Immunology Practice Questions

Q: A 45-year-old businesswoman arrives with vague abdominal complaints and has noticed melenic stool. A sigmoidoscopy reveals a 4-cm mass in the upper colon. Which tumor marker should be ordered?

Carcinoembryonic antigen (CEA). **Explanation:** The clinical presentation of a 45-year-old with vague abdominal symptoms, melena (indicating gastrointestinal bleeding), and a 4-cm colonic mass is highly suggestive of **Colorectal Carcinoma (CRC)**. **Why Carcinoembryonic Antigen (CEA) is correct:** CEA is an oncofetal glycoprotein normally produced during fetal development and found in very low levels in adults. It is the most widely used tumor marker for **adenocarcinomas of the colon**. While it lacks the sensitivity and specificity required for primary screening, it is the gold standard for **monitoring treatment response** and detecting **post-operative recurrence** in CRC patients. **Analysis of Incorrect Options:** * **A. $\alpha$-Fetoprotein (AFP):** This is the primary marker for **Hepatocellular Carcinoma (HCC)** and non-seminomatous germ cell tumors (e.g., Yolk sac tumors). * **B. Anti-tumor antibody:** While the immune system may produce antibodies against tumor antigens, these are not standardized or clinically used as diagnostic markers for colon cancer. * **C. Antitumor light chains:** This likely refers to Bence-Jones proteins (monoclonal free light chains), which are diagnostic markers for **Multiple Myeloma**, not solid gastrointestinal tumors. **NEET-PG High-Yield Pearls:** * **CEA** is also elevated in pancreatic, gastric, and breast cancers, as well as in smokers and patients with inflammatory bowel disease (IBD). * **CA 19-9:** Marker for Pancreatic and Cholangiocarcinoma. * **CA-125:** Marker for Ovarian cancer. * **PSA:** Marker for Prostate cancer. * **Rule of Thumb:** Tumor markers are generally used for **prognosis and monitoring**, not for definitive diagnosis (which requires biopsy).

Q: What is the function of T-lymphocytes?

All of the above. **Explanation:** T-lymphocytes (T-cells) are the primary mediators of **Cell-Mediated Immunity (CMI)**. They originate in the bone marrow and mature in the thymus, playing a central role in orchestrating the immune response. 1. **Lymphokine Production:** Upon activation by antigens presented by MHC molecules, T-helper (CD4+) cells secrete cytokines known as **lymphokines** (e.g., IL-2, IL-4, IFN-γ). These chemical messengers coordinate the activities of B-cells, macrophages, and cytotoxic T-cells. 2. **Production of Interferon:** Specifically, Th1 cells produce **Interferon-gamma (IFN-γ)**. This is a potent activator of macrophages and is crucial for controlling intracellular pathogens like *Mycobacterium tuberculosis*. 3. **Rosette Formation:** This is a classic laboratory characteristic. T-cells possess **CD2 receptors** (LFA-2) that bind to LFA-3 on sheep erythrocytes. When mixed, sheep red blood cells surround the T-cell, forming a "flower-like" pattern known as an **E-rosette**. While largely replaced by flow cytometry, it remains a high-yield diagnostic fact for identifying T-cells in exams. Since T-lymphocytes perform all these functions, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **CD3** is the definitive pan-T cell marker. * **MHC Restriction:** CD4+ cells recognize antigens with MHC II; CD8+ cells recognize antigens with MHC I (Rule of 8: 4×2=8 and 8×1=8). * **Th1 vs Th2:** Th1 cells (IFN-γ, IL-2) drive CMI; Th2 cells (IL-4, IL-5, IL-10) drive humoral immunity and allergic responses. * **Delayed-Type Hypersensitivity (Type IV):** This is primarily mediated by T-lymphocytes and macrophages.

Q: Which immunoglobulin has a pentameric structure?

IgM. **Explanation:** **IgM** is the correct answer because it is the only immunoglobulin that primarily exists as a **pentamer** in its secreted form. It consists of five basic H2L2 units held together by disulfide bonds and a specialized polypeptide called the **J-chain** (Joining chain). Due to its pentameric structure, it has 10 antigen-binding sites, giving it the highest **valency** and making it highly efficient at agglutination and complement activation via the classical pathway. **Analysis of Incorrect Options:** * **IgG:** The most abundant antibody in serum, it exists strictly as a **monomer**. It is the only antibody that crosses the placenta. * **IgA:** Primarily found in secretions (tears, saliva, colostrum). While it is a monomer in serum, **secretory IgA** exists as a **dimer** (connected by a J-chain and a secretory component). * **IgD:** Found on the surface of B-cells, it exists only as a **monomer** and acts as an antigen receptor. **High-Yield Clinical Pearls for NEET-PG:** * **First Responder:** IgM is the first antibody to appear in response to an initial exposure to an antigen (Primary Immune Response). * **Infection Marker:** Presence of specific IgM indicates a **recent/acute infection**, whereas IgG indicates past infection or chronic state. * **Molecular Weight:** IgM is the largest immunoglobulin ("Millionaire Molecule"), preventing it from crossing the placental barrier. * **Isohemagglutinins:** Naturally occurring anti-A and anti-B antibodies in the ABO blood group system are of the IgM class.

Q: Which cells are primarily involved in humoral immunity?

B-cells. **Explanation:** **Humoral immunity** is the aspect of the adaptive immune response mediated by macromolecules found in extracellular fluids (humors), primarily **antibodies**. **Why B-cells are correct:** B-cells are the primary mediators of humoral immunity. Upon encountering a specific antigen and receiving signals from T-helper cells, B-cells differentiate into **plasma cells**. These plasma cells act as "antibody factories," secreting immunoglobulins (IgG, IgM, IgA, IgE, IgD) that neutralize toxins, opsonize pathogens, and activate the complement system. **Why the other options are incorrect:** * **T-cells:** These are the primary mediators of **Cell-Mediated Immunity (CMI)**. They do not produce antibodies but instead destroy infected cells directly or coordinate the immune response. * **Helper cells (CD4+ T-cells):** While they are essential for activating B-cells (T-dependent response), they belong to the cell-mediated arm. They function by secreting cytokines rather than producing antibodies. * **Dendritic cells:** These are professional **Antigen-Presenting Cells (APCs)**. Their primary role is to bridge innate and adaptive immunity by capturing antigens and presenting them to T-cells in the lymph nodes. **NEET-PG High-Yield Pearls:** * **Origin:** Both B and T cells originate in the bone marrow, but B-cells mature in the **bone marrow**, while T-cells mature in the **thymus**. * **Markers:** CD19, CD20, and CD21 are characteristic surface markers for B-cells. * **Memory:** Humoral immunity provides long-term protection through **Memory B-cells**, which is the principle behind most vaccinations. * **Bursa of Fabricius:** In birds, B-cells mature here (hence the name "B" cell); in humans, the functional equivalent is the bone marrow.

Q: Which of the following is a chemoattractant?

C5a. **Explanation:** The complement system is a crucial component of innate immunity, consisting of a cascade of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens. **Why C5a is Correct:** C5a is a potent **anaphylatoxin** and the most powerful **chemoattractant** of the complement system. During the activation of the complement cascade, C5 is cleaved into C5a and C5b. C5a acts as a chemical signal that recruits neutrophils, monocytes, and macrophages to the site of inflammation (chemotaxis). It also triggers mast cell degranulation, leading to increased vascular permeability. **Why the other options are incorrect:** * **C1:** This is the starting component of the Classical Pathway. It recognizes antigen-antibody complexes but does not possess chemotactic activity. * **C3:** This is the most abundant complement protein. While its cleavage product **C3a** is an anaphylatoxin, C3 itself is an inactive precursor. Its other product, **C3b**, is primarily involved in **opsonization** (tagging pathogens for phagocytosis). * **C2:** This is a component of the Classical and Lectin pathways. Its cleavage products (C2a/C2b) are involved in forming the C3 convertase but do not act as chemoattractants. **High-Yield NEET-PG Pearls:** * **Potency of Chemoattractants:** C5a > LTB4 (Leukotriene B4) > IL-8 > Bacterial products (N-formyl peptides). * **Opsonization:** C3b is the major opsonin. * **Membrane Attack Complex (MAC):** Formed by C5b-C9; responsible for osmotic lysis of gram-negative bacteria. * **Anaphylatoxins:** C5a > C3a > C4a (in order of potency). * **Deficiency:** C5-C9 deficiency increases susceptibility to *Neisseria* infections.

Q: EAC rosette formation is a property of which of the following types of immune cells?

T-cells. **Explanation:** The formation of **rosettes** is a classic laboratory technique used to identify and enumerate specific lymphocyte populations based on their surface receptors. **1. Why T-cells are the correct answer:** T-cells possess a specific surface receptor known as **CD2**, which has a high affinity for **LFA-3** (CD58) found on the surface of sheep erythrocytes (SRBCs). When T-cells are incubated with SRBCs, the red cells adhere to the T-cell surface, forming a cluster that resembles a rose—hence the term **"E-rosette"** (Erythrocyte rosette). This is a gold-standard historical marker for identifying T-lymphocytes. **2. Analysis of Incorrect Options:** * **B-cells:** While B-cells do not form E-rosettes, they can form **EAC rosettes** (Erythrocyte-Antibody-Complement). This occurs because B-cells express receptors for the Fc portion of IgG and the C3b component of complement. However, the standard "rosette formation" mentioned in classical immunology questions specifically refers to the CD2-mediated E-rosette of T-cells. * **Macrophages:** Although macrophages have receptors for complement and Fc fragments, they are primarily identified by phagocytic activity and specific markers like CD14/CD68, not by spontaneous rosette formation with SRBCs. **3. High-Yield Clinical Pearls for NEET-PG:** * **E-Rosette:** Marker for **T-cells** (CD2 receptor). * **EAC-Rosette:** Marker for **B-cells** (CR1/C3b receptor). * **ANAE (Alpha-Naphthyl Acetate Esterase):** A histochemical stain used to differentiate T-cells (focal staining) from B-cells (negative). * **CD3:** The most specific pan-T-cell marker used in modern flow cytometry. * **Surface Immunoglobulin (sIg):** The most definitive marker for B-cells.

Q: Omenn syndrome represents which of the following immunological conditions?

Subset of T cell deficiency. **Explanation:** **Omenn Syndrome** is a rare, autosomal recessive form of **Severe Combined Immunodeficiency (SCID)**. It is primarily caused by hypomorphic (partial) mutations in the **RAG1 or RAG2 genes**. 1. **Why the correct answer is right:** In Omenn Syndrome, the RAG gene mutations lead to limited V(D)J recombination. This results in a profound deficiency of B cells, but a **paradoxical expansion of a few restricted, oligoclonal T-cell subsets**. These T cells are poorly regulated, autoreactive, and infiltrate peripheral tissues (skin, gut), leading to the characteristic clinical presentation of erythroderma and hepatosplenomegaly. Thus, it is classified as a specific subset of T-cell deficiency/dysfunction. 2. **Why the incorrect options are wrong:** * **HLA Class II Deficiency (Bare Lymphocyte Syndrome Type II):** Caused by mutations in transcription factors (like CIITA) required for HLA Class II expression. It presents with a lack of CD4+ T cells, not the oligoclonal T-cell expansion seen in Omenn. * **HLA Class I Deficiency (Bare Lymphocyte Syndrome Type I):** Usually due to TAP1/TAP2 mutations; it presents with chronic respiratory infections and skin ulcers, not SCID-like features. * **Purine Metabolism Deficiency:** Refers to **Adenosine Deaminase (ADA) deficiency**, which causes a complete absence of T, B, and NK cells (T-B-NK- SCID) due to toxic metabolite accumulation. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Erythroderma (peeling skin), Hepatosplenomegaly, and Lymphadenopathy. * **Laboratory Hallmarks:** Elevated **IgE** levels, **Eosinophilia**, and absent B cells. * **Genetics:** Most commonly **RAG1/RAG2** mutations (hypomorphic). * **Treatment:** Hematopoietic stem cell transplant (HSCT) is the definitive cure.

Question 1

A 45-year-old businesswoman arrives with vague abdominal complaints and has noticed melenic stool. A sigmoidoscopy reveals a 4-cm mass in the upper colon. Which tumor marker should be ordered?

Accepted Answer

Carcinoembryonic antigen (CEA)

Answer

a-Fetoprotein

Answer

Anti-tumor antibody

Answer

Antitumor light chains

Question 2

What is the function of T-lymphocytes?

Accepted Answer

All of the above

Answer

Production of interferon

Answer

Lymphokine production

Answer

Rosette formation

Question 3

Which of the following is characteristic of the mucosal immune system?

Accepted Answer

Tolerance to foreign antigens is the norm rather than the exception

Answer

A vigorous response is made to all nonself antigens encountered

Answer

Chronic inflammation makes an inhospitable environment for microbes

Answer

IL-2 and IFN-g contribute to a Th-1 like environment

Question 4

Which immunoglobulin has a pentameric structure?

Accepted Answer

IgM

Answer

IgG

Answer

IgA

Answer

IgD

Question 5

Which of the following is NOT a test for B cell function?

Accepted Answer

CH50 count

Answer

Amount of IgG

Answer

PPD testing

Answer

Cytokine levels

Question 6

Which of the following is FALSE about the primary immune response?

Accepted Answer

It is the immune response against subsequent antigenic challenge.

Answer

The lag period is longer.

Answer

It is slow, sluggish, and short-lived.

Answer

All of the above.

Question 7

Which cells are primarily involved in humoral immunity?

Accepted Answer

B-cells

Answer

T-cells

Answer

Helper cells

Answer

Dendritic cells

Question 8

Which of the following is a chemoattractant?

Accepted Answer

C5a

Answer

Cl

Answer

C3

Answer

C2

Question 9

EAC rosette formation is a property of which of the following types of immune cells?

Accepted Answer

T-cells

Answer

B-cells

Answer

Macrophages

Answer

All of the above

Question 10

Omenn syndrome represents which of the following immunological conditions?

Accepted Answer

Subset of T cell deficiency

Answer

Human Leukocyte antigen (HLA) Class II Deficiency

Answer

HLA Class I Deficiency

Answer

Purine Metabolism Deficiency

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?