Immunopathology — MCQs

A 35-year-old woman with end-stage renal disease of unknown etiology is transplanted with a cadaver kidney. The patient develops oliguria shortly after transplantation and a renal biopsy shows immediate (hyperacute) rejection. Immunosuppression improves renal function. Which of the following represents the principle target for immune attack directed against this patient's allograft?

Q308Medium

Candida infections are commonly seen in patients with which of the following conditions?

Q309Medium

Which of the following is NOT affected in Graft-Versus-Host reaction?

Q310Easy

What is the normal CD4:CD8 ratio?

Immunopathology Indian Medical PG Practice Questions and MCQs

Question 301: What is the mechanism of acute rheumatic fever?

A. Cross-reactivity with endogenous antigen (Correct Answer)
B. Innocent bystander effect
C. Due to toxin secretion by streptococci
D. Release of pyrogenic cytokines

Explanation: ### Explanation **Mechanism of Acute Rheumatic Fever (ARF)** Acute Rheumatic Fever is a classic example of a **Type II Hypersensitivity reaction** mediated by **molecular mimicry**. Following a Group A Streptococcal (GAS) pharyngitis, the body produces antibodies against the streptococcal **M-protein** [1]. Due to structural similarities, these antibodies cross-react with endogenous self-antigens in the heart (sarcolemmal membrane/myosin), joints, and brain [2]. This "cross-reactivity with endogenous antigens" leads to immune-mediated tissue destruction. **Analysis of Options:** * **Option A (Correct):** Molecular mimicry between the M-protein of *S. pyogenes* and cardiac myosin/laminin triggers an autoimmune attack on the heart valves and myocardium [1]. * **Option B (Incorrect):** The "innocent bystander effect" usually refers to Type II or III reactions where healthy cells are damaged during an immune response against a foreign pathogen or drug (e.g., drug-induced hemolysis), but it is not the primary mechanism of ARF. * **Option C (Incorrect):** While Streptococci secrete toxins (like Streptolysin O), these cause direct cellular damage or systemic symptoms (e.g., Scarlet Fever), not the delayed autoimmune sequelae seen in ARF. * **Option D (Incorrect):** Pyrogenic cytokines (IL-1, TNF) cause fever but do not explain the specific organ damage (carditis, chorea) characteristic of ARF. **High-Yield Clinical Pearls for NEET-PG:** * **Pathognomonic Finding:** **Aschoff bodies** (granulomatous inflammation) containing **Anitschkow cells** ("caterpillar cells" with condensed chromatin) [1]. * **Jones Criteria:** Used for diagnosis (Major: Joint, Carditis, Nodules, Erythema marginatum, Sydenham chorea). * **Key Association:** ARF follows **pharyngeal** infection only, whereas Post-Streptococcal Glomerulonephritis (PSGN) can follow either skin or pharyngeal infections. * **Valvular Involvement:** Mitral valve is most commonly affected (Mitral Stenosis is the most common chronic sequela). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, p. 566. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Infectious Diseases, pp. 372-374.

Question 302: Acute auto-graft rejection occurs within what timeframe?

A. A few hours
B. Less than 1 month
C. Less than 6 months (Correct Answer)
D. 6-12 months

Explanation: **Explanation:** The classification of transplant rejection is based on the timing and the underlying immunological mechanism. **Acute rejection** typically occurs within the first few days to **less than 6 months** post-transplantation [1]. 1. **Why Option C is correct:** Acute rejection is primarily a T-cell mediated process (Type IV hypersensitivity) involving CD8+ cytotoxic T cells and CD4+ helper T cells reacting against donor HLA antigens [1], [2]. It can also involve a humoral component (Type II hypersensitivity) with antibodies against donor endothelium [1]. While it can occur as early as 7–10 days, it is most commonly seen within the first 6 months. It is usually reversible with increased immunosuppression (e.g., corticosteroids) [2]. 2. **Analysis of Incorrect Options:** * **Option A (A few hours):** This describes **Hyperacute Rejection**. It occurs within minutes to hours due to pre-formed anti-donor antibodies (Type II hypersensitivity) leading to thrombosis and fibrinoid necrosis [1]. * **Option B (Less than 1 month):** While acute rejection can occur within a month, the standard clinical definition for the "acute" window extends up to 6 months. * **Option D (6-12 months):** Rejection occurring after 6 months is generally classified as **Chronic Rejection**. This involves slow, progressive fibrosis, intimal thickening of blood vessels (arteriosclerosis), and organ atrophy. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperacute Rejection:** Mediated by pre-formed antibodies (e.g., ABO incompatibility). Characterized by "White Graft" reaction [1]. * **Acute Cellular Rejection:** Histology shows mononuclear (lymphocytic) infiltrate and tubulitis (in kidneys) or endotheliitis [1]. * **Chronic Rejection:** Dominant mechanism is humoral and cytokine-mediated, leading to graft vascular sclerosis and interstitial fibrosis. * **Graft-versus-Host Disease (GVHD):** Occurs when donor T-cells attack host tissues; common in bone marrow transplants. Key targets: Skin, Liver, and GI tract. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 241-242. [2] 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. 180-181.

Question 303: Which of the following is a Type 2 Hypersensitivity reaction?

A. Chronic Kidney Rejection episode
B. Autoimmune Hemolytic anaemia (Correct Answer)
C. Arthus reaction
D. Mitsuda reaction

Explanation: **Explanation:** **Type 2 Hypersensitivity** is defined as **Antibody-Mediated Cytotoxicity**. In this reaction, IgG or IgM antibodies are directed against antigens present on the surface of specific cells or tissues. This leads to cell destruction via three mechanisms: Opsonization/Phagocytosis, Complement-mediated lysis, or Antibody-Dependent Cellular Cytotoxicity (ADCC) [1]. * **Why Option B is Correct:** In **Autoimmune Hemolytic Anemia (AIHA)**, antibodies (IgG in warm AIHA; IgM in cold AIHA) bind to antigens on the Red Blood Cell (RBC) membrane [2]. These "tagged" RBCs are then destroyed by splenic macrophages or complement lysis, making it a classic example of Type 2 hypersensitivity. **Analysis of Incorrect Options:** * **Option A (Chronic Kidney Rejection):** This is primarily a **Type 4 (Delayed-type)** hypersensitivity reaction involving T-cell mediated cytokine release and chronic inflammation leading to fibrosis. (Note: Hyperacute rejection is Type 2). * **Option C (Arthus Reaction):** This is a localized **Type 3** hypersensitivity reaction. It involves the formation of immune complexes (Antigen-Antibody) that deposit in vessel walls, causing focal vasculitis and tissue necrosis. * **Option D (Mitsuda Reaction):** This is a skin test used in Leprosy that represents a **Type 4** hypersensitivity reaction (Granulomatous inflammation). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity (ACID):** **A**naphylactic (Type 1), **C**ytotoxic (Type 2), **I**mmune-Complex (Type 3), **D**elayed-type (Type 4). * **Other Type 2 Examples:** Myasthenia Gravis, Graves’ Disease [1], Goodpasture Syndrome, and Rheumatic Fever. * **Key Distinction:** If the antigen is **soluble/circulating**, it is Type 3; if the antigen is **fixed on a cell surface**, it is Type 2. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 213-214. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 651-652.

Question 304: Which cytokine is responsible for activating macrophages and their conversion into epithelioid cells or giant cells?

A. Interleukin-2 (IL-2)
B. Interleukin-17 (IL-17)
C. Tumor Necrosis Factor-alpha (TNF-alpha)
D. Interferon-gamma (IFN-gamma) (Correct Answer)

Explanation: **Explanation:** The correct answer is **Interferon-gamma (IFN-gamma)**. This question tests the understanding of granulomatous inflammation and the Type IV hypersensitivity reaction [1]. **Why IFN-gamma is correct:** In the formation of a granuloma, CD4+ T-cells (specifically Th1 cells) recognize an antigen and secrete **IFN-gamma** [2]. This cytokine is the most potent activator of macrophages (Classical activation/M1 pathway) [1]. Under the influence of IFN-gamma, macrophages undergo structural changes: they increase their cytoplasm and organelles to become **epithelioid cells** (which have secretory rather than phagocytic functions) or fuse to form **multinucleated giant cells** (e.g., Langhans giant cells) [1]. **Why other options are incorrect:** * **IL-2:** Primarily acts as a T-cell growth factor, stimulating the proliferation of T-lymphocytes [2]. * **IL-17:** Produced by Th17 cells; its primary role is the recruitment of neutrophils to sites of inflammation. * **TNF-alpha:** While TNF-alpha is crucial for *maintaining* the structural integrity of a granuloma (preventing its breakdown), it is not the primary cytokine responsible for the initial transformation of macrophages into epithelioid cells. **NEET-PG High-Yield Pearls:** * **Granuloma Definition:** A focal collection of modified macrophages (epithelioid cells) surrounded by a rim of lymphocytes [1]. * **Key Cytokines:** **IL-12** (induces Th1 differentiation) → **IFN-gamma** (activates macrophages) → **TNF-alpha** (maintains granuloma). * **Clinical Correlation:** Anti-TNF drugs (e.g., Infliximab) can cause the breakdown of old granulomas, leading to the reactivation of latent Tuberculosis. * **Epithelioid cells** are the hallmark of granulomatous inflammation; they resemble epithelial cells due to their abundant pink cytoplasm and indistinct cell borders [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, p. 109. [2] 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. 173-174.

Question 305: Which cytokine has a predominant anti-inflammatory action?

A. Interleukin 6
B. GM-CSF
C. TGF-beta (Correct Answer)
D. TNF-Alpha

Explanation: **Explanation:** The balance between pro-inflammatory and anti-inflammatory cytokines is crucial for immune homeostasis. **TGF-beta (Transforming Growth Factor-beta)** is a potent **anti-inflammatory** cytokine. It functions by inhibiting the proliferation and activation of lymphocytes (T-cells and B-cells) and suppressing the production of pro-inflammatory cytokines [1]. Additionally, it plays a vital role in tissue repair and fibrosis by stimulating collagen synthesis [1]. Along with **IL-10**, TGF-beta is considered a primary mediator of immune tolerance and resolution of inflammation. **Analysis of Incorrect Options:** * **Interleukin 6 (IL-6):** A classic **pro-inflammatory** cytokine. It is a major inducer of the "acute phase response" in the liver (CRP production) and is involved in fever and the transition from innate to adaptive immunity [1]. * **GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor):** A growth factor that stimulates the bone marrow to produce granulocytes and monocytes [1]. It acts as a **pro-inflammatory** mediator by enhancing the survival and activation of mature macrophages and neutrophils. * **TNF-Alpha (Tumor Necrosis Factor-alpha):** A "master" **pro-inflammatory** cytokine produced primarily by macrophages. It mediates septic shock, induces endothelial activation, and recruits leukocytes to sites of infection [1]. **High-Yield Clinical Pearls for NEET-PG:** * **The "Anti-inflammatory Duo":** Always remember **IL-10 and TGF-beta** as the primary anti-inflammatory cytokines. * **TGF-beta Dual Role:** While anti-inflammatory, it is the most important cytokine for **fibrosis** (stimulates fibroblasts) [1]. * **IL-1 and TNF-alpha:** These are the primary mediators of acute inflammation [1]. * **IL-8:** The most potent chemotactic factor for **neutrophils**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 111-116.

Question 306: Memory T cells can be identified by using which of the following markers?

A. CD45RA
B. CD45RB
C. CD45RC
D. CD45RO (Correct Answer)

Explanation: **Explanation:** The correct answer is **CD45RO**. **1. Why CD45RO is correct:** CD45 (also known as Leukocyte Common Antigen) is a transmembrane protein tyrosine phosphatase essential for T-cell activation. It exists in several isoforms due to alternative splicing of exons 4, 5, and 6 (A, B, and C). * **CD45RO** is the shortest isoform (lacking exons A, B, and C). It is specifically expressed on **Memory T cells** [1] and activated effector T cells. Its structure allows for more efficient interaction with the T-cell receptor (TCR) complex, facilitating a rapid secondary immune response upon re-exposure to an antigen. **2. Why the other options are incorrect:** * **CD45RA:** This isoform contains the 'A' exon. It is the classic marker for **Naive T cells** (T cells that have not yet encountered their specific antigen) [1]. * **CD45RB and CD45RC:** These isoforms are typically expressed on naive B cells, subsets of T cells, and other myeloid cells. They are not specific markers used to differentiate memory T cells in a clinical or examination context. **3. NEET-PG High-Yield Pearls:** * **The Switch:** When a Naive T cell (CD45RA+) encounters an antigen, it undergoes activation and "switches" its expression to the memory phenotype (**CD45RO+**). * **CD4 vs. CD8:** Both Helper (CD4+) and Cytotoxic (CD8+) T cells can be subdivided into Naive and Memory subsets using these markers. * **Clinical Correlation:** In certain immunodeficiencies or lymphomas, the ratio of CD45RA to CD45RO can be diagnostically significant. * **Mnemonic:** "R**O**" for **O**ld (Memory) cells; "R**A**" for **A**mateur (Naive) cells. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 198.

Question 307: A 35-year-old woman with end-stage renal disease of unknown etiology is transplanted with a cadaver kidney. The patient develops oliguria shortly after transplantation and a renal biopsy shows immediate (hyperacute) rejection. Immunosuppression improves renal function. Which of the following represents the principle target for immune attack directed against this patient's allograft?

A. ABO antigens (Correct Answer)
B. Bacterial antigens
C. Glomerular basement membrane antigens
D. b2-Microglobulin

Explanation: ### Explanation **Correct Option: A (ABO antigens)** Hyperacute rejection occurs within minutes to hours of transplantation [1]. It is mediated by **pre-formed circulating antibodies** (Type II Hypersensitivity) in the recipient's blood that react against antigens on the donor vascular endothelium. The primary targets for these pre-formed antibodies are **ABO blood group antigens** and **HLA Class I antigens** [3]. Once these antibodies bind, they activate the complement system, leading to endothelial injury, fibrin-platelet thrombi, and rapid ischemic necrosis of the graft [1], [2]. **Analysis of Incorrect Options:** * **B (Bacterial antigens):** While infections can occur post-transplant due to immunosuppression, they do not mediate the immunological rejection of the graft itself. * **C (Glomerular basement membrane antigens):** Antibodies against GBM are characteristic of **Goodpasture Syndrome**. While this can cause renal failure in the native kidney, it is not the mechanism behind hyperacute allograft rejection. * **D (β2-Microglobulin):** This is a component of MHC Class I molecules and is also associated with dialysis-related amyloidosis in long-term ESRD patients. It is not the target of pre-formed antibodies in hyperacute rejection. **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** Hyperacute (minutes/hours), Accelerated (days), Acute (days/weeks), Chronic (months/years). * **Morphology:** Grossly, the kidney becomes cyanotic, mottled, and flaccid ("Blue kidney") [1]. Histologically, look for **neutrophilic infiltration** of arterioles and widespread **microvascular thrombosis** [2]. * **Prevention:** Hyperacute rejection is prevented by **cross-matching** (testing recipient serum against donor lymphocytes) and ABO typing [3]. * **Treatment:** Unlike acute rejection, hyperacute rejection is generally irreversible and requires immediate removal of the graft [1]. (Note: The question mentions immunosuppression improved function, which clinically suggests an acute component, but the "immediate/hyperacute" label points strictly to pre-formed antibody targets). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 241-242. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, p. 242. [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. 179-180.

Question 308: Candida infections are commonly seen in patients with which of the following conditions?

A. Chronic granulomatous disease
B. Chediak-Higashi syndrome
C. Myeloperoxidase deficiency (Correct Answer)
D. Lazy leukocyte syndrome

Explanation: **Explanation:** The correct answer is **Myeloperoxidase (MPO) deficiency**. **1. Why Myeloperoxidase Deficiency is Correct:** MPO is a lysosomal enzyme found in neutrophil granules that converts hydrogen peroxide ($H_2O_2$) and chloride ions into **hypochlorous acid (HOCl)**—the most potent bactericidal agent in the respiratory burst. In MPO deficiency, neutrophils can still produce superoxide and $H_2O_2$, but cannot produce HOCl. While most patients are asymptomatic because other killing mechanisms (like nitric oxide) compensate, the most characteristic clinical manifestation is **recurrent disseminated Candidiasis**, as HOCl is essential for effective fungal killing. **2. Why Other Options are Incorrect:** * **A. Chronic Granulomatous Disease (CGD):** Caused by a defect in **NADPH oxidase**, leading to an inability to produce any reactive oxygen species ($O_2^-$). Patients are highly susceptible to **Catalase-positive organisms** (e.g., *S. aureus, Aspergillus, Nocardia, Serratia*), but MPO deficiency is more specifically linked to *Candida* in exam vignettes. * **B. Chediak-Higashi Syndrome:** A defect in **LYST gene** (vesicle trafficking), resulting in giant lysosomal granules [2]. It presents with partial albinism, peripheral neuropathy, and recurrent pyogenic infections, but not specifically isolated *Candida* susceptibility [2]. * **C. Lazy Leukocyte Syndrome:** Characterized by defective **neutrophil chemotaxis** and abnormal inflammatory response. It involves a defect in the actin cytoskeleton, leading to neutropenia and recurrent infections, but is not the classic association for *Candida*. **High-Yield Clinical Pearls for NEET-PG:** * **MPO Deficiency:** Most common inherited defect of phagocytes; Nitroblue Tetrazolium (NBT) test is **Normal** (positive). * **CGD:** NBT test is **Abnormal** (negative/colorless) because no superoxide is produced. * **DHR (Dihydrorhodamine) Flow Cytometry:** Now the gold standard for diagnosing CGD. Note: Candida infections are also commonly seen in patients with immunosuppression such as AIDS and diabetes [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Infectious Diseases, pp. 394-395. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 245-246.

Question 309: Which of the following is NOT affected in Graft-Versus-Host reaction?

A. Skin
B. Gastrointestinal tract
C. Liver
D. Lung (Correct Answer)

Explanation: **Explanation:** Graft-Versus-Host Disease (GVHD) occurs when immunocompetent T-cells from a donor graft recognize the recipient’s (host) HLA antigens as foreign and initiate an immune attack [1]. This typically occurs in the setting of hematopoietic stem cell transplantation or solid organ transplants rich in lymphoid tissue. **Why Lung is the Correct Answer:** While GVHD is a systemic multisystem disorder, it characteristically targets specific "barrier" organs and the biliary system. The **Lung** is generally **not** considered a primary target organ in the classic triad of GVHD. While pulmonary complications (like bronchiolitis obliterans) can occur in chronic GVHD, the lung is not part of the diagnostic triad used to grade the severity of acute GVHD. **Analysis of Incorrect Options:** * **Skin (A):** This is the most common and usually the first organ affected [1]. It presents as a pruritic maculopapular rash, often starting on the palms, soles, and neck, which can progress to generalized erythroderma or toxic epidermal necrolysis. * **Gastrointestinal Tract (B):** The donor T-cells attack the intestinal mucosa, leading to profuse watery or bloody diarrhea, abdominal pain, and malabsorption [1]. * **Liver (C):** GVHD targets the small bile ducts, leading to cholestasis [1]. Clinical signs include jaundice and elevated alkaline phosphatase and bilirubin levels. **High-Yield Clinical Pearls for NEET-PG:** * **The Classic Triad:** Acute GVHD (occurring within 100 days) primarily affects the **Skin, Liver, and GIT** [1]. * **Prerequisites (Billingham’s Criteria):** 1. Graft must contain immunologically competent cells. 2. Recipient must possess antigens lacking in the donor. 3. Recipient must be immunosuppressed (cannot reject the graft). * **Mechanism:** Type IV Hypersensitivity reaction (Cell-mediated). * **Prevention:** Depletion of donor T-cells before transfusion/transplant can reduce GVHD risk but may increase the risk of graft failure or leukemia recurrence (loss of Graft-versus-leukemia effect). **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. 182-183.

Question 310: What is the normal CD4:CD8 ratio?

A. 3:1
B. 2:1 (Correct Answer)
C. 1:2
D. 1:3

Explanation: **Explanation:** In a healthy individual, the normal **CD4:CD8 ratio is approximately 2:1** [1]. This ratio represents the balance between T-helper cells (CD4+), which orchestrate the immune response, and T-cytotoxic cells (CD8+), which directly kill infected or malignant cells. In peripheral blood, CD4+ T cells typically make up about 60–70% of the T-cell population, while CD8+ T cells account for about 30%. **Analysis of Options:** * **Option B (2:1):** This is the physiological baseline [1]. Maintaining this ratio is crucial for effective immune surveillance without excessive immunosuppression or autoimmunity. * **Option A (3:1):** While some healthy individuals may have a slightly higher ratio, 2:1 is the standard clinical benchmark used in pathology and immunology. * **Options C & D (1:2 and 1:3):** These represent an **inverted ratio**. A ratio of less than 1:1 is clinically significant and pathological, indicating either a depletion of CD4 cells or an expansion of CD8 cells [1]. **Clinical Pearls for NEET-PG:** 1. **HIV/AIDS:** The hallmark of HIV progression is the progressive destruction of CD4+ T cells, leading to an **inverted CD4:CD8 ratio** (often <0.5) [1]. Monitoring this ratio is vital for staging the disease. 2. **Other Causes of Inversion:** Apart from HIV, an inverted ratio can be seen in viral infections (EBV, CMV), aging, chronic inflammation, and certain primary immunodeficiencies. 3. **Sarcoidosis:** A high-yield contrast is that in Sarcoidosis, the CD4:CD8 ratio in **Bronchoalveolar Lavage (BAL) fluid** is typically significantly **increased (>3.5:1)**, aiding in diagnosis. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Diseases Of The Urinary And Male Genital Tracts, pp. 555-556.

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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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