Autoimmune Diseases and Allergies

Q1 — Sample Band 6 response (8 marks), annotated

Multiple sclerosis is an organ-specific autoimmune disease in which autoreactive cytotoxic T cells and autoantibodies target the myelin sheath of neurons in the central nervous system. Demyelination disrupts saltatory nerve conduction, causing progressive neurological deficits. Normally, T cells reactive to myelin basic protein are destroyed in the thymus via clonal deletion (self-tolerance). In MS, this tolerance has failed — possibly due to defective regulatory T-cell (T-reg) suppression — allowing autoreactive clones to attack CNS myelin. [2 marks — target tissue + self-tolerance mechanism]

Stimulus A shows a clear inverse relationship between UV-B exposure and MS prevalence across Australian states: Queensland (UV-B 8.2) has the lowest prevalence (91/100,000) while Tasmania (UV-B 3.7) has the highest (243/100,000) — a 2.7-fold difference. This is consistent with the lesson's statement that MS prevalence increases with distance from the equator. The pattern suggests that higher UV-B (and thus greater vitamin D synthesis in the skin) may be protective against MS development. [2 marks — pattern with quantified evidence + interpretation]

Stimulus B proposes a molecular mechanism: vitamin D upregulates T-reg activity. T-regs normally suppress self-reactive lymphocytes that escaped thymic clonal deletion. If vitamin D is low, T-reg numbers or function decrease, leaving autoreactive T cells insufficiently suppressed — those clones can then proliferate and attack CNS myelin. Vitamin D also downregulates Th17 cells, which are pro-inflammatory and implicated in MS lesion formation. This provides a plausible biological pathway linking UV-B exposure (vitamin D synthesis) to MS risk via T-reg-mediated self-tolerance. [2 marks — cellular mechanism explicitly using T-reg and self-tolerance]

Limitations. Stimulus A: the correlation is observational — states also differ in genetic ancestry, migration patterns, diet and diagnostic rates; UV-B exposure is only a proxy for vitamin D status. Stimulus B: Mendelian randomisation controls for lifestyle confounders but cannot prove causation — the genetic variants used may have pleiotropic effects unrelated to vitamin D. [1 mark — one valid limitation per stimulus]

Judgement: The convergent evidence (ecological correlation + genetic causal inference study) is suggestive but not definitive. Recommending mandatory vitamin D supplementation is premature given the heterogeneity of MS causes (genetic HLA risk alleles, Epstein-Barr virus history, smoking). A randomised controlled trial of supplementation with MS incidence as an endpoint would be required. However, supplementation has low harm and high plausible benefit in vitamin D-deficient southern Australians — a cautious recommendation for at-risk individuals is defensible, but population-wide mandates are not yet justified. [1 mark — justified, nuanced judgement]

Marking criteria:

• 1 mark — Identifies MS target tissue as CNS myelin sheath and links disease to autoimmune attack on self-antigens.
• 1 mark — Links MS to failure of self-tolerance (clonal deletion, autoreactive T cells); mentions T-reg involvement.
• 1 mark — Describes the inverse UV-B / MS prevalence pattern from Stimulus A with at least two quantified figures.
• 1 mark — Interprets Stimulus A correctly (higher UV-B → lower MS prevalence) and notes distance-from-equator consistency.
• 1 mark — Uses Stimulus B to propose a cellular mechanism linking low vitamin D → reduced T-reg activity → less suppression of autoreactive T cells → autoimmune attack.
• 1 mark — Correctly identifies one limitation of Stimulus A (confounding, observational) and one of Stimulus B (pleiotropic effects, or Mendelian randomisation cannot prove causation).
• 1 mark — Reaches an evidence-based, nuanced judgement about the supplementation recommendation that acknowledges both the evidence strength and its limits.
• 1 mark — Response uses precise lesson terminology throughout (self-tolerance, clonal deletion, T-reg, myelin, autoimmune, cytotoxic T cells, autoreactive).

Q2 — Sample Band 6 response (7 marks), annotated

Errors identified:

(1) Wrong antibody class. The article states "IgG antibodies" — this is incorrect. The antibody class responsible for allergic (Type I hypersensitivity) reactions, including peanut allergy, is IgE, not IgG. IgG is the dominant serum antibody involved in conventional adaptive immunity and in autoimmune diseases (e.g. rheumatoid factor); it is not the effector antibody in Type I allergy. [1 mark]

(2) IgG does not bind mast cells. The article states that IgG binds to mast cells — this is incorrect. It is IgE that binds via high-affinity Fc-epsilon receptors (FcεRI) on the surface of mast cells and basophils. IgG does not bind to these receptors and does not sensitise mast cells for degranulation. [1 mark]

(3) First exposure cannot cause anaphylaxis. The article states "even a microscopic exposure the very first time can cause anaphylaxis." This is biologically impossible. Anaphylaxis requires prior sensitisation — the first exposure to the allergen causes B cells to produce IgE antibodies that then bind to mast cells (no symptoms at this stage). Anaphylaxis can only occur on a second or subsequent exposure when pre-bound IgE is cross-linked by the allergen, triggering simultaneous systemic degranulation. First exposure cannot trigger this response because no mast-cell-bound IgE yet exists. [2 marks — identifying the error AND explaining the correct two-stage mechanism]

What is correct: The article correctly identifies that mast cells release histamine during an allergic reaction, and that avoiding peanut exposure is clinically appropriate advice for a sensitised individual — the conclusion is correct even though the stated mechanism is wrong. [1 mark]

Clinical consequence of the final sentence: The advice "never be given peanuts even in tiny amounts" is clinically reasonable for a already-sensitised child, but the stated reason ("even a microscopic exposure the very first time can cause anaphylaxis") is scientifically wrong. Paradoxically, some current evidence (LEAP trial, 2015) supports early low-dose peanut introduction in non-sensitised infants to prevent sensitisation from occurring — the opposite of what the article implies. The advice is partially correct in outcome (avoid re-exposure once sensitised) but the mechanism given is entirely wrong and could mislead parents about when the risk begins. [1 mark]

Accurate reformulation: "Peanut allergy is caused by the immune system producing IgE antibodies against peanut proteins after a first exposure (sensitisation). These IgE antibodies bind to the surface of mast cells throughout the body. On a subsequent exposure, peanut proteins cross-link two adjacent IgE antibodies on mast cells, triggering rapid degranulation and release of histamine — causing symptoms ranging from hives to anaphylaxis. Anaphylaxis cannot occur on first exposure, because no mast-cell-bound IgE has yet been produced." [1 mark]

Marking criteria:

• 1 mark — Correctly identifies the antibody class error (IgG → IgE) and explains why IgE is correct (it is the allergen-specific antibody in Type I hypersensitivity).
• 1 mark — Identifies that IgG does not bind to mast cells; explains that only IgE binds via FcεRI receptors on mast cells and basophils, sensitising them for degranulation.
• 1 mark — Identifies the first-exposure error: anaphylaxis is impossible on first exposure because no mast-cell-bound IgE exists yet.
• 1 mark — Correctly explains the two-stage mechanism: first exposure → IgE production → IgE binds mast cells (no symptoms); re-exposure → cross-linking → degranulation → anaphylaxis.
• 1 mark — Identifies what is correct in the article (mast cell histamine release; advice to avoid re-exposure once sensitised).
• 1 mark — Assesses the clinical consequence of the final sentence: the advice is appropriate for a sensitised child but the reason is wrong; notes that first-exposure exposure is not dangerous and may even be beneficial (LEAP trial or equivalent reasoning accepted).
• 1 mark — Provides a biologically accurate reformulation of the central claim using correct terminology (IgE, sensitisation, mast cell, degranulation, histamine, cross-linking).