Immunology Practice Questions

Q: A 45-year-old homeless man presents with chronic cough and a cavitary lesion of the lungs. His sputum is positive for acid-fast bacilli. Which of the following represents the primary peripheral form of defense by which his body fights this infection?

Cell-mediated hypersensitivity. **Explanation:** The clinical presentation of chronic cough, cavitary lung lesions, and acid-fast bacilli (AFB) in sputum is diagnostic of **Pulmonary Tuberculosis** caused by *Mycobacterium tuberculosis*. **1. Why the correct answer is right:** *Mycobacterium tuberculosis* is an **obligate intracellular pathogen** that survives within macrophages. Because the bacteria are sequestered inside cells, humoral immunity (antibodies) cannot reach them. The body relies on **Type IV Hypersensitivity (Delayed-type/Cell-mediated hypersensitivity)**. In this process, CD4+ T-cells (Th1) recognize mycobacterial antigens and release **Interferon-gamma (IFN-γ)**, which activates macrophages to kill the intracellular bacteria. The formation of a granuloma (the "cavitary lesion") is the hallmark of this cell-mediated immune response. **2. Why incorrect options are wrong:** * **Option A:** Antibody-mediated immunity (Humoral) is effective against extracellular bacteria and toxins. It plays a negligible role in controlling TB. * **Option C:** IgA is primarily involved in mucosal immunity (preventing attachment at surfaces), but it does not clear an established intracellular infection. * **Option D:** IgE-mediated hypersensitivity (Type I) is involved in allergic reactions (asthma, anaphylaxis) and defense against helminthic parasites. **3. NEET-PG High-Yield Pearls:** * **Key Cytokine:** IFN-γ is the most critical cytokine for activating macrophages in TB. * **Diagnostic Test:** The Mantoux (PPD) test is a classic clinical example of a Type IV hypersensitivity reaction. * **Histology:** Look for **Langhans giant cells** (fused activated macrophages) and **caseating necrosis** in TB granulomas. * **Gold Standard:** Culture on **Lowenstein-Jensen (LJ) medium** remains the traditional gold standard, though NAAT (CBNAAT/GeneXpert) is the preferred initial test.

Q: A 3-year-old boy presented with recurrent attacks of pneumonia and otitis media since his first birthday. Diagnosis is X-linked agammaglobulinemia. What is the most likely offending organism?

Streptococcus pneumoniae. **Explanation:** **X-linked Agammaglobulinemia (Bruton’s Disease)** is a primary immunodeficiency caused by a mutation in the **BTK gene**, leading to a failure of B-cell maturation. This results in a profound deficiency of all immunoglobulin classes (IgG, IgA, IgM). **1. Why Streptococcus pneumoniae is correct:** Antibodies (immunoglobulins) are essential for the **opsonization** and clearance of **encapsulated bacteria**. Patients with B-cell defects are highly susceptible to recurrent infections with pyogenic encapsulated organisms. *Streptococcus pneumoniae* is the most common cause of pneumonia and otitis media in these patients. Other common pathogens include *Haemophilus influenzae* type b and *Staphylococcus aureus*. **2. Why incorrect options are wrong:** * **CMV (Cytomegalovirus):** This is a viral pathogen. Defense against viruses primarily relies on **T-cell (cell-mediated) immunity**. CMV is more common in SCID or HIV, not isolated B-cell defects. * **Mycoplasma:** While *Mycoplasma* can cause chronic lung disease in XLA, it is less common than pyogenic bacteria for acute recurrent pneumonia. * **Pneumocystis jiroveci:** This is an opportunistic fungus characteristic of **T-cell deficiencies** (e.g., SCID, HIV). In XLA, T-cell function remains intact, providing protection against *P. jiroveci*. **Clinical Pearls for NEET-PG:** * **Timing:** Symptoms typically start after **6 months of age**, once maternal IgG wanes. * **Physical Exam:** Characteristically shows **absent or hypoplastic tonsils** and lymph nodes (due to lack of germinal centers). * **Diagnosis:** Flow cytometry shows **absent B-cells (CD19+/CD20+)** with normal T-cell counts. * **Associated Risk:** These patients are also susceptible to **Enteroviruses** (e.g., Echovirus, Poliovirus), which can cause chronic meningoencephalitis. Live viral vaccines (like OPV) are contraindicated.

Q: Which of the following is a generic term for a protein or glycoprotein released by one cell population that acts as an intercellular mediator?

Cytokine. ### Explanation **Correct Answer: B. Cytokine** **Why it is correct:** Cytokines are low-molecular-weight, soluble **proteins or glycoproteins** secreted by various cell populations (primarily immune cells like macrophages and T-cells) that act as **intercellular mediators**. They function as signaling molecules that regulate the intensity and duration of the immune response by binding to specific receptors on target cells. They can act in an autocrine (same cell), paracrine (nearby cell), or endocrine (distant cell) fashion. **Analysis of Incorrect Options:** * **A. Hormone:** While hormones are also intercellular mediators, they are typically produced by specialized endocrine glands and travel through the bloodstream to act on distant organs. Unlike cytokines, which are produced by many cell types, hormones have specific glandular origins. * **C. Pheromone:** These are chemical substances produced and released into the **environment** by an animal to affect the behavior or physiology of others of its species. They do not act as internal mediators within a single organism. * **D. Antibody (Immunoglobulin):** These are proteins produced specifically by B-lymphocytes (plasma cells) in response to an antigen. Their primary role is to identify and neutralize foreign objects (like bacteria and viruses) rather than acting as general intercellular signaling mediators. **High-Yield Clinical Pearls for NEET-PG:** * **Pleiotropy:** A single cytokine having different biological effects on different target cells (e.g., IL-4 acting on B-cells, T-cells, and mast cells). * **Redundancy:** Multiple cytokines mediating the same function (e.g., IL-2, IL-4, and IL-5 all stimulating B-cell proliferation). * **Cytokine Storm:** An overproduction of cytokines (like IL-6, TNF-α) leading to systemic inflammation, often seen in severe COVID-19 or Sepsis. * **Key Cytokine:** **IL-1** is the endogenous pyrogen responsible for fever.

Q: MHC class I molecules are represented on which of the following cell types?

All nucleated cells. ### Explanation **1. Why "All nucleated cells" is correct:** Major Histocompatibility Complex (MHC) Class I molecules are essential for the immune system to distinguish "self" from "non-self." They are expressed on the surface of **all nucleated cells** and **platelets**. Their primary function is to present endogenous antigens (like viral proteins or tumor antigens) to **CD8+ Cytotoxic T-cells**. Since any nucleated cell in the body is susceptible to viral infection or malignant transformation, they must all possess the machinery to signal the immune system via MHC I. **2. Why the other options are incorrect:** * **Option A (All cells):** This is technically incorrect because it would include Red Blood Cells (RBCs). * **Option C (RBCs):** Mature human RBCs lack a nucleus and the necessary organelles (like the endoplasmic reticulum) to synthesize and express MHC molecules. This is clinically significant as it prevents RBCs from being targeted by cytotoxic T-cells, though it also means they cannot present malarial antigens in the same way. * **Option D (None):** Incorrect, as MHC I expression is a fundamental feature of vertebrate immunity. **3. High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I Structure:** Consists of an **alpha (α) heavy chain** (encoded on Chromosome 6) and a **β2-microglobulin** light chain (encoded on Chromosome 15). * **MHC Class II:** Expressed only on **Professional Antigen-Presenting Cells (APCs)**—namely Dendritic cells, Macrophages, and B-cells. They present exogenous antigens to **CD4+ Helper T-cells**. * **Rule of 8:** * MHC **I** × CD**8** = 8 * MHC **II** × CD**4** = 8 * **Exception:** While platelets are anucleated, they are a notable exception and **do** express MHC Class I.

Q: Complement attaches to immunoglobulin at which part?

Fc fragment. ### Explanation **1. Why the Correct Answer is Right (Fc Fragment):** The complement system (specifically the classical pathway) is initiated when the **C1q** component binds to the **CH2 domain** of IgG or the **CH3 domain** of IgM. These domains are located within the **Fc (Fragment crystallizable)** portion of the antibody. The Fc region is responsible for the biological effector functions of immunoglobulins, such as opsonization, placental transfer, and complement fixation, rather than antigen binding. **2. Why the Other Options are Wrong:** * **A. Aminoterminal:** This is the end of the polypeptide chain where the variable regions are located. It is involved in antigen recognition, not complement binding. * **B. Fab region (Fragment antigen-binding):** This region consists of one constant and one variable domain of each of the heavy and light chains. Its primary role is to bind specifically to the antigen. * **C. Variable region:** Located within the Fab region, this area contains the paratope (antigen-binding site). It determines the idiotype of the antibody and does not interact with complement proteins. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Complement Fixation Order:** IgM is the most potent activator of the classical pathway (due to its pentameric structure), followed by IgG3 > IgG1 > IgG2. **IgG4, IgA, IgD, and IgE** do not activate the classical pathway. * **Specific Binding Site:** For IgG, the specific binding site for C1q is the **CH2** domain. For IgM, it is the **CH3** domain. * **Mnemonic:** "C" for **C**omplement binds to the **C**onstant region (**Fc**). * **Alternative Pathway:** Does not require antibodies for activation; it is triggered directly by microbial surfaces (e.g., Endotoxin/LPS).

Q: Which of the following organisms does NOT produce a superantigen?

Enterococcus fecalis. **Explanation:** The core concept tested here is the mechanism of **Superantigens (SAgs)**. Unlike conventional antigens, superantigens do not undergo intracellular processing. Instead, they bind directly to the **MHC class II** molecules on Antigen-Presenting Cells (APCs) and the **Vβ chain of T-cell receptors (TCR)**. This bypasses the specificity of the immune response, leading to the massive activation of up to 20% of the body’s T-cells and a subsequent "cytokine storm" (IFN-γ, IL-1, IL-6, and TNF-α). * **Why Enterococcus faecalis is correct:** While *E. faecalis* is a significant human pathogen causing UTIs, endocarditis, and biliary tract infections, it is **not** known to produce superantigens. Its virulence factors primarily include surface proteins (aggregation substance), gelatinase, and cytolysin. * **Why the other options are incorrect:** * **Mycoplasma arthritidis:** Produces the *M. arthritidis* mitogen (MAM), a well-documented superantigen. * **Enterotoxin A (Staphylococcal):** Produced by *Staphylococcus aureus*, this is a classic superantigen responsible for staphylococcal food poisoning and toxic shock-like symptoms. * **Malassezia furfur:** This fungus produces superantigens that contribute to the inflammatory response in conditions like seborrheic dermatitis and atopic dermatitis. **High-Yield Clinical Pearls for NEET-PG:** 1. **Classic Examples of SAgs:** TSST-1 (*S. aureus*), Pyrogenic exotoxin A and C (*S. pyogenes*), and Exfoliative toxin (Scalded Skin Syndrome). 2. **Viral Superantigens:** Epstein-Barr Virus (EBV), Cytomegalovirus (CMV), and Rabies virus nucleocapsid. 3. **Key Feature:** SAgs bind to the **variable region of the beta chain (Vβ)** of the T-cell receptor. 4. **Result:** Massive release of **TNF-α and IL-1**, leading to shock and multi-organ failure.

Q: Which of the following is NOT an antigen-presenting cell?

Astrocytes. **Explanation:** Antigen-presenting cells (APCs) are specialized cells that process and present antigens via Major Histocompatibility Complex (MHC) molecules to T-cells. They are broadly categorized into **Professional APCs** (Dendritic cells, Macrophages, B-cells) and **Non-professional APCs** (various tissue cells). **Why Astrocytes are the correct answer:** While the central nervous system (CNS) was historically considered "immunologically privileged," it does contain APCs. However, **Microglia** are the primary resident professional APCs of the brain. **Astrocytes** are supportive glial cells; while they can express MHC-II under extreme inflammatory conditions in vitro, they are generally **not** considered functional APCs in a physiological or standard clinical context. In the context of NEET-PG, they are the least likely to function as APCs compared to the other options. **Analysis of other options:** * **Langerhans cells:** These are professional APCs (dendritic cells) found in the stratum spinosum of the epidermis. They are the most potent stimulators of naive T-cells. * **Endothelial cells:** These are well-recognized non-professional APCs. They express MHC-I and can be induced to express MHC-II, allowing them to present antigens to circulating T-lymphocytes. * **Epithelial cells:** Certain epithelial cells (e.g., thymic epithelial cells or intestinal epithelial cells) act as non-professional APCs to maintain local immune homeostasis. **High-Yield Clinical Pearls for NEET-PG:** * **Professional APCs:** Dendritic cells (most potent), Macrophages, and B-cells. They express **MHC-II** constitutively. * **MHC Restriction:** CD4+ T-cells recognize antigens with MHC-II; CD8+ T-cells recognize antigens with MHC-I. * **Langerhans Cells:** Contain characteristic **Birbeck granules** (tennis-racket shaped) on electron microscopy and are positive for **S-100 and CD1a**.

Q: What is the important step played by Interleukin in the activation of naive CD4+ T cells and initiation of cell-mediated immune response?

Interleukin-2. **Explanation:** The activation of naive CD4+ T cells is a two-signal process. Signal 1 involves the TCR-MHC II interaction, and Signal 2 involves costimulation (B7-CD28). Once these signals are received, the T cell undergoes **clonal expansion and differentiation**, a process driven primarily by **Interleukin-2 (IL-2)**. **Why Interleukin-2 is the correct answer:** IL-2 is known as the **T-cell growth factor**. Upon activation, the naive T cell begins to secrete IL-2 and simultaneously expresses the high-affinity IL-2 receptor (CD25). This creates an **autocrine loop** where IL-2 binds back to the same cell, triggering rapid cell division (proliferation) and survival. This step is essential for the initiation of a robust cell-mediated immune response. **Analysis of Incorrect Options:** * **Interleukin-1 (IL-1):** Produced by macrophages; it acts as a pyrogen and a pro-inflammatory mediator but is not the primary driver of T-cell clonal expansion. * **Interleukin-3 (IL-3):** Acts as a colony-stimulating factor (CSF) that supports the growth and differentiation of hematopoietic stem cells in the bone marrow. * **Interleukin-4 (IL-4):** Primarily involved in the differentiation of naive T cells into **Th2 cells** and promotes B-cell class switching to IgE; it is not the universal "activation" interleukin for all CD4+ T cells. **High-Yield NEET-PG Pearls:** * **CD25** is the alpha chain of the IL-2 receptor; it is a marker for activated T cells and Regulatory T cells (Tregs). * **Cyclosporine and Tacrolimus** (immunosuppressants) work by inhibiting calcineurin, which prevents the transcription of IL-2, thereby halting T-cell activation. * **Sirolimus (Rapamycin)** acts later by inhibiting the mTOR pathway, blocking the T-cell response *to* IL-2.

Question 1

A 45-year-old homeless man presents with chronic cough and a cavitary lesion of the lungs. His sputum is positive for acid-fast bacilli. Which of the following represents the primary peripheral form of defense by which his body fights this infection?

Accepted Answer

Cell-mediated hypersensitivity

Answer

Antibody-mediated immunity

Answer

IgA-mediated hypersensitivity

Answer

IgE-mediated hypersensitivity

Question 2

A 3-year-old boy presented with recurrent attacks of pneumonia and otitis media since his first birthday. Diagnosis is X-linked agammaglobulinemia. What is the most likely offending organism?

Accepted Answer

Streptococcus pneumoniae

Answer

CMV

Answer

Mycoplasma

Answer

Pneumocystis jiroveci

Question 3

Which of the following is a generic term for a protein or glycoprotein released by one cell population that acts as an intercellular mediator?

Accepted Answer

Cytokine

Answer

Hormone

Answer

Pheromone

Answer

Antibody

Question 4

MHC class I molecules are represented on which of the following cell types?

Accepted Answer

All nucleated cells

Answer

All cells

Answer

RBCs

Answer

None

Question 5

Complement attaches to immunoglobulin at which part?

Accepted Answer

Fc fragment

Answer

Aminoterminal

Answer

Fab region

Answer

Variable region

Question 6

Complement is a series of important host proteins which provide protection from invasion by foreign microorganisms. Which one of the following statements best describes complement?

Accepted Answer

Complement protects the host from pneumococcal and Haemophilus infection through complement components C1, C2, and C4

Answer

Complement inhibits phagocytosis

Answer

Microorganisms agglutinate in the presence of complement but do not lyse

Answer

Complement plays a minor role in the inflammatory response

Question 7

Which of the following organisms does NOT produce a superantigen?

Accepted Answer

Enterococcus fecalis

Answer

Mycoplasma arthridis

Answer

Enterotoxin A of food poisoning

Answer

Malassezia furfur

Question 8

Which of the following is NOT an antigen-presenting cell?

Accepted Answer

Astrocytes

Answer

Endothelial cells

Answer

Epithelial cells

Answer

Langerhans cells

Question 9

MHC restriction is a part of all except?

Accepted Answer

Autoimmune disorder

Answer

Antiviral cytotoxic T cell

Answer

Antibacterial helper T cell/cytotoxic cells

Answer

Allograft rejection

Question 10

What is the important step played by Interleukin in the activation of naive CD4+ T cells and initiation of cell-mediated immune response?

Accepted Answer

Interleukin-2

Answer

Interleukin-1

Answer

Interleukin-3

Answer

Interleukin-4

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?