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Microbiology and Immunology

Host immune response Practice Questions

20 free Host immune response practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 20 Medium

A patient is infected with an intracellular pathogen. Which immune component is most important for clearing this infection?

  1. A Neutrophils
  2. B CD8+ T cells
  3. C IgA antibodies
  4. D Basophils

Correct answer: CD8+ T cells

CD8+ cytotoxic T cells kill infected host cells harboring intracellular pathogens. This cell-mediated response is essential for viral control.

Question 2 of 20 Medium

A newborn receives maternal IgG antibodies through the placenta. What type of immunity does this represent?

  1. A Active natural immunity
  2. B Passive natural immunity
  3. C Active artificial immunity
  4. D Passive artificial immunity

Correct answer: Passive natural immunity

Maternal IgG crosses the placenta, providing passive natural immunity. The infant receives antibodies but does not generate them.

Question 3 of 20 Medium

A patient develops fever and neutrophilia during acute bacterial infection. Which cytokine is primarily responsible for neutrophil recruitment?

  1. A IL-2
  2. B IL-8
  3. C IL-4
  4. D TNF-beta

Correct answer: IL-8

IL-8 is a chemokine that attracts and activates neutrophils. It plays a key role in acute inflammation and bacterial clearance.

Question 4 of 20 Medium

Which immunoglobulin is predominantly produced during a secondary immune response?

  1. A IgM
  2. B IgD
  3. C IgE
  4. D IgG

Correct answer: IgG

Memory B cells formed during the primary response have already undergone class switching, so on re-exposure they secrete IgG rapidly and in far greater quantity than the IgM that dominates a primary response. Affinity maturation also leaves that IgG binding the antigen more tightly.

Question 5 of 20 Medium

Complement activation via the classical pathway requires which event?

  1. A MBL binding to mannose
  2. B Spontaneous C3 hydrolysis
  3. C Antigen–antibody complexes
  4. D C3b binding to pathogen surfaces

Correct answer: Antigen–antibody complexes

The classical pathway is triggered by IgG or IgM bound to antigens. This initiates complement cascade activation.

Question 6 of 20 Medium

A patient with parasitic worm infection shows elevated eosinophil levels. Which immunoglobulin drives eosinophil activation?

  1. A IgA
  2. B IgM
  3. C IgE
  4. D IgG

Correct answer: IgE

IgE binds to eosinophils, facilitating degranulation and parasite killing. This is characteristic of helminth responses.

Question 7 of 20 Medium

Natural killer (NK) cells primarily kill which type of cells?

  1. A Cells expressing high MHC I
  2. B Cells lacking MHC I
  3. C Neutrophils
  4. D Mast cells

Correct answer: Cells lacking MHC I

NK cells target cells with reduced or absent MHC I expression, typical of virus-infected or tumor cells.

Question 8 of 20 Medium

Which cytokine is most important for inducing fever during infection?

  1. A IL-1
  2. B IL-5
  3. C IL-10
  4. D IL-13

Correct answer: IL-1

IL-1 acts on the hypothalamus to increase prostaglandin E2, elevating body temperature. It is a potent endogenous pyrogen.

Question 9 of 20 Medium

A patient with recurrent Neisseria infections is found to have a complement deficiency. Which component is most likely deficient?

  1. A C1 inhibitor
  2. B C5-C9 components
  3. C Factor H
  4. D C3a

Correct answer: C5-C9 components

C5–C9 deficiencies impair the membrane attack complex, predisposing to Neisseria infections due to reduced bacterial lysis.

Question 10 of 20 Medium

Which immune cells are essential for granuloma formation in tuberculosis?

  1. A B cells and plasma cells
  2. B CD4+ Th1 cells and macrophages
  3. C Neutrophils only
  4. D Basophils and dendritic cells

Correct answer: CD4+ Th1 cells and macrophages

Th1 cells activate macrophages, leading to granuloma formation that walls off Mycobacterium tuberculosis.

Question 11 of 20 Medium

A patient receives a booster vaccine and mounts a rapid, robust immune response. Which cells are primarily responsible?

  1. A Naive B cells
  2. B Memory B cells
  3. C Neutrophils
  4. D Eosinophils

Correct answer: Memory B cells

Memory B cells rapidly proliferate and produce high-affinity antibodies upon re-exposure. This characterizes the secondary response.

Question 12 of 20 Medium

Type I interferons (IFN-α, IFN-β) help control viral infections by which mechanism?

  1. A Stimulating mucosal IgA production
  2. B Inhibiting viral protein synthesis
  3. C Increasing neutrophil chemotaxis
  4. D Activating eosinophil degranulation

Correct answer: Inhibiting viral protein synthesis

Type I interferons induce an antiviral state by blocking protein synthesis in neighboring cells. This limits viral spread.

Question 13 of 20 Medium

Which immunoglobulin is most effective at neutralizing pathogens on mucosal surfaces?

  1. A IgG
  2. B IgM
  3. C IgA
  4. D IgE

Correct answer: IgA

Dimeric IgA is secreted across mucosal surfaces. It prevents pathogen attachment and invasion.

Question 14 of 20 Medium

A patient with CGD (chronic granulomatous disease) has impaired killing of catalase-positive organisms. Which enzyme is deficient?

  1. A Myeloperoxidase
  2. B Superoxide dismutase
  3. C NADPH oxidase
  4. D Catalase

Correct answer: NADPH oxidase

CGD results from defective NADPH oxidase, preventing respiratory burst. Catalase-positive organisms neutralize any remaining peroxide.

Question 15 of 20 Medium

Which cytokine stimulates class switching to IgE in allergic responses?

  1. A IL-2
  2. B IL-4
  3. C IL-17
  4. D TNF-α

Correct answer: IL-4

IL-4 promotes B-cell class switching to IgE. It is central to type I hypersensitivity and Th2 responses.

Question 16 of 20 Medium

Which pattern recognition receptor detects LPS from Gram-negative bacteria?

  1. A TLR3
  2. B TLR4
  3. C TLR7
  4. D TLR9

Correct answer: TLR4

TLR4 binds LPS and activates innate immune responses. It triggers inflammatory cytokine production.

Question 17 of 20 Medium

A patient with viral hepatitis develops hepatocyte apoptosis mediated by perforin and granzyme. Which cells are responsible?

  1. A Plasma B cells
  2. B CD8+ T cells
  3. C Mast cells
  4. D Macrophages

Correct answer: CD8+ T cells

CD8+ cytotoxic T cells induce apoptosis via perforin/granzyme pathways. They target virus-infected hepatocytes.

Question 18 of 20 Medium

Which cell type presents antigen on MHC II molecules to CD4+ T cells?

  1. A All nucleated cells
  2. B Macrophages
  3. C Hepatocytes
  4. D Erythrocytes

Correct answer: Macrophages

Macrophages and other APCs express MHC II. They present extracellular antigens to CD4+ T cells.

Question 19 of 20 Medium

A patient with helminth infection shows elevated IL-5. Which function does IL-5 primarily support?

  1. A Macrophage activation and killing
  2. B Eosinophil growth
  3. C Th1 lymphocyte differentiation
  4. D NK cell inhibition

Correct answer: Eosinophil growth

IL-5 is essential for eosinophil maturation and activation. Eosinophils help kill parasites via toxic granule release.

Question 20 of 20 Medium

A patient vaccinated with a live attenuated virus develops long-lasting immunity. What is a key advantage of live vaccines?

  1. A They require repeated booster doses for protection
  2. B They stimulate only short-lived humoral immunity
  3. C They induce strong cellular and mucosal immunity
  4. D They cannot cause disease in immunocompromised hosts

Correct answer: They induce strong cellular and mucosal immunity

Live vaccines mimic natural infection and stimulate robust cellular, humoral, and mucosal immunity. This leads to long-lasting protection.

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