Immunopathology — MCQs

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 291: A neonate develops spastic contractions on the second postpartum day. Laboratory studies show hypocalcemia. MRI studies demonstrate aplasia of the thymus and parathyroid glands. What is the appropriate diagnosis?

A. Transient hypogammaglobulinemia of infancy
B. Adenosine deaminase deficiency
C. Common variable immunodeficiency
D. DiGeorge syndrome (Correct Answer)

Explanation: ### Explanation **Correct Option: D. DiGeorge Syndrome** DiGeorge syndrome (22q11.2 deletion syndrome) results from the failure of the **3rd and 4th pharyngeal pouches** to develop [1]. This leads to a classic triad (CATCH-22): 1. **Thymic Aplasia:** Results in T-cell deficiency and recurrent viral/fungal infections. 2. **Parathyroid Aplasia:** Leads to hypocalcemia, which presents as **tetany or spastic contractions** (as seen in this neonate) [3]. 3. **Congenital Heart Defects:** Specifically conotruncal anomalies (e.g., Tetralogy of Fallot, Truncus Arteriosus) [1]. **Analysis of Incorrect Options:** * **A. Transient hypogammaglobulinemia of infancy:** Characterized by a delay in the baby's own IgG production (usually around 6 months). It involves B-cells, not the thymus or parathyroid glands. * **B. Adenosine deaminase (ADA) deficiency:** This is a cause of **Severe Combined Immunodeficiency (SCID)**. It affects both B and T-cells due to toxic metabolite accumulation but does not cause hypocalcemia or anatomical aplasia of the parathyroid. * **C. Common variable immunodeficiency (CVID):** A defect in B-cell differentiation into plasma cells, leading to low antibody levels. It typically presents in the 2nd or 3rd decade of life, not the neonatal period. **NEET-PG High-Yield Pearls:** * **Mnemonic CATCH-22:** **C**ardiac defects, **A**bnormal facies, **T**hymic hypoplasia, **C**left palate, **H**ypocalcemia, **22**q11 deletion [1]. * **Immunology:** Patients have a deficient T-cell mediated response; on X-ray, look for the **absence of a thymic shadow**. * **Diagnosis:** Confirmed by **FISH** (Fluorescence In Situ Hybridization) for the 22q11.2 microdeletion [2]. * **Paracortex:** In lymph nodes, the paracortical area (T-cell zone) will be depleted in these patients. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 167-168. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 173. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Endocrine System, pp. 1107-1108.

Question 292: When tissue pretreated with potassium permanganate is stained with Congo-red stain, the apple-green birefringence is abolished. The tissue is likely to contain which type of amyloid?

A. AA amyloidosis (Correct Answer)
B. AL amyloidosis
C. Primary amyloidosis
D. Hereditary amyloidosis

Explanation: **Explanation:** The correct answer is **AA amyloidosis**. This question tests your knowledge of the **Potassium Permanganate (KMnO₄) Sensitivity Test**, a classic histochemical method used to differentiate between types of amyloid proteins. **Underlying Concept:** Amyloid proteins are characterized by their β-pleated sheet structure, which binds Congo-red dye and produces characteristic apple-green birefringence under polarized light [1]. However, the **AA (Amyloid Associated)** protein—seen in secondary amyloidosis—is unique because its affinity for Congo-red is sensitive to oxidation [2]. When tissue sections containing AA amyloid are pretreated with potassium permanganate, the ̢-pleated structure is disrupted, causing the protein to lose its ability to bind Congo-red. Consequently, the **apple-green birefringence is abolished (Permanganate Sensitive).** **Analysis of Options:** * **A. AA amyloidosis (Correct):** Derived from Serum Amyloid A (SAA), an acute-phase reactant [2]. It is "Permanganate Sensitive." * **B. AL amyloidosis (Incorrect):** Derived from immunoglobulin light chains (seen in Multiple Myeloma) [3]. AL amyloid is **"Permanganate Resistant,"** meaning it retains its birefringence even after treatment. * **C. Primary amyloidosis (Incorrect):** This is clinically synonymous with AL amyloidosis; therefore, it is permanganate resistant. * **D. Hereditary amyloidosis (Incorrect):** Most forms (e.g., Transthyretin/ATTR) are also permanganate resistant [1]. **NEET-PG High-Yield Pearls:** * **AA Amyloidosis:** Associated with chronic inflammatory conditions (Rheumatoid Arthritis, Bronchiectasis, Osteomyelitis) and Familial Mediterranean Fever (FMF) [2]. * **Stains for Amyloid:** Congo-red (Gold standard), Thioflavin T/S (Fluorescent), and Methyl Violet/Crystal Violet (Metachromatic) [1]. * **Most common site for biopsy:** Abdominal fat pad aspiration or rectal biopsy (due to high sensitivity and ease). * **Most common organ involved:** Kidney (presents as Nephrotic Syndrome) [3]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 264-269. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 267-268. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 533-534.

Question 293: Which of the following is true about Type II hypersensitivity reactions?

A. Immune complex mediated
B. Antigen-antibody mediated (Correct Answer)
C. Arthus phenomenon
D. Granulomatous reaction

Explanation: **Explanation:** Type II hypersensitivity, also known as **cytotoxic hypersensitivity**, is primarily mediated by **IgG or IgM antibodies** directed against antigens present on the surface of specific cells or tissues [1]. **Why Option B is correct:** In Type II reactions, antibodies bind to fixed cell-surface antigens. This binding triggers cell destruction through three main mechanisms: complement-mediated lysis, antibody-dependent cellular cytotoxicity (ADCC), or cellular dysfunction (e.g., Myasthenia Gravis) [1]. Since the core mechanism involves the direct interaction of antibodies with cellular antigens, it is classified as **antigen-antibody mediated**. **Why other options are incorrect:** * **Option A (Immune complex mediated):** This describes **Type III hypersensitivity**, where soluble antigen-antibody complexes circulate and deposit in tissues (like blood vessels or glomeruli), causing systemic inflammation [2]. * **Option C (Arthus phenomenon):** This is a localized form of **Type III hypersensitivity** characterized by tissue necrosis following the deposition of immune complexes in vessel walls [2]. * **Option D (Granulomatous reaction):** This is a feature of **Type IV (Delayed-type) hypersensitivity**, which is cell-mediated (T-cells and macrophages) rather than antibody-mediated. **High-Yield NEET-PG Pearls:** * **Mnemonic for Types:** **A-C-I-D** (Type I: **A**naphylactic; Type II: **C**ytotoxic; Type III: **I**mmune-complex; Type IV: **D**elayed). * **Classic Examples of Type II:** Erythroblastosis fetalis, Goodpasture syndrome, Pemphigus vulgaris, Rheumatic fever, and Graves' disease [1]. * **Key Difference:** Type II involves **fixed** antigens; Type III involves **soluble** antigens [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 208-214. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 214-215.

Question 294: c-ANCA is characteristic for which condition?

A. Polyarteritis Nodosa
B. Rapidly Progressive Glomerulonephritis (RPGN)
C. Henoch-Schonlein Purpura
D. Wegener's Granulomatosis (Correct Answer)

Explanation: **Wegener's Granulomatosis** (now known as Granulomatosis with Polyangiitis - GPA) is the correct answer because it is strongly associated with **c-ANCA** (cytoplasmic Antineutrophil Cytoplasmic Antibody) [2]. The primary target antigen for c-ANCA is **Proteinase-3 (PR3)** [2]. GPA is characterized by a triad of necrotizing granulomas of the respiratory tract, necrotizing vasculitis of small-to-medium vessels, and renal involvement (crescentic glomerulonephritis) [2]. **Analysis of Incorrect Options:** * **Polyarteritis Nodosa (PAN):** This is a systemic vasculitis of medium-sized muscular arteries. Crucially, PAN is **ANCA-negative** and is frequently associated with Hepatitis B surface antigen (HBsAg). * **Rapidly Progressive Glomerulonephritis (RPGN):** While Pauci-immune RPGN (Type III) is associated with ANCA, RPGN is a clinical syndrome, not a specific disease [1]. GPA is a *cause* of RPGN, but c-ANCA is specifically diagnostic for the underlying GPA pathology. * **Henoch-Schonlein Purpura (HSP):** This is an **IgA-mediated** small vessel vasculitis typically seen in children. It is characterized by IgA immune complex deposition, not ANCA. **High-Yield Clinical Pearls for NEET-PG:** * **c-ANCA (PR3-ANCA):** Specific for Granulomatosis with Polyangiitis (GPA) [2]. * **p-ANCA (MPO-ANCA):** Associated with Microscopic Polyangiitis (MPA) and Churg-Strauss Syndrome (Eosinophilic Granulomatosis with Polyangiitis). * **Rule of "C":** Wegener’s involves **C**-ANCA, presents with **C**-shaped distribution (Upper respiratory, Lower respiratory, and Kidney), and is treated with **C**yclophosphamide. * **Biopsy Gold Standard:** For GPA, a lung biopsy showing necrotizing granulomatous vasculitis is most definitive. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Kidney, pp. 917-918. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 519-520.

Question 295: What type of immunologic injury is seen in autoimmune hemolytic anemia (AIHA)?

A. Type I
B. Type II (Correct Answer)
C. Type III
D. Type IV

Explanation: ### Explanation **Correct Answer: B. Type II Hypersensitivity** **Mechanism:** Autoimmune Hemolytic Anemia (AIHA) is a classic example of **Type II Hypersensitivity (Antibody-mediated cytotoxicity)** [2]. In this condition, the body produces autoantibodies (IgG or IgM) directed against specific antigens located on the surface of its own Red Blood Cells (RBCs) [1]. * **Warm AIHA:** IgG antibodies coat RBCs, leading to their destruction primarily by splenic macrophages (extravascular hemolysis) via Fc-receptor interaction [4]. * **Cold AIHA:** IgM antibodies bind to RBCs and activate the classical complement pathway, leading to C3b opsonization or MAC-mediated lysis [3]. **Why other options are incorrect:** * **Type I (Immediate):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Asthma) [2]. It does not involve direct cell lysis by autoantibodies. * **Type III (Immune-complex):** Caused by the deposition of soluble antigen-antibody complexes in tissues (e.g., SLE, Post-streptococcal glomerulonephritis) [2]. In AIHA, the antigen is a fixed cell-surface component, not a soluble one. * **Type IV (Delayed-type):** Mediated by T-cells (CD4+ or CD8+), not antibodies (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Test:** The **Direct Coombs Test (Direct Antiglobulin Test)** is the gold standard for diagnosing AIHA; it detects antibodies or complement already bound to the patient's RBCs [3]. * **Morphology:** The hallmark finding on a peripheral blood smear in Warm AIHA is the presence of **Spherocytes** [4]. * **Associations:** Warm AIHA is often associated with **SLE and CLL**, while Cold AIHA is linked to ***Mycoplasma pneumoniae* and Infectious Mononucleosis** [4]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 214. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 208-210. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 651-652. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 602-603.

Question 296: Graves disease is an example of which type of immunologic response?

A. Type I (Correct Answer)
B. Type II
C. Type III
D. Type IV

Explanation: **Explanation:** **Graves’ Disease** is a classic example of **Type II Hypersensitivity (Antibody-mediated)** [1]. Specifically, it is a sub-type often referred to as "Type V" or "Stimulatory Hypersensitivity." In this condition, B cells produce autoantibodies (Thyroid Stimulating Immunoglobulins - TSI) that bind to the **TSH receptor** on thyroid follicular cells [1]. Instead of causing cell destruction, these antibodies mimic the action of TSH, leading to the overproduction of thyroid hormones (Hyperthyroidism) [1]. **Analysis of Options:** * **Type II (Correct):** Involves antibodies (IgG or IgM) directed against antigens on specific cell surfaces or tissues [1]. In Graves, the "antigen" is the TSH receptor. * **Type I:** This is an **IgE-mediated** immediate hypersensitivity (e.g., Anaphylaxis, Asthma) [1]. It involves mast cell degranulation and is not the mechanism in Graves. * **Type III:** This involves the deposition of **Ag-Ab (Immune) complexes** in tissues, leading to complement activation (e.g., SLE, Post-streptococcal Glomerulonephritis). * **Type IV:** This is **Cell-mediated (Delayed)** hypersensitivity involving T-lymphocytes, not antibodies (e.g., Mantoux test, Contact dermatitis). **High-Yield Clinical Pearls for NEET-PG:** * **Triad of Graves:** Hyperthyroidism, Exophthalmos (due to retro-orbital inflammation), and Pretibial Myxedema. * **Antibody Marker:** TSI (Thyroid Stimulating Immunoglobulin) is the most specific [1]. * **Morphology:** Diffuse hypertrophy and hyperplasia of follicles with **scalloping** of colloid edges. * **Other Type II Examples:** Myasthenia Gravis (blocking antibodies), Goodpasture syndrome, and Hemolytic disease of the newborn [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 208-214.

Question 297: Toll-like receptors recognize bacterial products and stimulate an immune response by:

A. Perforin and granzyme mediated apoptosis
B. FADD ligand apoptosis
C. Transcription of nuclear factor mediated by NF-κB, which recruits cytokines (Correct Answer)
D. Cyclin

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Toll-like receptors (TLRs) are a class of **Pattern Recognition Receptors (PRRs)** located on cell membranes and endosomes [1]. They recognize **Pathogen-Associated Molecular Patterns (PAMPs)**, such as Lipopolysaccharide (LPS) or viral RNA, triggering pro-inflammatory responses [1]. When a TLR binds to its ligand, it triggers a signaling cascade (most commonly via the adapter protein **MyD88**). This leads to the activation of the transcription factor **Nuclear Factor-kappa B (NF-κB)**. NF-κB then translocates to the nucleus to stimulate the transcription of genes encoding **pro-inflammatory cytokines** (like TNF, IL-1, and IL-6) and co-stimulatory molecules. This process is the cornerstone of the innate immune response and the bridge to adaptive immunity [2]. **2. Why the Other Options are Incorrect:** * **Options A & B (Perforin/Granzyme & FADD):** These are mechanisms of **apoptosis** (programmed cell death). Perforins and granzymes are used by Cytotoxic T-cells and NK cells to kill infected cells directly. FADD (Fas-Associated Death Domain) is involved in the extrinsic pathway of apoptosis. TLRs are primarily involved in *activation* and *inflammation*, not direct induction of apoptosis. * **Option D (Cyclin):** Cyclins are proteins that regulate the **cell cycle** (mitosis). They are not involved in the immediate signaling pathway of innate immune recognition. **3. High-Yield Clinical Pearls for NEET-PG:** * **TLR-4:** Specifically recognizes **LPS** (Endotoxin) from Gram-negative bacteria. * **TLR-3:** Recognizes **double-stranded RNA** (viral). * **TLR-5:** Recognizes **Flagellin**. * **TLR-9:** Recognizes unmethylated **CpG DNA**. * **Location:** TLRs 1, 2, 4, 5, and 6 are on the **plasma membrane**; TLRs 3, 7, 8, and 9 are on **endosomal membranes**. * **IRFs:** Besides NF-κB, TLR signaling can activate **Interferon Regulatory Factors (IRFs)**, which stimulate the production of Type I Interferons (Antiviral response). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Hemodynamic Disorders, Thromboembolic Disease, and Shock, p. 142. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 196-198.

Question 298: Which of the following is the most common form of primary immunodeficiency?

A. Acquired immunodeficiency syndrome (AIDS)
B. Wiskott-Aldrich syndrome
C. Common variable immunodeficiency
D. Selective IgA immunodeficiency (Correct Answer)

Explanation: **Explanation:** **Selective IgA Immunodeficiency** is the most common primary immunodeficiency disorder, occurring in approximately 1 in 600 individuals of European descent. It is characterized by serum IgA levels less than 7 mg/dL with normal levels of IgG and IgM. The underlying defect is a failure of B cells to differentiate into IgA-secreting plasma cells [1]. Most patients are asymptomatic, but some present with recurrent sinopulmonary infections or diarrhea (due to lack of mucosal immunity). **Analysis of Options:** * **Common Variable Immunodeficiency (CVID):** While it is the most common *clinically significant* (symptomatic) primary antibody deficiency requiring medical intervention, its overall prevalence is much lower than Selective IgA deficiency [1]. * **Wiskott-Aldrich Syndrome:** This is a rare X-linked recessive disorder characterized by the triad of thrombocytopenia (small platelets), eczema, and recurrent infections [2]. * **AIDS:** This is an **acquired** (secondary) immunodeficiency caused by HIV. The question specifically asks for a **primary** (congenital/genetic) immunodeficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Anaphylaxis Risk:** Patients with Selective IgA deficiency are at high risk for severe anaphylactic reactions when receiving blood transfusions containing IgA, as they develop anti-IgA antibodies. * **Associations:** It is frequently associated with autoimmune diseases (SLE, Rheumatoid Arthritis) and Celiac disease. * **False Positive:** It can cause a false-positive result in pregnancy tests due to heterophile antibodies. * **Diagnosis:** Diagnosis is usually made after the age of 4 to exclude transient physiological delays in immunoglobulin production [3]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 249-250. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 250-251. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 166-167.

Question 299: Hereditary angioneurotic edema is due to a deficiency of which of the following?

A. C1 deficiency
B. C2 and C4 deficiency
C. C1 esterase inhibitor (Correct Answer)
D. C9 deficiency

Explanation: **Explanation:** **Hereditary Angioneurotic Edema (HANE)** is an autosomal dominant disorder caused by a deficiency or functional defect of the **C1 esterase inhibitor (C1-INH)**. **Why C1 esterase inhibitor is correct:** C1-INH is a crucial regulatory protein that inhibits the classical complement pathway by inactivating C1r and C1s. More importantly, it also inhibits the **Kallikrein-Kinin system**. In its absence, there is uncontrolled activation of kallikrein, leading to excessive production of **Bradykinin**. Bradykinin is a potent vasodilator that increases vascular permeability [1], resulting in the characteristic episodes of non-pitting edema in the skin, larynx (causing life-threatening airway obstruction), and gastrointestinal tract. **Why other options are incorrect:** * **C1 deficiency:** Rare; typically associated with an increased risk of systemic lupus erythematosus (SLE) and recurrent pyogenic infections, not angioedema. * **C2 and C4 deficiency:** These are the most common complement deficiencies [1]. While HANE leads to *secondary* low levels of C2 and C4 (because they are consumed by the overactive C1), the primary defect is the inhibitor, not the components themselves. * **C9 deficiency:** This affects the formation of the Membrane Attack Complex (MAC) [1]. It primarily predisposes individuals to recurrent *Neisseria* infections. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Low **C4 levels** are the most consistent screening finding, even between attacks. * **Clinical Feature:** Characterized by recurrent edema **without urticaria** (hives) or pruritus (distinguishes it from allergic angioedema). * **Treatment:** Acute attacks are treated with C1-INH concentrate or **Icatibant** (Bradykinin B2 receptor antagonist). Prophylaxis often involves **Danazol** (androgens increase hepatic synthesis of C1-INH). * **ACE Inhibitors:** These are contraindicated in HANE patients as they prevent bradykinin breakdown, worsening the condition. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 99-101.

Question 300: Nucleotidase deficiency is associated with which of the following?

A. Humoral immunity deficiency (Correct Answer)
B. Acquired immunity deficiency
C. Severe combined immunodeficiency (SCID)
D. Cell mediated immunity deficiency

Explanation: **Explanation:** The correct answer is **Humoral immunity deficiency (Option A)**. **Understanding the Concept:** The enzyme **5'-nucleotidase** (specifically ecto-5'-nucleotidase or CD73) is a marker for mature B-lymphocytes. It plays a crucial role in the final stages of B-cell maturation and the production of antibodies. A deficiency in this enzyme leads to a failure in the maturation of B-cells into plasma cells, resulting in **hypogammaglobulinemia**. This primarily affects the humoral (antibody-mediated) immune response [1]. It is frequently observed in patients with Common Variable Immunodeficiency (CVID) and X-linked agammaglobulinemia (Bruton’s) [1]. **Analysis of Incorrect Options:** * **Option B (Acquired immunity deficiency):** While humoral immunity is a component of acquired immunity, "acquired immunity deficiency" is too broad and non-specific. Nucleotidase deficiency specifically targets the B-cell lineage. * **Option C (SCID):** Severe Combined Immunodeficiency is typically associated with **Adenosine Deaminase (ADA) deficiency** or Purine Nucleoside Phosphorylase (PNP) deficiency. While these are also enzymes in the purine pathway, their deficiency affects both T and B cells severely, whereas 5'-nucleotidase deficiency is more restricted to B-cell dysfunction. * **Option D (Cell-mediated immunity deficiency):** This refers to T-cell dysfunction. Nucleotidase deficiency primarily impacts B-cell maturation; T-cell function usually remains relatively intact. **NEET-PG High-Yield Pearls:** * **ADA Deficiency:** Most common cause of Autosomal Recessive SCID (affects T, B, and NK cells). * **PNP Deficiency:** Primarily affects T-cells (Cell-mediated immunity). * **5'-Nucleotidase:** Marker for mature B-cells; deficiency = Humoral/B-cell defect. * **Clinical Correlation:** Low levels of 5'-nucleotidase are a classic laboratory finding in **Common Variable Immunodeficiency (CVID)** [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 249-250.

Practice by Chapter

Cells and Tissues of the Immune System

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Innate Immunity

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Adaptive Immunity

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Hypersensitivity Reactions

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Autoimmune Diseases

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Immunodeficiency Disorders

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Transplantation Immunopathology

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Immune Response to Infections

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Immunologic Laboratory Techniques

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Tumor Immunology

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