Immunology Practice Questions

Q: Which cells secrete Interferon gamma?

CD4 T cells. **Explanation:** Interferon-gamma (IFN-γ) is a critical cytokine in the cell-mediated immune response, primarily known as **Type II Interferon**. It is produced by specific immune cells to activate macrophages and enhance MHC expression. **1. Why CD4 T cells are correct:** CD4+ T cells, specifically the **Th1 (T-helper 1) subset**, are the primary producers of IFN-γ. Upon stimulation by IL-12 from macrophages, Th1 cells secrete IFN-γ to "classically activate" macrophages (M1 phenotype), increasing their microbicidal activity. While CD8+ T cells and Natural Killer (NK) cells also produce IFN-γ, CD4+ Th1 cells are the classic physiological source emphasized in immunology. **2. Why other options are incorrect:** * **CD8 cells:** While CD8+ T cells can produce IFN-γ, in the context of standard medical examinations, CD4+ Th1 cells are considered the prototypical source. (Note: If "NK cells" were an option, they would also be a major source). * **RBCs:** Red blood cells are non-nucleated cells involved in gas transport; they do not possess the machinery for cytokine synthesis or secretion. * **Neutrophils:** These are primary phagocytes of the innate system. While they respond to cytokines, they are not significant producers of IFN-γ. **High-Yield Clinical Pearls for NEET-PG:** * **Type I Interferons (IFN-α and IFN-β):** Produced by almost all nucleated cells (especially plasmacytoid dendritic cells) in response to viral infections. * **The IFN-γ/IL-12 Axis:** Essential for controlling intracellular pathogens like *Mycobacterium tuberculosis*. A defect in this axis leads to Mendelian Susceptibility to Mycobacterial Diseases (MSMD). * **Granuloma Formation:** IFN-γ is the key cytokine responsible for transforming macrophages into epithelioid cells and multinucleated giant cells in granulomatous inflammation.

Q: Perforins are produced by which of the following cells?

Cytotoxic T cells. **Explanation:** **Correct Answer: A. Cytotoxic T cells** Perforins are pore-forming cytolytic proteins found in the granules of **Cytotoxic T lymphocytes (CD8+ T cells)** and **Natural Killer (NK) cells**. When a Cytotoxic T cell recognizes a target cell (e.g., a virus-infected or tumor cell) via MHC Class I molecules, it releases perforins through exocytosis. These proteins insert themselves into the target cell's plasma membrane, polymerizing to form functional pores. This allows the entry of **granzymes**, which trigger programmed cell death (apoptosis). **Analysis of Incorrect Options:** * **B. Suppressor T cells (Regulatory T cells):** These cells function to modulate the immune system and maintain tolerance; they do not typically utilize the perforin-granzyme pathway for direct cytolysis. * **C. Memory helper T cells:** These are CD4+ cells that "remember" antigens for a faster secondary response. Their primary role is cytokine secretion to coordinate the immune response, not direct cell-mediated killing. * **D. Plasma cells:** These are terminally differentiated B cells responsible for the mass production of **antibodies** (humoral immunity), not cytolytic proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Mechanism:** Cytotoxic T cells kill via two main pathways: the **Perforin-Granzyme pathway** and the **Fas-Fas Ligand (FasL) pathway**. * **NK Cells:** Remember that NK cells also produce perforins and are part of the innate immune system (MHC-independent). * **Calcium Dependency:** The polymerization of perforin to form pores is a calcium-dependent process. * **Clinical Correlation:** Mutations in the perforin gene (*PRF1*) are associated with **Familial Hemophagocytic Lymphohistiocytosis (FHL)**, a life-threatening hyperinflammatory condition.

Q: A 27-year-old male patient with blood group O presents with massive intestinal bleeding and has lost half his blood volume. He is transfused with blood group AB blood. Within minutes, he develops fever, chills, dyspnea, and a dramatic drop in blood pressure. What is the most likely cause of this reaction?

Preformed anti-A and anti-B antibodies in the recipient. **Explanation:** This clinical scenario describes an **Acute Hemolytic Transfusion Reaction (AHTR)**, which is a classic example of a **Type II Hypersensitivity Reaction**. **Why Option C is correct:** Individuals with **Blood Group O** naturally possess preformed, high-titer IgM (and some IgG) antibodies against both A and B antigens (anti-A and anti-B). When blood group AB (which expresses both A and B antigens) is transfused into this patient, these preformed antibodies immediately bind to the donor's red blood cells. This triggers the **classical complement pathway**, leading to rapid intravascular hemolysis, release of inflammatory cytokines, and the clinical triad of fever, hypotension, and potential DIC. **Why other options are incorrect:** * **Option A:** Cell-mediated responses (Type IV Hypersensitivity) are delayed (taking days) and do not involve the immediate antibody-antigen binding seen in ABO incompatibility. * **Option B:** This describes a primary immune response. Production of new IgG takes 7–14 days; it cannot account for a reaction occurring "within minutes." * **Option C vs D:** In a major mismatch, the **recipient’s antibodies** attacking the **donor’s cells** (Option C) is the primary cause of life-threatening hemolysis. While the donor's plasma (Option D) contains no antibodies in this specific case (AB is the universal recipient), even if it did, the volume is usually too diluted to cause such a dramatic systemic collapse. **High-Yield Clinical Pearls for NEET-PG:** * **Universal Donor:** Blood Group O (Negative). * **Universal Recipient:** Blood Group AB (Positive). * **Mechanism:** Type II Hypersensitivity (Complement-mediated lysis). * **First Step in Management:** Stop the transfusion immediately and initiate aggressive IV fluid resuscitation to maintain renal perfusion. * **Commonest cause of AHTR:** Clerical/Administrative error (mislabeled samples).

Q: All of the following agents have polysaccharide capsule related antigen-antibody responses, EXCEPT?

Bordetella Pertussis. **Explanation:** The core concept tested here is the nature of the protective antigens in encapsulated bacteria. **Bordetella pertussis** is the correct answer because it is **not a polysaccharide encapsulated organism** in the context of vaccine-induced immunity. Its primary virulence factors and the antigens used in vaccines (like DTaP/Tdap) are **proteins**, specifically Pertussis Toxin (PT), Filamentous Hemagglutinin (FHA), and Pertactin. **Analysis of Options:** * **A, B, and C (Pneumococcus, Meningococcus, H. influenzae):** These are the classic "encapsulated organisms." Their primary virulence factor is a polysaccharide capsule that inhibits phagocytosis. Immunity against these pathogens is mediated by antibodies (opsonins) directed specifically against these **capsular polysaccharides**. This is why vaccines for these three (e.g., PCV-13, Hib vaccine) are either pure polysaccharide or polysaccharide-protein conjugates. * **D (Bordetella pertussis):** While it may have a glycocalyx, it does not possess a clinical polysaccharide capsule that dictates its serotyping or vaccine strategy. Immunity is directed against its protein exotoxins and adhesion factors. **High-Yield Clinical Pearls for NEET-PG:** * **Quellung Reaction:** Used to identify encapsulated bacteria (Capsular swelling). * **Asplenia Risk:** Patients with sickle cell disease or splenectomy are highly susceptible to the "Big Three" encapsulated organisms (A, B, and C) because the spleen is the primary site for filtering opsonized encapsulated bacteria. * **Conjugate Vaccines:** Polysaccharides are T-cell independent antigens (poor memory). Conjugating them to a protein (like Diphtheria toxoid) converts them to T-cell dependent antigens, making them effective in children <2 years old.

Q: The image shows a cellular finding in a peripheral blood smear. This finding is most characteristic of which condition?

Systemic Lupus Erythematosus. ***Systemic Lupus Erythematosus*** - The image shows an **LE cell** (Lupus Erythematosus cell), which is a neutrophil or macrophage containing an engulfed **hematoxylin body** (nuclear chromatin material). - This finding is pathognomonic for **SLE** and results from anti-nuclear antibodies coating cell nuclei, leading to phagocytosis by neutrophils. *Tuberculosis* - Characterized by **epithelioid cell granulomas** with **Langhans giant cells** and **caseous necrosis** in tissue sections. - Peripheral blood smears do not show specific cellular findings like LE cells, but may show nonspecific changes like anemia. *Leprosy* - Microscopic findings include **acid-fast bacilli** within **foamy macrophages** (Virchow cells) in tissue sections. - Blood smears are typically normal and do not demonstrate the **nuclear chromatin engulfment** seen in LE cells. *Malaria* - Blood smears show **ring-form trophozoites**, **schizonts**, or **gametocytes** within **red blood cells**. - The parasitic forms are intracellular within RBCs, not the **neutrophilic engulfment of nuclear material** characteristic of LE cells.

Q: The Weil-Felix reaction is based on the sharing of antigens between which of the following?

Rickettsia antigens and Proteus antigens. The **Weil-Felix reaction** is a classic example of a **heterophile antibody test**. It is based on the principle of **cross-reactivity**, where antibodies produced during a Rickettsial infection react with specific non-motile strains of *Proteus vulgaris* (OX-19, OX-2) and *Proteus mirabilis* (OX-K). This occurs because these organisms share common alkali-stable carbohydrate antigens. ### Explanation of Options: * **Option C (Correct):** Rickettsial species (except *R. akari*) induce antibodies that agglutinate *Proteus* antigens. For example, Epidemic typhus reacts with OX-19, while Scrub typhus (caused by *Orientia tsutsugamushi*) reacts specifically with OX-K. * **Option A (Incorrect):** The sharing of antigens between **Sheep RBCs and EBV** (Epstein-Barr Virus) is the basis for the **Paul-Bunnell Test**, used to diagnose Infectious Mononucleosis. * **Option B (Incorrect):** The sharing of antigens between **Mycoplasma pneumoniae and human RBCs** (specifically the I-antigen) leads to the production of **Cold Agglutinins**, which can cause autoimmune hemolytic anemia. ### High-Yield Clinical Pearls for NEET-PG: 1. **Specific Strains:** * **OX-19 & OX-2:** Positive in Typhus group and Spotted Fever group. * **OX-K:** Positive only in **Scrub Typhus**. 2. **Negative Weil-Felix:** It is important to remember that **Rickettsialpox** (*R. akari*) and **Q Fever** (*Coxiella burnetii*) show a **negative** Weil-Felix reaction. 3. **Current Status:** While historically significant, it is now largely replaced by more sensitive tests like Indirect Immunofluorescence (IFA).

Q: Which ion controls the activity of the classical complement pathway (C1)?

Calcium. **Explanation:** The **Classical Pathway** of the complement system is initiated by the binding of the C1 complex to antigen-antibody complexes (IgM or IgG). The C1 complex is a large, multi-subunit macromolecule consisting of one molecule of **C1q** and two molecules each of **C1r** and **C1s** ($C1q r_2 s_2$). **Why Calcium is the correct answer:** The structural integrity and catalytic activity of the C1 complex are strictly **Calcium ($Ca^{2+}$) dependent**. Calcium ions are required to hold the C1q, C1r, and C1s subunits together. In the absence of Calcium (e.g., when using chelating agents like EDTA), the C1 complex dissociates, thereby inactivating the classical pathway. **Analysis of Incorrect Options:** * **Magnesium ($Mg^{2+}$):** While Magnesium is essential for the complement system, it specifically stabilizes the **C3 convertase** of the **Alternative Pathway** ($C3bBb$) and the formation of the C3 convertase in the Classical/Lectin pathways ($C4b2a$). * **Manganese and Phosphate:** These ions do not play a primary regulatory or structural role in the activation of the complement cascades. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **C**lassical = **C**alcium; **A**lternative = **M**agnesium (**C-C; A-M**). * **EDTA vs. Heparin:** In laboratory medicine, EDTA prevents complement activation by chelating both $Ca^{2+}$ and $Mg^{2+}$, whereas Heparin inhibits the formation of the Membrane Attack Complex (MAC). * **C1 Esterase Inhibitor:** Deficiency of this regulatory protein leads to **Hereditary Angioedema**, characterized by uncontrolled activation of the classical pathway and the kinin system. * **C1q** is the recognition unit, while **C1s** carries the enzymatic (esterase) activity.

Question 1

Which cells secrete Interferon gamma?

Accepted Answer

CD4 T cells

Answer

CD8 cells

Answer

RBCs

Answer

Neutrophils

Question 2

What reaction is due to the lysis of bacterial cell wall and necrotic cell products?

Accepted Answer

Jarisch-Herxheimer reaction

Answer

Arthus reaction

Answer

Serum sickness

Answer

Infectious mononucleosis-ampicillin reaction

Question 3

Perforins are produced by which of the following cells?

Accepted Answer

Cytotoxic T cells

Answer

Suppressor T cells

Answer

Memory helper T cells

Answer

Plasma cells

Question 4

Which one of the following statements best describes immunological tolerance?

Accepted Answer

It is prolonged by administration of immunosuppressive drugs.

Answer

Immunologic maturity of the host does not play a major role.

Answer

It occurs only with polysaccharide antigens.

Answer

It is related to the concentration of antibody.

Question 5

A 27-year-old male patient with blood group O presents with massive intestinal bleeding and has lost half his blood volume. He is transfused with blood group AB blood. Within minutes, he develops fever, chills, dyspnea, and a dramatic drop in blood pressure. What is the most likely cause of this reaction?

Accepted Answer

Preformed anti-A and anti-B antibodies in the recipient

Answer

A cell-mediated response against AB antigens

Answer

IgG production by the recipient in response to AB antigens

Answer

Preformed anti-A and anti-B antibodies of the blood donor

Question 6

All are true regarding superantigens except?

Accepted Answer

Activate very large numbers of T cells

Answer

Bind outside the antibody binding groove

Answer

Are medium sized proteins

Answer

Cause release of cytokines

Question 7

All of the following agents have polysaccharide capsule related antigen-antibody responses, EXCEPT?

Accepted Answer

Bordetella Pertussis

Answer

Pneumococcus

Answer

Meningococcus

Answer

Haemophilus Influenza

Question 8

The image shows a cellular finding in a peripheral blood smear. This finding is most characteristic of which condition?

Accepted Answer

Systemic Lupus Erythematosus

Answer

Tuberculosis

Answer

Leprosy

Answer

Malaria

Question 9

The Weil-Felix reaction is based on the sharing of antigens between which of the following?

Accepted Answer

Rickettsia antigens and Proteus antigens

Answer

Sheep RBC and EBV

Answer

Mycoplasma and human RBC

Answer

None of the above

Question 10

Which ion controls the activity of the classical complement pathway (C1)?

Accepted Answer

Calcium

Answer

Magnesium

Answer

Manganese

Answer

Phosphate

Immunology — MCQs

Immunology — MCQs

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