Immunopathology Practice Questions

Q: Which cell type lacks HLA antigen?

Red blood cell. The Human Leukocyte Antigen (HLA) system is the human version of the Major Histocompatibility Complex (MHC). **MHC Class I** antigens (HLA-A, B, and C) are expressed on the surface of almost all **nucleated cells** in the body, as well as on platelets. **Why Red Blood Cells (RBCs) are the correct answer:** Mature erythrocytes (RBCs) lack a nucleus and most organelles [1]. Consequently, they **do not express HLA antigens** on their surface. Instead, RBCs express their own specific blood group antigens (ABO and Rh systems). This lack of HLA expression is clinically significant as it prevents HLA-mediated graft-versus-host reactions during simple packed red cell transfusions. **Analysis of Incorrect Options:** * **Monocytes (A) and Neutrophils (C):** As nucleated white blood cells, both are rich in MHC Class I antigens. Monocytes, being professional antigen-presenting cells (APCs), also express **MHC Class II** (HLA-DR, DP, DQ). * **Thrombocytes (B):** Although platelets (thrombocytes) are anucleated fragments of megakaryocytes, they **do express MHC Class I** antigens. This is a high-yield distinction; HLA antibodies can lead to "platelet refractoriness" in multi-transfused patients. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I:** Found on all nucleated cells + Platelets (except RBCs). * **MHC Class II:** Found only on Antigen Presenting Cells (Macrophages, B-cells, Dendritic cells) and activated T-cells. * **Exceptions:** Neurons, corneal endothelium, and placental trophoblasts also show very low or absent HLA expression, contributing to their "immune-privileged" status. * **Gene Locus:** HLA genes are located on the **Short arm of Chromosome 6**. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 577-586.

Q: What percentage of peripheral lymphocytes are null cells?

10-15%. **Explanation:** In the context of immunopathology, peripheral blood lymphocytes are categorized based on their surface markers and lineage. **Null cells** are a population of lymphocytes that lack the characteristic surface markers of either T-cells (CD3) or B-cells (surface Immunoglobulins). The majority of these null cells are **Natural Killer (NK) cells**, which are identified by markers such as CD16 and CD56. In a healthy individual, the distribution of peripheral lymphocytes is approximately: * **T-cells:** 60–70% [1] * **B-cells:** 10–20% [1] * **Null cells (NK cells):** 10–15% [1] **Analysis of Options:** * **Option C (10-15%):** This is the correct physiological range for null cells in the peripheral blood. * **Option A (0-1%):** This value is too low; it might represent rare progenitor cells but does not account for the significant NK cell population. * **Option B (5-10%):** While closer, this underestimates the standard clinical range (10-15%) cited in major pathology textbooks like Robbins. * **Option D (15-20%):** This range overlaps more closely with the B-cell population rather than null cells. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Function:** Unlike T and B cells, NK cells are part of the **innate immune system** and do not require prior sensitization. * **MHC Restriction:** NK cells are **not MHC-restricted**. They kill cells that show "missing self" (downregulation of MHC Class I), a common tactic used by viruses and tumors. * **Markers:** For MCQ purposes, remember **CD16** (FcγRIII) and **CD56** as the definitive markers for NK cells. * **Large Granular Lymphocytes (LGLs):** Morphologically, null cells often appear as LGLs in peripheral smears. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 579-580.

Q: Which of the following is a pan-leukocyte marker?

CD45. **Explanation:** **CD45**, also known as the **Leukocyte Common Antigen (LCA)**, is a transmembrane protein tyrosine phosphatase essential for T and B cell receptor signaling. It is expressed on the surface of **all hematopoietic cells** (except mature erythrocytes and platelets), making it the definitive **pan-leukocyte marker**. In histopathology, immunohistochemistry (IHC) for CD45 is the primary tool used to differentiate undifferentiated malignant tumors of lymphoid origin (lymphomas) from carcinomas or sarcomas. **Analysis of Incorrect Options:** * **CD19:** This is a specific marker for **B-lymphocytes** [1]. It is expressed from the early stages of B-cell development until the plasma cell stage, making it a pan-B cell marker, not a pan-leukocyte marker [1]. * **CD3:** This is the definitive marker for **T-lymphocytes** [1]. It is part of the T-cell receptor (TCR) complex and is used to identify cells of T-lineage [1]. * **CD45 RO:** This is a specific isoform of CD45. While CD45 is the broad marker, the "RO" isoform is specifically expressed on **memory T-cells** and a subset of B-cells/monocytes. It is not universal to all leukocytes. **High-Yield Clinical Pearls for NEET-PG:** * **CD45 (LCA):** If a biopsy is "LCA positive," the diagnosis is almost certainly a **Lymphoma**. * **CD34:** Marker for hematopoietic **stem cells** (used in leukemia diagnosis). * **CD15 & CD30:** Classic markers for **Reed-Sternberg cells** in Hodgkin Lymphoma (except the lymphocyte-predominant type). * **CD68:** Marker for **Macrophages/Monocytes**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, p. 598.

Question 1

Which cell type lacks HLA antigen?

Accepted Answer

Red blood cell

Answer

Monocyte

Answer

Thrombocyte

Answer

Neutrophil

Question 2

Which of the following antibody patterns is seen most commonly?

Accepted Answer

IgG

Answer

IgA

Answer

IgM

Answer

IgE

Question 3

Nitro-blue tetrazolium test is done for which condition?

Accepted Answer

Chronic granulomatous disorder

Answer

Chronic glomerulonephritis

Answer

Acute granulomatous disease

Answer

Chediak Higashi syndrome

Question 4

Contact dermatitis is an example of which hypersensitivity reaction?

Accepted Answer

Type IV hypersensitivity

Answer

Type I hypersensitivity

Answer

Type II hypersensitivity

Answer

Type III hypersensitivity

Question 5

Which interleukin is secreted by Th1 cells?

Accepted Answer

IL-2

Answer

IL-4

Answer

IL-10

Answer

IL-13

Question 6

What percentage of peripheral lymphocytes are null cells?

Accepted Answer

10-15%

Answer

0-1%

Answer

5-10%

Answer

15-20%

Question 7

Which type of hypersensitivity reaction is Rh incompatibility?

Accepted Answer

Type 2 hypersensitivity reaction

Answer

Type 1 hypersensitivity reaction

Answer

Type 3 hypersensitivity reaction

Answer

Type 4 hypersensitivity reaction

Question 8

Human leukocyte antigen (HLA), which plays an important role in graft rejection, is present on which chromosome?

Accepted Answer

Short arm of chromosome 6

Answer

Long arm of chromosome 6

Answer

Short arm of chromosome 3

Answer

Long arm of chromosome 3

Question 9

What is the hallmark of AIDS in terms of cell count reduction?

Accepted Answer

CD4

Answer

CD3

Answer

CD8

Answer

CD20

Question 10

Which of the following is a pan-leukocyte marker?

Accepted Answer

CD45

Answer

CD19

Answer

CD3

Answer

CD45 RO

Immunopathology — MCQs

Immunopathology — MCQs

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