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).