Hypersensitivity questions become easier when you stop trying to match every disease to a memorised Roman numeral. Instead, identify what recognises the antigen, where the antigen is located and how the immune response causes harm. Those three questions turn a list of conditions into a usable reasoning method.
This guide compares the four Gell and Coombs types, works through commonly confused presentations and finishes with five original single-best-answer questions. Use it alongside your MRCEM Primary revision plan.
1. Start with the mechanism, then name the type
Where this fits in MRCEM Primary revision
The published RCEM Basic Sciences Syllabus includes hypersensitivity reactions under pathology and immune responses. RCEM states that the updated syllabus applies from the 21 April 2027 examination onwards. Check the official syllabus for your sitting rather than assuming older subject weightings still apply. No particular number of hypersensitivity questions is promised. [1]
Ask three questions about the stem
First, identify the immune component: does the question describe IgE, another antibody or T cells? Second, locate the target: is it attached to a cell or tissue, or present as a soluble antigen forming complexes? Third, identify the consequence: mediator release, cell injury, altered receptor function or inflammation around deposited complexes?
Write a short mechanism statement before looking at the answers. For example, “antibody binding directly to a receptor and stimulating it” is more useful than “an autoimmune disease”. The mechanism statement gives you something specific to compare with each option.
A comparison table to organise recall
Type | Central mechanism | Useful example | Decisive distinction |
|---|---|---|---|
I | IgE-associated mast-cell activation | IgE-mediated immediate allergy | Allergen triggers release of inflammatory mediators |
II | Antibody against a cell-surface or tissue target | Immune haemolysis; anti-GBM disease | Antibody recognises a target already present on the cell or tissue |
III | Inflammation associated with antigen–antibody complexes | Classical serum sickness | Soluble antigen participates in complexes that accumulate in tissues |
IV | T-cell-mediated injury or inflammation | Allergic contact dermatitis | Antibody is not the defining effector mechanism |
The British Society for Immunology presents these categories using the immune reactant, antigen and effector. Use those features as the foundation of your answer. A familiar disease name is a useful clue, but the mechanism described in the question should decide the classification. [2]
2. Type I: IgE, mast cells and immediate reactions
Separate sensitisation from the reaction
Sensitisation establishes an allergen-specific immune response. IgE can bind to high-affinity receptors on mast cells. On a subsequent relevant exposure, allergen cross-linking of bound IgE can activate those cells. Release of mediators, including histamine, helps explain rapid symptoms such as itching, wheals and bronchoconstriction. [3]
What the timing contributes
A rapid onset after exposure supports an immediate mechanism, but “fast” is not a substitute for the rest of the stem. Look for the trigger, symptom pattern and any explicit evidence of IgE involvement. Avoid selecting type I solely because a reaction occurred during a treatment or infusion.
Anaphylaxis is a clinical diagnosis
IgE-mediated anaphylaxis is a classic type I example, but anaphylaxis can also arise through other mechanisms. The clinical emergency must be recognised and treated without waiting to establish its immunological pathway. For emergency management, use the Resuscitation Council UK anaphylaxis guidance; this article focuses on classification. [4]
3. Type II: direct antibody targets and receptor effects
Cell-surface and tissue antigens
In type II reactions, antibodies recognise targets on cells or within tissues. The resulting effects may include complement-mediated injury or removal of antibody-coated cells. A drug-associated immune haemolysis stem therefore describes a different mechanism from an immediate IgE-mediated reaction to a drug. The medication name alone cannot identify the hypersensitivity type. [5]
Why “cytotoxic” is an incomplete shortcut
Antibodies can change receptor function as well as promote tissue injury. In Graves’ disease, stimulating antibodies act on thyroid cells and drive excessive thyroid hormone production. Within the conventional four-type framework, this is a type II functional antibody effect. The key event is receptor stimulation, rather than destruction of the thyroid. [6]
This is a useful test of your understanding: if you have learned type II only as “cells being killed”, receptor-stimulation questions will feel contradictory. Expand the category in your notes to “antibody against a fixed target, causing injury or altered function”.
Anti-GBM disease: the target is already in the tissue
Anti-glomerular basement membrane disease involves antibodies directed against basement-membrane antigens. Kidney and lung involvement may occur. The antibody target is part of the tissue, which is why this belongs with direct antibody-mediated disease rather than being classified simply as an immune-complex disorder because the kidney is involved. [7]
4. Type III: soluble antigens and immune complexes
Why complexes can cause inflammation
Antibodies can bind soluble antigens to form complexes. When these accumulate in tissues, they can activate complement and recruit inflammatory cells. The resulting injury may affect several sites. Classical serum sickness and a local Arthus reaction are useful examples of this mechanism. [3]
Type II versus type III: compare the sequence
Question to ask | Type II pattern | Type III pattern |
|---|---|---|
What does the antibody recognise? | A cell-surface or tissue target | A soluble antigen participating in complexes |
What makes the tissue a target? | The relevant antigen is present there | Complexes accumulate there and provoke inflammation |
Is complement enough to distinguish them? | No | No |
Best clue in a short stem | Antibody binding directly to a specified structure | Immune-complex deposition explicitly described |
Complement involvement is not exclusive to one category. When both options seem plausible, return to the location of the antigen and the sequence that produces injury. This distinction is reflected in the mechanistic classification used by BSACI. [5]
Use clinical clusters without over-reading them
Fever, rash and joint symptoms after an exposure may suggest serum sickness, but the symptom cluster is not itself proof of circulating immune complexes. In an educational question, explicit laboratory or mechanistic information can make the classification clearer. Do not add unprovided findings merely to make your preferred answer fit.
5. Type IV: T cells and delayed reactions
What makes this category different?
Type IV reactions are defined by T-cell-mediated mechanisms. These can involve inflammatory cytokines and recruited cells, or direct cellular cytotoxicity. Allergic contact dermatitis is a useful example. Unlike the first three categories, the classification does not depend on an antibody being the principal mediator. [8]
Allergic contact dermatitis versus irritation
A rash at a contact site does not automatically establish allergy. Irritant contact dermatitis results from direct damage to the skin barrier; allergic contact dermatitis involves an immune response to an allergen. Nickel is a familiar trigger of the allergic form. History and appropriate assessment distinguish them, and the two processes can coexist. [9]
Do not turn “delayed” into a rigid stopwatch rule
Delayed contact reactions commonly evolve over days, but timing varies with the reaction and exposure history. A 48–72-hour association is a useful teaching pattern, not a universal rule for every T-cell-mediated drug reaction. The question’s mechanism should outweigh an oversimplified timing mnemonic. [8]
6. Work through two common SBA traps
The same product can be associated with different reactions
Gloves provide a helpful comparison. Natural rubber latex proteins can provoke immediate type I allergy. Chemicals used in glove manufacture can cause delayed allergic contact dermatitis, while glove use can also be associated with irritation. Therefore, “reaction to gloves” is not a complete mechanistic diagnosis. HSE explicitly distinguishes these possibilities. [10]
Different tests answer different questions
Skin-prick testing and allergen-specific IgE testing assess aspects of immediate sensitisation. Patch testing is used to investigate allergic contact dermatitis. A positive sensitisation test must be interpreted alongside the clinical history; it does not independently establish that an allergen caused a particular episode. Match the proposed test to the mechanism being investigated. [8]
7. Five original MRCEM Primary practice SBAs
These are original educational questions, not official or recalled RCEM examination material. Attempt all five before reading the answers. For each, write down the decisive clue and the closest competing option.
Question 1: identify the immediate pathway
A patient with established peanut-specific IgE develops wheeze and widespread urticaria within minutes of accidental exposure. Which event best explains the immediate immunological response?
Soluble immune complexes depositing in glomeruli
Allergen cross-linking IgE bound to mast cells
Antibodies stimulating thyroid receptors
Memory T cells producing a delayed contact reaction
Direct chemical injury to the skin barrier
Question 2: classify a functional antibody effect
A patient has biochemical hyperthyroidism. Testing identifies antibodies that stimulate the TSH receptor. Using the conventional four-type hypersensitivity classification, which mechanism is most appropriate?
Type I, because a hormone is released
Type III, because antibodies circulate in blood
Type IV, because the target is a human tissue
Type II, with antibody-mediated receptor stimulation
No hypersensitivity category can include altered receptor function
Question 3: distinguish a fixed tissue target
A patient has rapidly progressive glomerulonephritis. Investigations identify antibodies directed against an antigen within the glomerular basement membrane. Which feature most strongly supports a type II rather than a type III mechanism?
The patient has kidney injury
The disease involves the immune system
Antibody binds directly to an antigen already present in tissue
An antibody can be detected in the circulation
The patient has an abnormal urine test
Question 4: follow the complexes
Following exposure to a foreign protein, a patient develops systemic symptoms. Investigation demonstrates complexes of soluble antigen and antibody accumulating in small vessels, with complement activation and inflammation. Which classification best fits the described mechanism?
Type III hypersensitivity
Type I hypersensitivity
Type II receptor stimulation
Type IV hypersensitivity
Non-immune irritation
Question 5: interpret delayed contact inflammation
A patient develops an eczematous eruption beneath a nickel-containing watch strap. Specialist assessment identifies a delayed positive patch-test reaction to nickel, consistent with allergic contact dermatitis. Which mechanism best explains the finding?
IgE cross-linking as the defining mechanism
Antibody-dependent destruction of erythrocytes
Soluble immune complexes depositing in renal tissue
Antibody stimulation of a hormone receptor
T-cell-mediated inflammation
8. Answers and explanations
Question 1: B — allergen cross-linking mast-cell-bound IgE
The supplied sensitisation and immediate symptoms support B. A describes an immune-complex process; C describes a functional receptor antibody; D describes a delayed cellular response; E describes non-allergic irritation. The transferable point is to connect the antibody class with the effector cell, rather than choosing an answer from the exposure alone. [2]
Question 2: D — type II receptor stimulation
The stem specifies a stimulating antibody against a receptor, making D the best answer. A incorrectly treats hormone release as proof of IgE involvement. B confuses circulating antibodies with deposited immune complexes. C mistakes a self-target for evidence of T-cell mediation. E excludes a recognised functional antibody mechanism. [6]
Question 3: C — direct recognition of a tissue antigen
C supplies the discriminating mechanism. A and E describe consequences of kidney disease. B is too broad to separate immune categories. D identifies where an antibody is measured, not what it does. In anti-GBM disease, the relevant antibody target is within the basement membrane. Organ involvement alone cannot classify an immune response. [7]
Question 4: A — type III hypersensitivity
The explicit formation and tissue accumulation of complexes makes A correct. B would require an IgE-associated immediate pathway; C would require direct receptor stimulation; D would make T cells the defining mechanism; E conflicts with the documented immune process. The question is deliberately solvable from its mechanism even without naming a disease. [3]
Question 5: E — T-cell-mediated inflammation
The confirmed delayed allergic contact reaction supports E. A describes an immediate IgE mechanism, B a cellular antibody target, C immune-complex deposition and D a receptor antibody effect. None explains the specified patch-test finding as well as E. Do not confuse a patch test with a skin-prick test. [8]
9. Frequently asked questions and revision strategy
Are types I–III all immediate reactions?
No. They share antibody involvement, but that does not give them one universal onset time. Use timing as supporting evidence and classify the actual immune mechanism. In particular, do not convert the word “antibody” into an automatic type I answer.
Can one disease involve more than one mechanism?
Yes. The four-type framework is a teaching classification, and complex diseases need not fit every aspect of their biology into one box. Answer the mechanism described in the stem. Avoid replacing a precise finding with a broad rule learned from a disease list.
How should I revise mistakes?
Make a three-column error log: decisive finding, incorrect assumption and corrected mechanism. If you confuse types II and III, write where the antigen is located. If you confuse types I and IV, identify whether the stem centres on IgE or T cells. Reattempt the question later without seeing the original answer.
Then mix these questions with other pathology topics. A correct answer immediately after reading a heading is less informative than a correct answer when the subject is not announced. Explain why the closest distractor is wrong before moving on.
Put the mechanisms into regular SBA practice
The StudyMedical MRCEM Primary question bank offers over 7,800 questions, explanations with teaching diagrams, custom topic-based quizzes, timed quizzes and the option to revisit previously incorrect questions. Try free sample questions without signing up, then choose the plan that suits your preparation. Full mock-paper access varies by plan. [11]
References and further reading
Links checked 2 November 2026. Clinical examples teach mechanisms; they are not complete diagnostic or treatment pathways.
