A 3-year-old boy in Aligarh has had recurrent sinusitis and pneumonia, and now has Pneumocystis jirovecii pneumonia. He is HIV-negative. Serum IgM is 420 mg/dL (high), and IgG and IgA are very low. B-cell numbers in blood are normal. His maternal uncle died of a similar illness in childhood. What is the most likely defect?

Bruton tyrosine kinase in B cells

Adenosine deaminase in lymphocytes

A 16-year-old schoolboy in Sambalpur was bitten deeply on the face by a stray dog and, as human rabies immunoglobulin was unavailable, received equine rabies immunoglobulin with his vaccine. Nine days later he returns with fever, an itchy rash, painful knees and wrists, and enlarged lymph nodes. Urine shows 1+ protein, and serum C3 is low. What is the mechanism of this illness?

Deposition of circulating immune complexes

Sensitised T cells releasing cytokines

Antibodies destroying his red cells

Release of histamine from mast cells

A 34-year-old woman in Bengaluru develops an itchy, oozing, scaly rash along her hairline and forehead two days after using a new black hair dye. She had used the same brand once before without trouble. Which test best confirms the cause?

Skin prick test read at 20 minutes

Immunity, hypersensitivity and vaccines

26 NEET-PG practice questions on Immunity, hypersensitivity and vaccines, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A 19-year-old hostel student is admitted for the second time in three years with meningococcal meningitis. He has had no other serious infections, and his immunoglobulin levels are normal. Deficiency of which component is most likely?
1. AC1 esterase inhibitor
2. BTerminal components C5-C9
3. CComplement component C3
4. DEarly classical pathway C2
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Answer: B · Terminal components C5-C9
Neisseria species are killed mainly by the membrane attack complex, which punches holes in their thin outer membrane. People lacking any of C5 to C9 cannot form the complex and suffer recurrent meningococcal or gonococcal infections, often with uncommon serogroups. The screening test is a total haemolytic complement (CH50) assay, which is very low. Patients should be given meningococcal vaccines; the same risk applies to people treated with eculizumab, which blocks C5.
- A: C1 esterase inhibitor deficiency causes hereditary angioedema, not infections.
- C: C3 deficiency causes severe, recurrent pyogenic infections with many organisms, not Neisseria alone.
- D: Early classical pathway deficiencies such as C2 mainly cause SLE-like disease.
Question 2 · Hard
A 3-year-old boy in Aligarh has had recurrent sinusitis and pneumonia, and now has Pneumocystis jirovecii pneumonia. He is HIV-negative. Serum IgM is 420 mg/dL (high), and IgG and IgA are very low. B-cell numbers in blood are normal. His maternal uncle died of a similar illness in childhood. What is the most likely defect?
1. ABruton tyrosine kinase in B cells
2. BCD40 ligand on helper T cells
3. CAdenosine deaminase in lymphocytes
4. DC1 inhibitor in plasma
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Answer: B · CD40 ligand on helper T cells
High IgM with very low IgG and IgA, normal B-cell numbers and X-linked inheritance indicate X-linked hyper-IgM syndrome. CD40 ligand on activated helper T cells must bind CD40 on B cells for immunoglobulin class switching, so B cells keep making only IgM. CD40L also activates macrophages, which is why these boys get opportunistic infections such as Pneumocystis pneumonia and Cryptosporidium. Treatment is immunoglobulin replacement, Pneumocystis prophylaxis and, where possible, stem cell transplantation.
- A: BTK deficiency causes absent B cells and low IgM as well as low IgG and IgA.
- C: ADA deficiency causes SCID with profound T-cell lymphopenia.
- D: C1 inhibitor deficiency causes hereditary angioedema, not immunodeficiency.
Question 3 · Easy
A 23-year-old primigravida at a PHC in Bareilly received two doses of Td vaccine during this pregnancy. Her baby is born at home and the cord is cut with an unboiled blade, yet the baby does not develop neonatal tetanus. What protected the baby?
1. AFetal IgM made in utero
2. BSecretory IgA from colostrum
3. CMaternal IgG antitoxin
4. DMaternal IgM antitoxin
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Answer: C · Maternal IgG antitoxin
IgG is the only immunoglobulin class actively transported across the placenta, through the neonatal Fc receptor (FcRn) on syncytiotrophoblast. Maternal antitoxin IgG produced after Td vaccination neutralises tetanus toxin in the newborn for the first months of life. This is why Td in pregnancy is a key strategy for eliminating neonatal tetanus, along with clean delivery practices.
- A: The fetus makes IgM only when infected in utero, not after maternal vaccination.
- B: Milk IgA protects the gut lumen and does not neutralise toxin in the tissues.
- D: IgM is a large pentamer and does not cross the placenta.
Question 4 · Medium
A 16-year-old schoolboy in Sambalpur was bitten deeply on the face by a stray dog and, as human rabies immunoglobulin was unavailable, received equine rabies immunoglobulin with his vaccine. Nine days later he returns with fever, an itchy rash, painful knees and wrists, and enlarged lymph nodes. Urine shows 1+ protein, and serum C3 is low. What is the mechanism of this illness?
1. ASensitised T cells releasing cytokines
2. BAntibodies destroying his red cells
3. CRelease of histamine from mast cells
4. DDeposition of circulating immune complexes
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Answer: D · Deposition of circulating immune complexes
This is serum sickness, a type III hypersensitivity reaction to the foreign horse proteins in equine rabies immunoglobulin. About 1-2 weeks after the dose, he makes antibodies while the antigen is still circulating; the resulting immune complexes deposit in vessels, joints and glomeruli, activate complement and attract neutrophils, which explains the low C3. It is usually self-limiting and is treated with antihistamines, analgesics and, if severe, a short course of steroids.
- A: T cell-mediated injury is type IV and does not lower complement.
- B: Antibody against red cells is type II and would cause haemolysis, not arthritis and rash.
- C: Mast cell histamine release is type I, which occurs within minutes of the dose.
Question 5 · Medium
A 34-year-old woman in Bengaluru develops an itchy, oozing, scaly rash along her hairline and forehead two days after using a new black hair dye. She had used the same brand once before without trouble. Which test best confirms the cause?
1. ASerum total IgE level
2. BIntradermal tuberculin test
3. CPatch test read at 48 hours
4. DSkin prick test read at 20 minutes
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Answer: C · Patch test read at 48 hours
An eczematous rash at the site of contact, appearing 1-2 days after re-exposure to a hair dye (paraphenylenediamine is the usual sensitiser), is allergic contact dermatitis, a type IV delayed hypersensitivity. The first use sensitised her T cells; on re-exposure, memory T cells release cytokines that recruit macrophages over 48-72 hours. A patch test reproduces this delayed reaction under occlusion and identifies the allergen.
- A: Total IgE is non-specific and reflects type I, not type IV, allergy.
- B: The tuberculin test checks sensitisation to mycobacterial antigens, not hair dye.
- D: Skin prick tests detect IgE-mediated (type I) allergy, which appears within minutes.
Question 6 · Medium
A 22-year-old college student in Hyderabad has had sneezing, watery rhinorrhoea and itchy eyes every year from February to April for 5 years. She wants to know exactly which pollen she is allergic to, so that immunotherapy can be considered. Which test should be done first?
1. APatch test read at 48 and 96 hours
2. BSerum total IgE estimation
3. CSkin prick test with a panel of allergens
4. DNasal smear for neutrophils
Answer and explanation in LearnFree account
Question 7 · Medium
A 10-month-old boy in Hubli has had three episodes of pneumonia and one of otitis media since the age of 6 months. His tonsils are tiny and no lymph nodes are palpable. Serum IgG, IgA and IgM are all very low, and flow cytometry shows almost no CD19-positive cells in blood; T cell numbers are normal. What is the underlying defect?
1. AMutation in Bruton tyrosine kinase
2. BDefect in CD40 ligand on T cells
3. CDeletion on chromosome 22q11.2
4. DDeficiency of adenosine deaminase
Answer and explanation in LearnFree account
Question 8 · Medium
During a training session at a PHC in Sangli, the medical officer reviews the UIP vaccines that infants receive. Consider four statements: 1. BCG is a live attenuated bacterial vaccine. 2. Hepatitis B vaccine is a recombinant subunit vaccine. 3. Fractional IPV is a live attenuated viral vaccine. 4. The oral rotavirus vaccine is a live attenuated viral vaccine. Which statements are correct?
1. A1 and 2 only
2. B1, 2 and 4 only
3. C2, 3 and 4 only
4. D1 and 3 only
Answer and explanation in LearnFree account
Question 9 · Medium
A 30-year-old man with recurrent sinusitis and giardiasis develops flushing, wheeze and hypotension a few minutes into his first blood transfusion after a road accident in Vijayawada. Serum IgA is undetectable, and IgG and IgM are normal. How should future transfusions be given to him?
1. AWashed red cells or IgA-deficient donor blood
2. BFresh frozen plasma before each unit
3. CLeucoreduced red cells from any donor
4. DIrradiated red cells from a relative
Answer and explanation in LearnFree account
Question 10 · Medium
At a 6-week immunisation session in a PHC in Dharwad, a mother who is a science teacher asks why the Hib part of the pentavalent vaccine works in such a young infant, when plain polysaccharide vaccines do not work below 2 years of age. What is the reason?
1. AThe polysaccharide is linked to a protein carrier
2. BThe Hib bacteria in the vaccine are live attenuated
3. CThe vaccine contains a larger dose of polysaccharide
4. DThe aluminium adjuvant makes it a T-independent antigen
Answer and explanation in LearnFree account
Question 11 · Medium
A 5-day-old girl in a NICU in Lucknow has repeated tetany and seizures. Her serum calcium is 6.2 mg/dL with a low PTH. Echocardiography shows truncus arteriosus, and a chest X-ray shows no thymic shadow. She has low-set ears and a short philtrum. Which arm of immunity is chiefly deficient in her?
1. AClassical complement pathway
2. BT cell-mediated immunity
3. CPhagocytic killing by neutrophils
4. DAntibody production by B cells
Answer and explanation in LearnFree account
Question 12 · Easy
A 42-year-old farmer in Wayanad is stung by several honeybees. Within 10 minutes he develops generalised urticaria, wheeze and a BP of 80/50 mmHg. Which antibody, bound to his mast cells, mediated this reaction?
1. AIgD
2. BIgM
3. CIgE
4. DIgG
Answer and explanation in LearnFree account
Question 13 · Medium
A 55-year-old woman of blood group O is wrongly given a unit of group A red cells in a ward in Kanpur. Within 15 minutes she has fever, chills, flank pain and hypotension, and her urine turns dark red. Which type of hypersensitivity explains this reaction?
1. AType IV, T cell-mediated delayed
2. BType II, antibody-mediated cytotoxic
3. CType III, immune complex-mediated
4. DType I, IgE-mediated immediate
Answer and explanation in LearnFree account
Question 14 · Hard
A 24-year-old woman in Kolkata has a photosensitive malar rash and arthralgia, and has had two episodes of pneumococcal pneumonia. Complement studies: CH50 undetectable, alternative pathway (AH50) activity normal, C3 normal, C4 normal, C1q normal. Which component is most likely deficient?
1. AC8
2. BC3
3. CC2
4. DFactor B
Answer and explanation in LearnFree account
Question 15 · Medium
A paediatric resident in Kottayam explains antibodies to the parents of a 3-month-old exclusively breastfed baby. Consider four statements: 1. Antibodies the baby got across the placenta are of the IgG class. 2. Antibody in breast milk is a dimer joined by a J chain and carries a secretory component. 3. IgE is the most abundant immunoglobulin in serum. 4. IgM is the class most efficient at activating the classical complement pathway. Which statements are correct?
1. A1, 2 and 3 only
2. B1 and 3 only
3. C1, 2 and 4 only
4. D2, 3 and 4 only
Answer and explanation in LearnFree account
Question 16 · Easy
Tetanus toxoid is prepared by treating tetanus toxin with which agent?
1. ABeta-propiolactone
2. BFormaldehyde
3. CEthylene oxide
4. DPhenol
Answer and explanation in LearnFree account
Question 17 · Medium
In a Navi Mumbai slum with a high incidence of typhoid, a 9-month-old girl is brought to the urban health centre for her measles-rubella vaccine. Her father asks for typhoid protection as well. Which typhoid vaccine can be given at her age?
1. AVi capsular polysaccharide vaccine
2. BWhole-cell heat-killed vaccine
3. CTyphoid conjugate vaccine
4. DOral live Ty21a capsules
Answer and explanation in LearnFree account
Question 18 · Hard
A 4-month-old boy in Jodhpur, the child of a consanguineous marriage, has persistent oral thrush, chronic diarrhoea and failure to thrive. A firm, enlarging swelling has appeared at his left axillary nodes on the side of his BCG vaccination. His absolute lymphocyte count is 700/µL and the chest X-ray shows no thymic shadow. Which vaccine due on the UIP schedule must not be given to him?
1. APentavalent vaccine
2. BFractional inactivated polio vaccine
3. CPneumococcal conjugate vaccine
4. DOral rotavirus vaccine
Answer and explanation in LearnFree account
Question 19 · Easy
A 5-year-old boy with acute lymphoblastic leukaemia is on maintenance chemotherapy at a cancer centre in Mumbai. His mother asks whether he can receive the vaccines given in a school catch-up campaign. Which of these vaccines must be withheld while he is on chemotherapy?
1. AMeasles-rubella vaccine
2. BRecombinant hepatitis B vaccine
3. CInactivated poliovirus vaccine
4. DTetanus-diphtheria (Td) vaccine
Answer and explanation in LearnFree account
Question 20 · Medium
A 45-year-old construction worker in Ludhiana has received tetanus toxoid in casualty after almost every minor injury, five times in the past three years. Eight hours after another booster he has a painful, red, swollen and indurated area at the injection site, with a small central area of necrosis. What type of reaction is this?
1. ADelayed-type hypersensitivity (type IV)
2. BArthus reaction (local type III)
3. CAntibody-dependent cytotoxicity (type II)
4. DLocal anaphylaxis (type I)
Answer and explanation in LearnFree account
Question 21 · Hard
A 32-year-old man in Vellore has episodes of dark morning urine, fatigue and a portal vein thrombosis. Hb is 7.8 g/dL, LDH is high, the direct antiglobulin test is negative, and flow cytometry shows red cells and neutrophils lacking CD55 and CD59. The haematologist plans eculizumab. What must be done before the first dose?
1. AVaccinate against Neisseria meningitidis
2. BScreen for latent tuberculosis with IGRA
3. CGive hepatitis B immunoglobulin
4. DGive penicillin prophylaxis alone
Answer and explanation in LearnFree account
Question 22 · Medium
A 19-year-old woman in Kochi has had repeated episodes of painless, non-itchy swelling of her lips and hands, and bouts of colicky abdominal pain, each lasting 2-3 days. An episode began the day after a tooth extraction. Antihistamines have never helped and there is no urticaria. Her father had similar attacks. Which pattern of complement results is expected?
1. ALow C3 and low C4
2. BNormal C4 with low CH50
3. CLow C4 with normal C3
4. DLow C3 with normal C4
Answer and explanation in LearnFree account
Question 23 · Easy
A newborn at a district hospital in Gaya has bilateral cataracts, a continuous murmur of patent ductus arteriosus and a low birth weight. Cord blood total IgM is markedly raised. Why does the raised cord IgM point to infection in utero?
1. AIgM does not cross the placenta
2. BMaternal IgM rises in all pregnancies
3. CIgM is made only after birth
4. DIgM has the longest half-life
Answer and explanation in LearnFree account
Question 24 · Medium
A 24-year-old man in Delhi, who had dengue fever two years ago, is admitted on day 4 of a new febrile illness with platelets of 60,000/µL. NS1 antigen is positive. Dengue IgG ELISA is strongly positive at a high titre, and dengue IgM is weakly positive. How is this antibody pattern best interpreted?
1. AOld dengue with no current infection
2. BSecondary dengue infection
3. CPrimary dengue infection
4. DFalse-positive IgG from cross-reaction
Answer and explanation in LearnFree account
Question 25 · Hard
A medical intern on a ward round in Indore sees four patients and classifies each illness by hypersensitivity type. Consider four statements: 1. A child with nephritis three weeks after streptococcal pharyngitis has a type III reaction. 2. A newborn with haemolytic disease from maternal anti-D has a type II reaction. 3. A woman with eczema at the site of a nickel earring has a type IV reaction. 4. A farmer with fever and breathlessness hours after handling mouldy hay has a type I reaction. Which statements are correct?
1. A1, 2 and 3 only
2. B1 and 2 only
3. C2 and 4 only
4. D1 and 3 only
Answer and explanation in LearnFree account

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Immunity, hypersensitivity and vaccines

A 19-year-old hostel student is admitted for the second time in three years with meningococcal meningitis. He has had no other serious infections, and his immunoglobulin levels are normal. Deficiency of which component is most likely?

A 23-year-old primigravida at a PHC in Bareilly received two doses of Td vaccine during this pregnancy. Her baby is born at home and the cord is cut with an unboiled blade, yet the baby does not develop neonatal tetanus. What protected the baby?

A 34-year-old woman in Bengaluru develops an itchy, oozing, scaly rash along her hairline and forehead two days after using a new black hair dye. She had used the same brand once before without trouble. Which test best confirms the cause?

A 22-year-old college student in Hyderabad has had sneezing, watery rhinorrhoea and itchy eyes every year from February to April for 5 years. She wants to know exactly which pollen she is allergic to, so that immunotherapy can be considered. Which test should be done first?

A 30-year-old man with recurrent sinusitis and giardiasis develops flushing, wheeze and hypotension a few minutes into his first blood transfusion after a road accident in Vijayawada. Serum IgA is undetectable, and IgG and IgM are normal. How should future transfusions be given to him?

At a 6-week immunisation session in a PHC in Dharwad, a mother who is a science teacher asks why the Hib part of the pentavalent vaccine works in such a young infant, when plain polysaccharide vaccines do not work below 2 years of age. What is the reason?

A 5-day-old girl in a NICU in Lucknow has repeated tetany and seizures. Her serum calcium is 6.2 mg/dL with a low PTH. Echocardiography shows truncus arteriosus, and a chest X-ray shows no thymic shadow. She has low-set ears and a short philtrum. Which arm of immunity is chiefly deficient in her?

A 42-year-old farmer in Wayanad is stung by several honeybees. Within 10 minutes he develops generalised urticaria, wheeze and a BP of 80/50 mmHg. Which antibody, bound to his mast cells, mediated this reaction?

A 55-year-old woman of blood group O is wrongly given a unit of group A red cells in a ward in Kanpur. Within 15 minutes she has fever, chills, flank pain and hypotension, and her urine turns dark red. Which type of hypersensitivity explains this reaction?

A 24-year-old woman in Kolkata has a photosensitive malar rash and arthralgia, and has had two episodes of pneumococcal pneumonia. Complement studies: CH50 undetectable, alternative pathway (AH50) activity normal, C3 normal, C4 normal, C1q normal. Which component is most likely deficient?

Tetanus toxoid is prepared by treating tetanus toxin with which agent?

In a Navi Mumbai slum with a high incidence of typhoid, a 9-month-old girl is brought to the urban health centre for her measles-rubella vaccine. Her father asks for typhoid protection as well. Which typhoid vaccine can be given at her age?

A 5-year-old boy with acute lymphoblastic leukaemia is on maintenance chemotherapy at a cancer centre in Mumbai. His mother asks whether he can receive the vaccines given in a school catch-up campaign. Which of these vaccines must be withheld while he is on chemotherapy?

A newborn at a district hospital in Gaya has bilateral cataracts, a continuous murmur of patent ductus arteriosus and a low birth weight. Cord blood total IgM is markedly raised. Why does the raised cord IgM point to infection in utero?