Immunology Practice Questions

Q: Which immunoglobulin crosses the placenta?

IgG. **Explanation:** **IgG** is the only class of immunoglobulin capable of crossing the placenta. This occurs through a specialized active transport mechanism involving **neonatal Fc receptors (FcRn)** located on the syncytiotrophoblast of the placenta. This transfer provides the fetus and newborn with **passive immunity**, protecting them during the first few months of life while their own immune system matures. **Analysis of Incorrect Options:** * **IgA (Option A):** Primarily found in secretions (tears, saliva, colostrum) as a dimer. It does not cross the placenta but is the main antibody transferred via **breast milk**, providing local intestinal immunity to the infant. * **IgE (Option B):** Involved in Type I hypersensitivity and parasitic infections. It has a very low serum concentration and lacks the specific Fc region required for placental transport. * **IgM (Option D):** It is a large pentamer (the "millionaire molecule"). Due to its high molecular weight and lack of specific receptors, it cannot cross the placenta. **Clinical Note:** The presence of IgM in a neonate’s blood indicates an **intrauterine infection** (e.g., TORCH), as it signifies the fetus's own immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Subclasses:** Only IgG1, IgG3, and IgG4 cross the placenta efficiently; IgG2 crosses poorly. * **Rh Incompatibility:** Because IgG crosses the placenta, anti-D antibodies (IgG) cause Hemolytic Disease of the Newborn (HDN). * **Half-life:** IgG has the longest half-life (approx. 23 days) among all immunoglobulins. * **Abundance:** IgG is the most abundant immunoglobulin in the serum (75-80%).

Q: Allergic reactions are a type of hypersensitivity. To which type of hypersensitivity reaction do common allergies like hay fever and asthma belong?

Type I. **Explanation:** **Correct Answer: A. Type I Hypersensitivity** Type I hypersensitivity, also known as **Immediate Hypersensitivity**, is mediated by **IgE antibodies**. When a predisposed individual is exposed to an allergen (like pollen or dander), IgE binds to the surface of **mast cells and basophils** via Fc receptors. Upon re-exposure, the allergen cross-links these IgE molecules, triggering degranulation and the release of vasoactive amines like **histamine**. This leads to rapid clinical manifestations such as vasodilation and bronchoconstriction, characteristic of hay fever, extrinsic asthma, and anaphylaxis. **Why the other options are incorrect:** * **Type II (Cytotoxic):** Mediated by **IgG or IgM** antibodies directed against antigens on specific cell surfaces or tissues (e.g., ABO incompatibility, Rh incompatibility, Myasthenia Gravis). * **Type III (Immune-Complex):** Caused by the deposition of **antigen-antibody complexes** in tissues, leading to complement activation and inflammation (e.g., SLE, Post-streptococcal glomerulonephritis, Arthus reaction). * **Type IV (Delayed-type):** This is **cell-mediated** (T-cells), not antibody-mediated. It takes 48–72 hours to manifest (e.g., Mantoux test, contact dermatitis, graft rejection). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (ACID):** **A**naphyalctic (I), **C**ytotoxic (II), **I**mmune-Complex (III), **D**elayed (IV). * **Key Cells:** Mast cells are the primary effectors in Type I; T-lymphocytes/Macrophages in Type IV. * **Atopy:** Refers to the genetic predisposition to develop Type I reactions. * **Coombs and Gell Classification:** This is the formal name for the four-type classification system of hypersensitivity.

Q: What is the distinct amino acid sequence at the antigen combining site called?

Idiotype. ### Explanation The correct answer is **Idiotype**. **1. Why Idiotype is correct:** An **Idiotype** refers to the unique set of antigenic determinants (idiotopes) located within the **Variable (V) regions** of an antibody or T-cell receptor. These are determined by the specific **amino acid sequence** at the antigen-binding site. Since each clone of B-cells produces antibodies with a unique V-region to fit a specific antigen, the idiotype serves as a signature for that specific clone. **2. Why other options are incorrect:** * **Allotype:** Refers to antigenic variations in the **Constant (C) region** of antibodies that differ between individuals of the same species due to allelic inheritance (e.g., Gm markers in IgG). * **Epitope:** Also known as the antigenic determinant, this is the specific part of the **antigen** molecule to which an antibody binds. * **Paratope:** This is the actual **physical pocket** or site on the antibody (formed by the Hypervariable regions) that binds to the epitope. While closely related to idiotype, the "sequence" itself that defines the specificity is the idiotype. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isotype:** Determines the class/subclass of the antibody (IgG, IgM, etc.) based on heavy chain constant regions; same in all individuals of a species. * **Hypervariable Regions (HVRs):** Also called **Complementarity Determining Regions (CDRs)**. There are 3 CDRs in the light chain and 3 in the heavy chain. These form the idiotype. * **Monoclonal Gammopathy:** In conditions like Multiple Myeloma, all myeloma cells produce antibodies with the **same idiotype**, appearing as an 'M-band' on electrophoresis.

Q: Which of the following statements is FALSE regarding Natural Killer (NK) cells?

NK cell activity is MHC restricted.. **Explanation:** **Why Option D is the correct (False) statement:** Natural Killer (NK) cells are a component of the **innate immune system**. Unlike T-cells, which require the presentation of antigens via Major Histocompatibility Complex (MHC) molecules to recognize targets, NK cells are **MHC-unrestricted**. In fact, NK cells operate on the "Missing Self" hypothesis: they preferentially kill cells that have *downregulated* or lost their MHC Class I expression (a common evasion tactic used by viruses and tumors). **Analysis of other options:** * **Option A:** Interferons (IFN-α and IFN-β) and Interleukin-12 are potent activators of NK cells, enhancing their cytotoxic capabilities during an immune response. * **Option B:** NK cells are the first line of defense against viral infections. They provide immediate non-specific cytotoxicity before the adaptive immune system (CD8+ T-cells) can mount a specific response. * **Option C:** NK cells play a crucial role in "immunosurveillance." They identify and destroy malignant cells that display stress-induced ligands or abnormal surface proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** NK cells are large granular lymphocytes derived from the lymphoid progenitor but do not express T-cell receptors (TCR) or CD3. * **Markers:** The definitive surface markers for NK cells are **CD56** and **CD16** (FcγRIII). CD16 allows them to perform **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. * **Mechanism of Killing:** They induce apoptosis in target cells using **perforins** (to create pores) and **granzymes** (to activate caspases). * **Receptors:** They possess Killer Immunoglobulin-like Receptors (KIRs). Inhibitory KIRs bind to MHC I on healthy cells to prevent "self" destruction.

Q: What is true about the WIC (Western Immunogenetics Conference) complex?

Class III has complement components.. The question refers to the **Major Histocompatibility Complex (MHC)**, often discussed in the context of immunogenetics. The MHC is a cluster of genes essential for the immune system to recognize foreign molecules. ### **Explanation of Options** * **Correct Answer: C. Class III has complement components.** The MHC locus is divided into three classes. **Class III genes** are located between Class I and Class II regions. Unlike Class I and II, they do not encode antigen-presenting molecules. Instead, they encode various secreted proteins involved in the immune response, most notably **complement components (C2, C4A, C4B)** and **Factor B**, as well as cytokines like TNF-α and TNF-β. * **A is incorrect:** The MHC (Human Leukocyte Antigen or HLA system in humans) is located on the short arm of **Chromosome 6**, not chromosome 4. * **B is incorrect:** **Class I** comprises the A, B, and C loci. **Class II** comprises the **DP, DQ, and DR** loci. * **D is incorrect:** The **Mixed Leukocyte Reaction (MLR)** is primarily a measure of **Class II MHC** incompatibility (specifically HLA-DR). Class I molecules are involved in presenting endogenous antigens to CD8+ T-cells. ### **High-Yield Clinical Pearls for NEET-PG** * **MHC Restriction:** CD8+ T-cells are MHC Class I restricted, while CD4+ T-cells are MHC Class II restricted (Rule of 8: 8×1=8; 4×2=8). * **Structure:** Class I has one heavy chain and a **β2-microglobulin** (encoded on Chromosome 15). Class II consists of two polypeptide chains (α and β), both encoded within the MHC locus. * **Ankylosing Spondylitis:** Strongly associated with **HLA-B27** (Class I). * **Narcolepsy:** Strongly associated with **HLA-DR2/DQB1** (Class II).

Q: Nude mice are able to accept xenografts because they lack:

T cells. **Explanation:** The **Nude Mouse** is a laboratory strain characterized by a genetic mutation in the *FOXN1* gene. This mutation results in two primary phenotypic features: the absence of hair (hence "nude") and a **congenital absence of the thymus (thymic aplasia)**. 1. **Why T cells is correct:** The thymus is the primary site for T-cell maturation. Without a functional thymus, nude mice cannot produce mature, functional T lymphocytes. Since **T cells (specifically Cytotoxic T cells)** are the primary mediators of Cell-Mediated Immunity (CMI) and are responsible for the rejection of foreign tissue (allografts and xenografts), their absence allows these mice to accept grafts even from different species (xenografts). 2. **Why other options are incorrect:** * **B-cells:** Nude mice possess a normal population of B-cells. While their antibody response to T-dependent antigens is impaired, the B-cells themselves are present. * **NK cells:** Natural Killer (NK) cell levels are actually **normal or even elevated** in nude mice. They provide a baseline level of innate immunity against tumors and infections. * **LAK cells:** Lymphokine-activated killer cells are derived primarily from NK cells in the presence of IL-2. Since NK cells are present, the potential for LAK cell activity remains. **High-Yield Clinical Pearls for NEET-PG:** * **DiGeorge Syndrome:** The human clinical counterpart to the nude mouse, characterized by thymic hypoplasia and T-cell deficiency. * **Xenograft:** A transplant between different species (e.g., pig to human). * **Role in Research:** Nude mice are extensively used in oncology to grow human tumor explants (xenografts) because their lack of T-cell-mediated rejection prevents them from destroying the human cancer cells.

Q: Which of the following are examples of live vaccines?

Rubella and Yellow fever. **Explanation:** Live attenuated vaccines contain pathogens that have been weakened (attenuated) in a laboratory so they can replicate and induce a robust immune response without causing the actual disease in healthy individuals. **1. Why Option A is Correct:** Both **Rubella** and **Yellow fever** are classic examples of live attenuated viral vaccines. Rubella uses the **RA 27/3 strain**, while Yellow fever uses the **17D strain**. These vaccines provide long-lasting immunity, often with a single dose, by mimicking a natural infection. **2. Why Other Options are Incorrect:** * **Option B:** While Oral Polio Vaccine (Sabin) is live, **TAB** (Typhoid, Paratyphoid A and B) is a killed whole-cell vaccine (now largely replaced by newer typhoid vaccines). Note that Injectable Polio Vaccine (Salk) is also killed. * **Option C:** **Diphtheria and Tetanus** are **Toxoid** vaccines. They are prepared by detoxifying bacterial exotoxins using formaldehyde, inducing immunity against the toxin rather than the bacteria itself. * **Option D:** **Hepatitis A** and **Rabies** are **Killed (Inactivated)** vaccines. They contain pathogens that have been destroyed by heat or chemicals and cannot replicate. **Clinical Pearls for NEET-PG:** * **Mnemonic for Live Vaccines:** "**BOY** **R**omeo **I**s **V**ery **M**uch **L**oving **T**yping **I**n **S**mart **P**hone" (**B**CG, **O**PV, **Y**ellow Fever, **R**otavirus, **I**nfluenza (Intranasal), **V**aricella, **M**easles, **M**umps, **L**ive Typhoid (Ty21a), **S**mallpox). * **Contraindication:** Live vaccines are generally contraindicated in **pregnancy** and **immunocompromised** states (e.g., HIV with low CD4 count) due to the risk of uncontrolled viral replication. * **Yellow Fever:** It is a mandatory vaccine for international travel to endemic zones; immunity starts after 10 days and lasts for life.

Question 1

Which immunoglobulin crosses the placenta?

Accepted Answer

IgG

Answer

IgA

Answer

IgE

Answer

IgM

Question 2

Cell fusion is an innovative technique for preparing specific antibodies. Which of the following techniques is NOT used to induce cell fusion?

Accepted Answer

Reducing the viscosity of the membrane

Answer

Adhering some inactivated viruses to the surface membrane

Answer

Adding polyethylene glycol

Answer

Applying a mild electric shock

Question 3

Which of the following is a component of innate immunity?

Accepted Answer

NK cells

Answer

T lymphocytes

Answer

Complement proteins

Answer

B lymphocytes

Question 4

Allergic reactions are a type of hypersensitivity. To which type of hypersensitivity reaction do common allergies like hay fever and asthma belong?

Accepted Answer

Type I

Answer

Type II

Answer

Type III

Answer

Type IV

Question 5

What is the distinct amino acid sequence at the antigen combining site called?

Accepted Answer

Idiotype

Answer

Allotype

Answer

Epitope

Answer

Paratope

Question 6

Which of the following statements is FALSE regarding Natural Killer (NK) cells?

Accepted Answer

NK cell activity is MHC restricted.

Answer

NK cell activity is augmented by interferons.

Answer

NK cells provide the initial response in viral infections.

Answer

NK cells are active against tumor cells.

Question 7

What is true about the WIC (Western Immunogenetics Conference) complex?

Accepted Answer

Class III has complement components.

Answer

It is present on chromosome 4.

Answer

Class II comprises A, B, and C loci.

Answer

Class I is involved in the mixed leukocyte reaction.

Question 8

Nude mice are able to accept xenografts because they lack:

Accepted Answer

T cells

Answer

B-cells

Answer

NK cells

Answer

LAK cells

Question 9

Which of the following are examples of live vaccines?

Accepted Answer

Rubella and Yellow fever

Answer

Polio and TAB

Answer

Diphtheria and Tetanus

Answer

Hepatitis A and Rabies

Question 10

All of the following are disorders of phagocyte function, except?

Accepted Answer

X-linked SCID

Answer

Chronic granulomatous disease

Answer

Chediak-Higashi syndrome

Answer

Myeloperoxidase deficiency

Immunology — MCQs

Immunology — MCQs

On this page

Practice by Chapter

Want unlimited practice?