Sample response. Anaphylaxis is a Type I hypersensitivity reaction that requires prior sensitisation. During first exposure, the allergen is processed and presented to helper T cells (Th2 subset), which stimulate B cells to produce IgE antibodies specific to the allergen. These IgE antibodies then bind to high-affinity receptors on the surface of mast cells throughout the body — no symptoms occur at this stage. Anaphylaxis requires a second or subsequent allergen exposure to cross-link the already-bound IgE on mast cells, triggering simultaneous systemic degranulation. On first exposure, no mast-cell-bound IgE yet exists, so degranulation cannot be triggered.

Marking notes. 1 mark for stating that anaphylaxis requires prior sensitisation (IgE must already be produced and bound to mast cells). 1 mark for explaining that first exposure produces IgE but no symptoms. 1 mark for explaining that re-exposure cross-links IgE to trigger degranulation — impossible on first exposure because no bound IgE exists.

Sample response. Adrenaline (epinephrine) acts via two receptor types. Via alpha-adrenergic receptors it causes vasoconstriction — reversing the dangerous systemic vasodilation and blood pressure drop of anaphylactic shock by redirecting blood to vital organs. Via beta-adrenergic receptors it causes bronchodilation (relaxing smooth muscle in bronchioles) — reversing the severe bronchoconstriction that impairs breathing — and also increases cardiac contractility and heart rate, restoring cardiac output.

Marking notes. 1 mark for vasoconstriction via alpha receptors / reversal of vasodilation or blood pressure drop. 1 mark for bronchodilation via beta receptors / reversal of bronchoconstriction. 1 mark for improved cardiac output or additional mechanism description. Accept: "reverses vasodilation restoring blood pressure" + "reverses bronchoconstriction" + "cardiac stimulation" each as separate points.

Sample response. Antihistamines (e.g. cetirizine) competitively block H1 histamine receptors on target cells, preventing histamine from binding and causing its effects (vasodilation, increased permeability, mucus secretion). They do not prevent mast cell degranulation or histamine release — they act downstream of the allergy mechanism, relieving symptoms without modifying the underlying IgE sensitisation. Allergen immunotherapy (desensitisation) involves administering escalating subcutaneous doses of pollen allergen over months to years. This gradually shifts the immune response from a Th2/IgE pathway to a Th1/Treg pathway, increasing allergen-specific IgG "blocking antibodies" and reducing IgE production. Desensitisation is the only disease-modifying allergy treatment because it re-educates the immune system to tolerate the allergen, reducing the underlying sensitisation itself rather than blocking a downstream mediator.

Marking notes. 1 mark for antihistamine mechanism (H1 receptor blockade on target cells; does not prevent histamine release). 1 mark for stating antihistamines are symptom-relieving / not disease-modifying. 1 mark for desensitisation mechanism (escalating allergen doses → Th2 to Th1/Treg shift → IgG blocking antibodies). 1 mark for correctly identifying desensitisation as disease-modifying because it reduces underlying IgE sensitisation, not just downstream symptoms.

Sample response (a) — trend description. Food-induced anaphylaxis admissions in Australian children aged 0–4 increased substantially from approximately 28 per million in 1998 to approximately 95–100 per million in 2012 — a roughly 3.4-fold increase over 14 years. The rate of increase was steepest between 1998 and 2010. After approximately 2012, the rate plateaued at around 95–100 per million and did not continue to rise substantially through 2018.

Marking notes for (a). 1 mark for identifying the overall increasing trend to 2012 with approximate figures. 1 mark for identifying the plateau from ~2012 to 2018.

Sample response (b) — IgE mechanism consistent with rising rate. A rising rate of anaphylaxis admissions in a child population over 20 years reflects an increase in the proportion of children who have undergone sensitisation — i.e. who have produced allergen-specific IgE antibodies following first-food exposure. As the sensitised proportion of the cohort grows (due to increased exposure to allergenic foods or to changes in immune priming in early life, such as reduced microbial exposure under the hygiene hypothesis), a greater number of children carry mast-cell-bound IgE. When those children are subsequently re-exposed, cross-linking of IgE on a large number of mast cells simultaneously triggers anaphylaxis. The population-level rise reflects a rising cohort of individuals progressing from the sensitisation stage to the re-exposure stage with each year.

Marking notes for (b). 1 mark for connecting the rise to an increasing proportion of sensitised children (IgE produced). 1 mark for using the two-stage mechanism to explain why sensitised children are at risk on re-exposure. 1 mark for linking population-level rise to a growing pool of individuals who have undergone sensitisation over the 20-year period (e.g. hygiene hypothesis or increased early-life allergen exposure changes accepted).

Sample response (c) — plateau reason. Possible biological/public-health reasons include: (i) Introduction of ASCIA early allergen introduction guidelines around 2010–2012, recommending that high-risk infants be given small amounts of peanut and egg from 4–6 months. Early oral exposure induces tolerance (shifts Th2 to Th1/Treg) rather than sensitisation, reducing the proportion of new children becoming IgE-sensitised. (ii) Improved community access to EpiPen auto-injectors and better emergency management may have reduced fatalities without reducing admissions. Accept other biologically plausible explanations (e.g. expanded allergen labelling reducing re-exposure, or diagnostic ceiling).

Marking notes for (c). 1 mark for a plausible reason with a biological or public-health mechanism. 1 mark for linking the proposed mechanism to a reduction in the rate of new sensitisations or improved management.

Sample response. The claim captures a genuine commonality — both autoimmune diseases and allergies involve the immune system mounting a response that is inappropriate given the target. However, reducing them to "two versions of the same problem" obscures critical differences in target, mechanism, effector cells, antibody class and treatment that are central to understanding each condition.

The shared characteristic is immune dysregulation: in both conditions, immune tolerance has broken down in a broad sense — either towards self-antigens (autoimmune) or towards harmless non-self antigens (allergy). This is why both are classified as non-infectious immune disorders, not as immune deficiency.

However, the mechanisms are fundamentally different. In autoimmune disease (e.g. multiple sclerosis), autoreactive cytotoxic T cells that escaped clonal deletion in the thymus attack the body's own myelin sheath. The effectors are primarily T cells or IgG/IgM autoantibodies; the disease is chronic and progressive; and treatment focuses on broadly suppressing the immune response (corticosteroids, methotrexate, anti-TNF-α biologics such as adalimumab) or depleting B cells (rituximab). In Type I allergy (e.g. peanut allergy, bee venom anaphylaxis), Th2-driven B cells produce IgE antibodies against an external allergen. These IgE antibodies sensitise mast cells by binding their surface Fc receptors. On re-exposure, the allergen cross-links IgE, triggering mast cell degranulation and histamine release — causing effects ranging from hayfever to anaphylaxis. Treatment aims to block histamine receptors downstream (antihistamines) or, uniquely, to re-educate the immune system toward Th1/IgG through desensitisation — the only disease-modifying allergy treatment.

A critical distinction is the target: autoimmunity targets self-antigens (the immune system attacks the body it is supposed to protect), while allergy targets non-self, harmless antigens that pose no actual biological threat. The damage in autoimmunity is irreversible organ destruction (e.g. permanent beta cell loss in Type 1 diabetes); the damage in allergy is largely mediated by inflammatory mediators that are rapidly reversible (e.g. histamine-induced vasodilation reversed by adrenaline within minutes). The antibody classes also differ categorically — IgG/IgM versus IgE — which determines the effector cell involved (antigen-presenting cells and T cells vs mast cells and basophils) and the time course (chronic vs immediate).

Evaluating the claim: it is partially defensible as a conceptual starting point — both represent the immune system "responding when it should not." But this framing is too simplistic for HSC purposes. The claim obscures the mechanistic differences that drive entirely different clinical presentations, organ systems, antibody classes and treatment paradigms. A more accurate statement is: autoimmune diseases and allergies are both forms of immune dysregulation, but they differ in the nature of the antigen target (self vs harmless non-self), the effector pathway (T cells/IgG vs IgE/mast cells), the time course (chronic vs immediate) and the appropriate therapeutic strategy (immunosuppression vs receptor blockade or desensitisation).

Marking criteria.

• 1 mark — Identifies the correct commonality: both involve inappropriate immune activation (immune dysregulation) rather than immune deficiency.
• 1 mark — Names and correctly describes the mechanism of a specific autoimmune disease (e.g. MS — autoreactive T cells target CNS myelin due to failed self-tolerance).
• 1 mark — Names and correctly describes the Type I allergy mechanism (IgE-sensitisation → re-exposure → mast cell degranulation → histamine) with a named example (peanut, bee venom, pollen).
• 1 mark — Compares the immune targets: self-antigens (autoimmune) vs harmless non-self allergen (allergy).
• 1 mark — Compares the antibody classes: IgG/IgM or T cells (autoimmune) vs IgE (allergy) and explains the role of the effector cell in each (cytotoxic T cell or B cell / mast cell).
• 1 mark — Compares treatment approaches: immunosuppressants/biologics targeting the chronic immune response (autoimmune) vs antihistamines or desensitisation for allergy, and notes that desensitisation is the only disease-modifying allergy treatment.
• 1 mark — Evaluates the claim: acknowledges what is correct (both are immune dysregulation, not deficiency), but identifies where it breaks down (mechanism, target, antibody class and treatment are fundamentally different).
• 1 mark — Reaches an explicit, justified overall evaluation that neither fully accepts nor fully rejects the claim — instead formulates a more precise distinction using correct terminology (self-tolerance, allergen, IgE, mast cell, clonal deletion, T-reg, immunosuppression, desensitisation).