MCQs Immunology (41-55): Basic Immunology with Answers

I have here presented fifteen Multiple Choice Questions from the Immunology sections. These MCQs are collected via various sources, and some of the MCQs I have prepared myself. I have posted the key (correct answers) to some questions and left others for you to try and explore.

### Immunology Practice Questions (41–55)

41. Pattern recognition receptors (PRR) include:

a. Lectin-like molecules

b. Lipoteichoic acid

c. LPS

d. PAMPs

e. Unmethylated CpG sequences

42. The mononuclear phagocyte system does not include:

a. Endothelial cells

b. Kidney mesangial cells

c. Kupffer cells

d. Lymph node medullary macrophages

e. Monocytes

43. A polymorphonuclear neutrophil (PMN):

a. Is a bone marrow stem cell

b. Is closely similar to a mast cell

c. Contains microbicidal cytoplasmic granules

d. Is not a professional phagocytic cell

e. Has granules that stain with eosin

44. Plasma cells are a specific type of immune cells that play major roles in antibody-mediated immunity. This plasma cell secretes:

a. Antibody of a single specificity related to that on the surface of the parent B-cell

b. Antibody of two antigen specificities

c. The antigen it recognizes

d. Many different types of antibody

e. Lysozyme

45. Neutrophil defensins are:

a. Anti-toxins

b. Oxygen-dependent

c. Enzymes

d. Glycolipids

e. Peptide antibiotics

46. Lysozyme:

a. Is a cytoplasmic organelle

b. Activates complement

c. Is a proteolytic enzyme

d. Splits peptidoglycan

e. Is released by mast cells

47. Which of the following is not an acute phase protein?

a. Chondroitin sulfate

b. C-reactive protein

c. Fibrinogen

d. Mannose-binding lectin

e. Serum amyloid P component

48. Interferons:

a. Are divided into 5 main families

b. Are found only in mammalian species

c. Are specific for individual viruses

d. Induce enzyme synthesis in the target cell

e. Only affect infected cells

49. Natural killer (NK) cells do not:

a. Contain perforin

b. Contain serine proteases

c. Contain tumor necrosis factor (TNF)

d. Kill only by damaging the target cell outer membrane

e. Respond to interferon

50. Polymorphonuclear neutrophils attack bacteria:

a. By phagocytosis

b. By secreting complement

c. By secreting interferon

d. Exclusively by oxygen-dependent mechanisms

e. Exclusively by oxygen-independent mechanisms

51. Acute inflammation can be initiated by:

a. An increase in vascular permeability

b. C3

c. Influx of neutrophils

d. Lysozyme

e. Mast cell activation

52. Clonal selection occurs when antigen is encountered by:

a. Basophils

b. Eosinophils

c. Mast cells

d. Neutrophils

e. T-cells

53. The major function of plasma cells is to secrete large amounts of antibody. Which of the following statements regarding plasma cells is correct?

a. Are derived from T-cells

b. Develop into B-cells

c. Have a highly developed rough endoplasmic reticulum

d. Have a thin layer of cytoplasm

e. Secrete large amounts of gamma interferon

54. After contact with foreign antigens, the body produces specific antibody. These specific antibodies are readily detectable in serum following primary contact with antigen after:

a. 10 min

b. 1 h

c. 5–7 days

d. 3–5 weeks

e. Only following a second contact with antigen

55. Adoptive transfer of acquired immune responsiveness involves the transfer of:

a. Antibody

b. Complement

c. Phagocytes

d. Lymphocytes

e. Serum

Answer Keys of Basic Immunology Concepts Questions:

Here are the completed explanations for questions 41 to 55 based on your key. For the questions marked as "Try yourself," I have filled in the correct microbiological answers and provided the detailed rationale for you.

Immunology MCQs: Answer Key & Explanations

**41. Correct Answer: a (Lectin-like molecules)**

• Explanation: Pattern Recognition Receptors (PRRs) are receptors expressed by cells of the innate immune system (like macrophages and dendritic cells) to identify pathogen-associated molecular patterns (PAMPs). Lectin-like molecules (such as the mannose receptor) are host PRRs. Lipoteichoic acid, LPS, and unmethylated CpG sequences are all PAMPs—the microbial targets that PRRs bind to.

**42. Correct Answer: a (Endothelial cells)**

• Explanation: The mononuclear phagocyte system (MPS) consists of monocytes circulating in the blood and tissue-fixed macrophages. This includes Kupffer cells (liver), mesangial cells (kidney), and alveolar/medullary macrophages. Endothelial cells line the blood vessels and are structural/vascular cells, not professional phagocytes belonging to the MPS lineage.

**43. Correct Answer: c (Contains microbicidal cytoplasmic granules)**

• Explanation: Polymorphonuclear neutrophils (PMNs) are front-line granulocytes packed with primary (azurophilic) and secondary (specific) cytoplasmic granules filled with antimicrobials like defensins, lysozyme, and myeloperoxidase. They are highly active, short-lived professional phagocytes whose granules stain neutrally, unlike eosinophils (eosin/acidic) or basophils (basic).

**44. Correct Answer: a (Antibody of a single specificity related to that on the surface of the parent B-cell)**

• Explanation: A plasma cell is a terminally differentiated B cell dedicated to a single job. It functions as a factory that secretes highly specific monoclonal antibodies matching the precise idiotype and antigen-binding specificity of the original B-cell receptor (BCR) that encountered the antigen.

**45. Correct Answer: e (Peptide antibiotics)**

• Explanation: Defensins are small, cationic, arginine-rich antimicrobial peptides (AMPs) found within neutrophil granules. They act as endogenous "peptide antibiotics" by inserting themselves into microbial membranes, disrupting the lipid bilayer, and causing cellular lysis through pore formation.

**46. Correct Answer: d (Splits peptidoglycan)**

• Explanation: Lysozyme is an antibacterial enzyme abundant in tears, saliva, and cytoplasmic granules. It targets bacteria by specifically hydrolyzing the $\beta(1\rightarrow4)$ glycosidic bonds between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG), effectively splitting the peptidoglycan backbone of the bacterial cell wall.

**47. Correct Answer: a (Chondroitin sulfate)**

• Explanation: Acute-phase proteins (like CRP, Fibrinogen, MBL, and Serum Amyloid P) are proteins whose plasma concentrations significantly increase or decrease in response to inflammation, synthesized mainly by the liver under IL-6 stimulation. Chondroitin sulfate is a structural sulfated glycosaminoglycan found in cartilage and connective tissues, not an acute-phase reactant.

**48. Correct Answer: d (Induce enzyme synthesis in the target cell)**

• Explanation: Interferons (IFNs) do not neutralize viruses directly. Instead, they bind to specific receptors on neighboring, uninfected host cells, signaling them to activate transcription factors. This triggers the synthesis of antiviral enzymes (such as $2',5'$-oligoadenylate synthetase and protein kinase R) that block viral replication if infection occurs.

**49. Correct Answer: d (Kill only by damaging the target cell outer membrane)**

• Explanation: Natural Killer (NK) cells do not restrict their mechanism to simple outer membrane damage. While they use perforin to form pores in the target membrane, they simultaneously deliver granzymes (serine proteases) into the cell to trigger the intracellular caspase cascade, inducing targeted apoptosis (programmed cell death).

**50. Correct Answer: a (By phagocytosis)**

• Explanation: The primary, hallmark mechanism by which PMNs attack invading bacteria is phagocytosis—engulfing the microbe into a phagosome, which then fuses with lysosomes. Inside this phagolysosome, they eliminate the bacteria using a combination of both oxygen-dependent (respiratory burst) and oxygen-independent pathways.

**51. Correct Answer: e (Mast cell activation)**

• Explanation: While an increase in vascular permeability and neutrophil influx are critical structural components of acute inflammation, the initiation event is typically driven by chemical mediators. Mast cell activation triggered by tissue trauma or tissue-bound IgE causes immediate degranulation, releasing pre-formed histamine that initiates the downstream inflammatory cascades.

**52. Correct Answer: e (T-cells)**

• Explanation: Clonal selection is a fundamental feature exclusive to the adaptive immune system (T-cells and B-cells). When an antigen binds to a specific lymphocyte clone carrying a matching receptor, that distinct cell is selected to proliferate and differentiate into an army of effector and memory cells. Granulocytes (neutrophils, eosinophils, basophils) and mast cells lack rearranged, antigen-specific receptors.

**53. Correct Answer: c (Have a highly developed rough endoplasmic reticulum)**

• Explanation: Because a plasma cell's sole purpose is the mass production and secretion of glycoprotein molecules (antibodies), its morphology changes drastically from a naive B cell. It expands its cytoplasm to house a highly developed rough endoplasmic reticulum (RER) and a prominent Golgi apparatus to sustain intense protein synthesis.

**54. Correct Answer: c (5–7 days)**

• Explanation: Following the primary exposure to a novel antigen, there is a distinct lag phase (typically lasting 3 to 5 days) where naive lymphocytes recognize the antigen, undergo clonal expansion, and differentiate into plasma cells. Specific antibodies (initially IgM) only become readily detectable in serum after roughly 5 to 7 days.

**55. Correct Answer: d (Lymphocytes)**

• Explanation: Adoptive immunity refers to the transfer of immunity from an immune donor to a non-immune recipient specifically through the transfer of sensitized lymphocytes (T-cells or B-cells), rather than pre-formed chemical or protein elements. Transferring serum or antibodies alone is classified as passive immunity.