Immunology MCQs: Innate Immunity and Immune Cells (With Answers)

Fifteen multiple choice questions with answers and explanations on innate immunity and the cells and molecules that drive it: pattern recognition receptors, phagocytes, neutrophils, defensins, lysozyme, acute-phase proteins, interferons, natural killer cells, and inflammation. Write down your answers first, then check them against the key at the end.

1. Pattern recognition receptors (PRRs) include:
a. Lectin-like molecules
b. Lipoteichoic acid
c. LPS
d. PAMPs
e. Unmethylated CpG sequences 2. The mononuclear phagocyte system does not include:
a. Endothelial cells
b. Kidney mesangial cells
c. Kupffer cells
d. Lymph node medullary macrophages
e. Monocytes 3. 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 4. A 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 5. Neutrophil defensins are:
a. Antitoxins
b. Oxygen-dependent
c. Enzymes
d. Glycolipids
e. Peptide antibiotics 6. Lysozyme:
a. Is a cytoplasmic organelle
b. Activates complement
c. Is a proteolytic enzyme
d. Splits peptidoglycan
e. Is released by mast cells 7. 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 8. Interferons:
a. Are divided into five main families
b. Are found only in mammalian species
c. Are specific for individual viruses
d. Induce enzyme synthesis in the target cell
e. Affect only infected cells 9. 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 10. 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 11. Acute inflammation can be initiated by:
a. An increase in vascular permeability
b. C3
c. Influx of neutrophils
d. Lysozyme
e. Mast cell activation 12. Clonal selection occurs when antigen is encountered by:
a. Basophils
b. Eosinophils
c. Mast cells
d. Neutrophils
e. T cells 13. Which of the following statements regarding plasma cells is correct?
a. They are derived from T cells
b. They develop into B cells
c. They have a highly developed rough endoplasmic reticulum
d. They have a thin layer of cytoplasm
e. They secrete large amounts of gamma interferon 14. After a primary contact with antigen, specific antibodies become readily detectable in serum after:
a. 10 minutes
b. 1 hour
c. 5 to 7 days
d. 3 to 5 weeks
e. Only following a second contact with antigen 15. Adoptive transfer of acquired immune responsiveness involves the transfer of:
a. Antibody
b. Complement
c. Phagocytes
d. Lymphocytes
e. Serum

Answer key

1. a. Lectin-like molecules
2. a. Endothelial cells
3. c. Contains microbicidal cytoplasmic granules
4. a. Antibody of a single specificity related to that on the surface of the parent B cell
5. e. Peptide antibiotics
6. d. Splits peptidoglycan
7. a. Chondroitin sulfate
8. d. Induce enzyme synthesis in the target cell
9. d. Kill only by damaging the target cell outer membrane
10. a. By phagocytosis
11. e. Mast cell activation
12. e. T cells
13. c. They have a highly developed rough endoplasmic reticulum
14. c. 5 to 7 days
15. d. Lymphocytes

Why these are the answers

1. Lectin-like molecules. Pattern recognition receptors are the host's sensors on innate immune cells (such as macrophages and dendritic cells) that detect conserved microbial structures. Lectin-like receptors, such as the mannose receptor, are host PRRs. The other options, lipoteichoic acid, LPS, and unmethylated CpG DNA, are PAMPs (pathogen-associated molecular patterns), the microbial targets that PRRs recognize, not the receptors themselves. The trap is to confuse the sensor with what it senses.

2. Endothelial cells. The mononuclear phagocyte system is made up of blood monocytes and the tissue macrophages derived from them, including Kupffer cells in the liver, mesangial cells in the kidney, and medullary macrophages in lymph nodes. Endothelial cells line blood vessels and are structural, not phagocytic cells of this lineage, so they are the exception.

3. Contains microbicidal cytoplasmic granules. Neutrophils are professional phagocytes packed with granules full of antimicrobial contents (defensins, lysozyme, myeloperoxidase). They are not stem cells, they are not similar to mast cells, and their granules stain neutrally, unlike the eosin-staining granules of eosinophils. The defining feature among these options is their microbicidal granules.

4. Antibody of a single specificity related to that on the surface of the parent B cell. A plasma cell is a terminally differentiated B cell devoted to one job: secreting antibody of the single specificity that its parent B cell displayed as a receptor. It does not make many different antibodies, does not secrete the antigen, and its product matches the original B-cell receptor's specificity.

5. Peptide antibiotics. Defensins are small cationic antimicrobial peptides stored in neutrophil granules. They act like the body's own peptide antibiotics, inserting into microbial membranes and forming pores that disrupt the membrane and kill the microbe. They are not enzymes, glycolipids, or antitoxins.

6. Splits peptidoglycan. Lysozyme is an enzyme found in tears, saliva, and phagocyte granules that attacks bacteria by hydrolyzing the bond between the sugars (NAM and NAG) in the peptidoglycan cell wall, breaking the wall apart. It is not an organelle, it does not activate complement, and it is not a proteolytic enzyme (it cleaves a sugar backbone, not protein).

7. Chondroitin sulfate. Acute-phase proteins are made mainly by the liver (under IL-6 signaling) and rise or fall sharply during inflammation; they include C-reactive protein, fibrinogen, mannose-binding lectin, and serum amyloid P. Chondroitin sulfate is a structural glycosaminoglycan of cartilage and connective tissue, not an acute-phase reactant, so it is the exception.

8. Induce enzyme synthesis in the target cell. Interferons do not kill viruses directly. They bind receptors on neighboring uninfected cells and switch on antiviral enzymes (such as 2',5'-oligoadenylate synthetase and protein kinase R) that block viral replication if that cell becomes infected. The distractors are wrong: interferons are grouped into three types (I, II, and III), not five families; they are not limited to mammals; they are not virus-specific (a single interferon acts against many viruses); and they act on neighboring cells, not only infected ones.

9. Kill only by damaging the target cell outer membrane. This is the false statement, so it is the exception. NK cells do more than punch the membrane: they release perforin to form pores and then deliver granzymes (serine proteases) that trigger the target cell's own apoptosis program. They also contain TNF and respond to interferon. So it is wrong to say they kill only by outer-membrane damage.

10. By phagocytosis. The hallmark way neutrophils attack bacteria is phagocytosis: they engulf the microbe into a phagosome, which fuses with granules to form a phagolysosome. Inside, they kill using both oxygen-dependent (respiratory burst) and oxygen-independent mechanisms, so neither "exclusively oxygen-dependent" nor "exclusively oxygen-independent" is correct. Neutrophils do not secrete complement or interferon as their attack mechanism.

11. Mast cell activation. Increased vascular permeability and neutrophil influx are central events of acute inflammation, but they are downstream results; the question asks what initiates it. Mast cell activation, triggered by tissue injury or bound IgE, causes immediate degranulation and release of preformed histamine, which sets the inflammatory cascade in motion. So mast cell activation is the initiating event among these options.

12. T cells. Clonal selection is a property of the adaptive immune system, the T cells and B cells, which carry rearranged, antigen-specific receptors. When antigen binds a matching receptor, that specific clone is selected to proliferate and differentiate. Of the options given, only T cells belong to the adaptive system; basophils, eosinophils, mast cells, and neutrophils are innate cells without antigen-specific receptors.

13. They have a highly developed rough endoplasmic reticulum. A plasma cell exists to mass-produce and secrete antibody, so its cytoplasm is filled with rough endoplasmic reticulum and a prominent Golgi to support intense protein synthesis. It is derived from a B cell (not a T cell), does not turn back into a B cell, has abundant (not thin) cytoplasm, and secretes antibody rather than gamma interferon.

14. 5 to 7 days. After first meeting a new antigen, there is a lag while naive lymphocytes recognize it, expand, and differentiate into plasma cells. Specific antibody (initially IgM) becomes readily detectable in serum only after roughly 5 to 7 days. The very short intervals are far too early, and detectable antibody does not require waiting for a second exposure.

15. Lymphocytes. Adoptive transfer means giving a non-immune recipient the cells that carry acquired immunity, namely sensitized lymphocytes (T or B cells). Transferring antibody or serum instead would be passive immunity, not adoptive transfer, and complement or phagocytes do not carry antigen-specific memory.