Immunology Practice Questions

Q: Transplantation of the host's own tissue is known as?

Autograft. **Explanation:** The correct answer is **Autograft**. This term refers to the transplantation of tissue from one site to another within the same individual. Since the donor and recipient are the same person, the MHC (Major Histocompatibility Complex) molecules are identical, resulting in **zero risk of graft rejection**. Common clinical examples include skin grafting for burns or using the saphenous vein for coronary artery bypass grafting (CABG). **Analysis of Incorrect Options:** * **Isograft (Syngeneic graft):** This involves transplantation between genetically identical individuals, such as **monozygotic (identical) twins**. Like autografts, these are usually accepted without rejection. * **Allograft (Homograft):** This is the most common clinical transplant type, occurring between genetically different members of the **same species** (e.g., human to human). These require immunosuppression to prevent rejection due to HLA mismatch. * **Xenograft (Heterograft):** This involves transplantation between members of **different species** (e.g., pig heart valve to a human). These are prone to rapid, hyperacute rejection. **NEET-PG High-Yield Pearls:** 1. **Order of Rejection Risk:** Autograft = Isograft < Allograft < Xenograft. 2. **MHC/HLA:** The primary target of the immune response in graft rejection is the **MHC Class II** molecules on the donor's dendritic cells (Direct Pathway). 3. **Privileged Sites:** Certain areas like the **cornea, anterior chamber of the eye, and testes** have low risk of rejection because they are "immunologically privileged" (sequestered from immune surveillance).

Q: All of the following are categorized as secondary lymphoid organs except?

Thymus. ### Explanation Lymphoid organs are categorized into two types based on their function in lymphocyte development: **Primary** and **Secondary**. **1. Why Thymus is the Correct Answer:** The **Thymus** and **Bone Marrow** are **Primary Lymphoid Organs**. These are the sites where lymphocytes are produced (lymphopoiesis) and undergo antigen-independent maturation. In the thymus, T-cell progenitors from the bone marrow differentiate into mature, immunocompetent T-cells while learning self-tolerance. Since the question asks for the exception to secondary organs, the Thymus is the correct choice. **2. Analysis of Incorrect Options (Secondary Lymphoid Organs):** Secondary lymphoid organs are sites where mature lymphocytes reside, encounter antigens, and initiate an immune response (antigen-dependent proliferation). * **Lymph Nodes (Option A):** Filter lymph and are the primary site for local immune responses to tissue-borne antigens. * **Spleen (Option B):** Filters blood and is the chief site of immune responses to blood-borne antigens. * **Subepithelial collections (Option D):** These include **MALT** (Mucosa-Associated Lymphoid Tissue) such as Peyer’s patches in the intestine, tonsils, and appendix. They protect mucosal surfaces. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **B-cell Maturation:** Occurs in the Bone Marrow (Primary). * **T-cell Maturation:** Occurs in the Thymus (Primary). * **Hassall’s Corpuscles:** Characteristic histological feature of the Thymic medulla. * **DiGeorge Syndrome:** Congenital failure of the 3rd and 4th pharyngeal pouches to develop, leading to thymic aplasia and T-cell deficiency. * **Mnemonic:** Primary organs are where cells are **"Born and Trained"** (Bone marrow/Thymus); Secondary organs are where they **"Fight"** (Spleen/Nodes/MALT).

Q: A 24-year-old man is diagnosed with disseminated histoplasmosis after developing symptoms of fever, lymphadenopathy, hepatosplenomegaly, and pancytopenia. Which of the following is the body's major immunologic defense against histoplasmosis?

Mononuclear leukocytes. **Explanation:** **1. Why Mononuclear Leukocytes are Correct:** *Histoplasma capsulatum* is a **dimorphic intracellular fungus**. The primary defense against intracellular pathogens (fungi, mycobacteria, and certain protozoa) is **Cell-Mediated Immunity (CMI)**. When *Histoplasma* spores are inhaled, they are phagocytosed by alveolar macrophages. In a healthy immune system, T-helper cells (CD4+) recognize fungal antigens and release cytokines like **IFN-gamma**. This activates **mononuclear leukocytes** (macrophages and monocytes), enhancing their phagocytic and fungicidal activity to contain the infection within granulomas. In disseminated disease (as seen in this patient), a failure of this T-cell/macrophage axis allows the yeast to spread throughout the reticuloendothelial system. **2. Why Other Options are Incorrect:** * **Options A & D (IgG and IgM):** Humoral immunity (antibodies) plays a minimal role in controlling intracellular fungal infections. While antibodies are produced and can be used for diagnosis (serology), they do not provide protective clearance. * **Option C (Complement):** The complement system is vital for opsonization and lysis of extracellular bacteria, but it is ineffective against the intracellular yeast form of *Histoplasma* residing inside macrophages. **3. NEET-PG Clinical Pearls:** * **Intracellular Niche:** *Histoplasma* is the only fungus that is an **obligate intracellular** parasite of the reticuloendothelial system (found inside macrophages). * **Morphology:** On biopsy (Grocott’s Methenamine Silver stain), look for small, oval budding yeasts inside macrophages. * **Risk Factor:** Disseminated histoplasmosis is a classic **AIDS-defining illness** (usually when CD4 <150 cells/mm³). * **Mimicry:** It clinically mimics Tuberculosis (fever, weight loss, granulomas, and hepatosplenomegaly).

Q: Thymic hypoplasia is seen in which of the following conditions?

All of the above. **Explanation:** The correct answer is **D. All of the above**. Thymic hypoplasia refers to the underdevelopment of the thymus, leading to impaired T-cell maturation and cell-mediated immunity. 1. **DiGeorge Syndrome (Option A):** This is the classic cause of thymic hypoplasia. It results from a microdeletion on chromosome **22q11**, leading to the failure of the **3rd and 4th pharyngeal pouches** to develop. This manifests as the CATCH-22 mnemonic: Cardiac defects, Abnormal facies, **Thymic hypoplasia**, Cleft palate, and Hypocalcemia (due to parathyroid hypoplasia). 2. **Ataxia Telangiectasia (Option C):** This is an autosomal recessive DNA repair defect (ATM gene). While primarily known for cerebellar ataxia and oculocutaneous telangiectasia, it also involves **thymic hypoplasia/dysplasia**, leading to combined immunodeficiency and an increased risk of malignancies. 3. **Common Variable Immunodeficiency (CVID) (Option B):** While CVID is primarily characterized by hypogammaglobulinemia (B-cell defect), a significant subset of patients (up to 20-30%) exhibit T-cell abnormalities, including reduced thymic output and functional thymic impairment, which can manifest as varying degrees of thymic atrophy or hypoplasia. **NEET-PG High-Yield Pearls:** * **Chest X-ray finding:** The absence of a "Thymic Shadow" in a neonate is a classic sign of DiGeorge Syndrome or SCID. * **Nezelof Syndrome:** An autosomal recessive condition characterized by thymic hypoplasia with normal or near-normal immunoglobulin levels. * **SCID (Severe Combined Immunodeficiency):** Also presents with a vestigial/hypoplastic thymus, but is not listed as a standalone option here. * **Hassall’s Corpuscles:** These are characteristically absent or reduced in cases of thymic dysplasia/hypoplasia.

Q: What is true about IgM?

Fixes complement. ### Explanation **Correct Option: A (Fixes complement)** IgM is the most potent activator of the **Classical Complement Pathway**. It exists primarily as a pentamer (connected by a J-chain), providing multiple binding sites for the C1q component of the complement cascade. A single pentameric IgM molecule bound to an antigen can initiate the cascade, whereas IgG requires at least two molecules in close proximity. **Analysis of Incorrect Options:** * **B. Increased in primary response:** While IgM is the *first* antibody produced in a primary immune response, the statement is technically incomplete or less "true" than its complement-fixing ability in a comparative sense. However, in many exam contexts, IgM is the hallmark of the **acute/primary phase**, while IgG dominates the secondary (anamnestic) response. * **C. The Fab region is composed of the variable region:** This is partially incorrect. The Fab (Fragment antigen-binding) region is composed of **both** the entire variable region (VH and VL) and the first constant region (CH1 and CL). * **D. The Fc region is antibody-binding:** This is incorrect. The **Fab** region binds to the antigen (antibody-binding), while the **Fc** (Fragment crystallizable) region binds to host cell receptors (Fc receptors) and mediates effector functions like complement activation and opsonization. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** IgM is a **pentamer** in serum (10 binding sites) but a **monomer** on the surface of B-cells (BCR). * **Molecular Weight:** It is the largest immunoglobulin (**"Millionaire Molecule"**), preventing it from crossing the placenta. * **Clinical Significance:** Presence of **IgM in a newborn** indicates intrauterine infection (e.g., TORCH), as maternal IgM cannot cross the placenta. * **Agglutination:** Due to its high valency, IgM is the most efficient antibody for agglutination (e.g., ABO blood grouping).

Q: Papain acts on gamma globulin to form which of the following?

2 Fab fragments. **Explanation:** The structure of an immunoglobulin (IgG) molecule consists of two heavy chains and two light chains held together by disulfide bonds. The enzymatic cleavage of this molecule is a high-yield topic in immunology. **Why Option B is Correct:** When the enzyme **Papain** acts on a gamma globulin (IgG) molecule, it cleaves the heavy chains at the **hinge region**, specifically *above* the interchain disulfide bonds. This proteolytic cleavage results in three distinct fragments: 1. **Two Fab fragments** (Fragment Antigen Binding): Each fragment consists of one light chain and a portion of one heavy chain. These fragments are monovalent, meaning they can bind to an antigen but cannot precipitate or cross-link them. 2. **One Fc fragment** (Fragment Crystallizable): This consists of the remaining portions of the two heavy chains held together. It is responsible for biological effector functions like complement fixation and opsonization. **Why Other Options are Incorrect:** * **Option A:** Papain produces only **one** Fc fragment, not two. The two heavy chain segments in the Fc region remain joined by disulfide bonds. * **Option C:** Papain produces **two** separate Fab fragments because it cuts above the site where the heavy chains are linked to each other. **NEET-PG High-Yield Pearls:** * **Pepsin Digestion:** Unlike Papain, Pepsin cleaves *below* the hinge region. This results in **one F(ab')₂ fragment** (bivalent, can precipitate antigens) and degraded Fc subfragments. * **Mercaptoethanol:** This reducing agent breaks all disulfide bonds, resulting in four separate polypeptide chains (2 Heavy, 2 Light). * **Memory Aid:** **Pa**pain = **Pa**rted (splits the two Fabs apart); **Pe**psin = **Pe**rmanent (keeps the two Fabs together as one unit).

Question 1

Which is the rich source of IgG?

Accepted Answer

Small intestine

Answer

Stomach

Answer

Visceral peritoneum

Answer

Large intestine

Question 2

Which of the following statements about macrophages is false?

Accepted Answer

Macrophages are involved in type III hypersensitivity reactions.

Answer

Macrophages can harbor mycobacteria.

Answer

Macrophages are derived from blood monocytes.

Answer

Macrophages produce tumor necrosis factor and interleukins.

Question 3

Transplantation of the host's own tissue is known as?

Accepted Answer

Autograft

Answer

Isograft

Answer

Allograft

Answer

Xenograft

Question 4

All of the following are categorized as secondary lymphoid organs except?

Accepted Answer

Thymus

Answer

Lymph nodes

Answer

Spleen

Answer

Subepithelial collections of lymphocytes

Question 5

A 24-year-old man is diagnosed with disseminated histoplasmosis after developing symptoms of fever, lymphadenopathy, hepatosplenomegaly, and pancytopenia. Which of the following is the body's major immunologic defense against histoplasmosis?

Accepted Answer

Mononuclear leukocytes

Answer

Immunoglobulin G (IgG) antibodies

Answer

Complement

Answer

Immunoglobulin M (IgM) antibody

Question 6

Which of the following cells is primarily involved in T cell independent antigen presentation?

Accepted Answer

B cell

Answer

T cell

Answer

Macrophage

Answer

CD8+ cytotoxic T lymphocyte

Question 7

Infection with Epstein-Barr virus (EBV) results in the development of virus-specific antibodies. The pattern of these antibodies helps to stage the illness. Which statement regarding EBV-VCA (IgG) Ab is correct?

Accepted Answer

Arises early in the course of the illness; detectable titers persist a lifetime

Answer

Appears 2 weeks to several months after onset and is present more often in atypical cases of infectious mononucleosis

Answer

Appears 3 to 4 weeks after onset; titers correlate with severity of clinical illness

Answer

Appears late in the course of the disease and persists a lifetime

Question 8

Thymic hypoplasia is seen in which of the following conditions?

Accepted Answer

All of the above

Answer

DiGeorge syndrome

Answer

Common variable immunodeficiency

Answer

Ataxia telangiectasia

Question 9

What is true about IgM?

Accepted Answer

Fixes complement

Answer

Increased in primary response

Answer

The Fab region is composed of the variable region

Answer

The Fc region is antibody-binding

Question 10

Papain acts on gamma globulin to form which of the following?

Accepted Answer

2 Fab fragments

Answer

2 Fc fragments

Answer

1 Fab fragment

Answer

None of the above

Immunology — MCQs

Immunology — MCQs

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