Test your knowledge on cellular immunology with MCQs on TLRs, leukocyte recruitment, and tissue repair. Includes detailed explanations for medical students.
SECTION 1: RECOGNITION AND INITIATION OF INFLAMMATION 1. A 45-year-old man develops a wound infection. The tissue macrophages recognize the bacteria through pattern recognition receptors. Which of the following best describes the mechanism by which Toll-like receptors (TLRs) contribute to the inflammatory response? a) They directly phagocytose bacteria and present antigens to T cellsb) They recognize pathogen-associated molecular patterns and trigger production of inflammatory mediatorsc) They form pores in bacterial cell walls leading to osmotic lysisd) They bind to antibodies coating bacteria to enhance complement activatione) They release histamine from preformed granules upon bacterial contact Answer: b) They recognize pathogen-associated molecular patterns and trigger production of inflammatory mediators Explanation: TLRs are pattern recognition receptors expressed on sentinel cells like macrophages, dendritic cells, and epithelial cells. They recognize conserved microbial structures (PAMPs) in different cellular compartments - plasma membrane for extracellular microbes, endosomes for ingested microbes, and cytosol for intracellular microbes. Upon engagement, TLRs trigger signaling pathways that lead to production of inflammatory mediators including cytokines, adhesion molecules, and chemokines. They don't directly phagocytose (that's the role of phagocytic receptors), don't form membrane pores (that's complement's membrane attack complex), don't bind antibodies (that's Fc receptors), and don't release histamine (that's mast cells). 2. A research study examines cellular responses to tissue necrosis from various causes including ischemia, trauma, and chemical injury. Which sensor mechanism is responsible for detecting damaged cells regardless of the cause of injury? a) Mannose-binding lectin recognizing microbial sugarsb) TLR-4 detecting lipopolysaccharidec) NOD-like receptors detecting uric acid and ATPd) Complement C3b opsonizing cellular debrise) Scavenger receptors binding oxidized LDL Answer: c) NOD-like receptors detecting uric acid and ATP Explanation: NOD-like receptors (NLRs) are cytosolic receptors that recognize diverse molecules released from damaged cells, making them the universal sensors of cell damage. These include uric acid (DNA breakdown product), ATP (from damaged mitochondria), reduced intracellular K+ (from membrane injury), and cytoplasmic DNA. NLRs activate the inflammasome, which produces IL-1, triggering inflammation. The other options are more specific: mannose-binding lectin and TLR-4 recognize microbes specifically, C3b is a complement product that assists in phagocytosis, and scavenger receptors primarily handle modified lipoproteins. Only NLRs detect damage from ANY cause - ischemia, trauma, or toxins. 3. During an inflammatory response to bacterial infection, which sequence correctly describes the temporal pattern of leukocyte infiltration? a) Lymphocytes → neutrophils → monocytes → plasma cellsb) Eosinophils → basophils → neutrophils → macrophagesc) Neutrophils (6-24h) → monocytes (24-48h) → lymphocytes (later stages)d) Monocytes → neutrophils → eosinophils → mast cellse) Mast cells → neutrophils → lymphocytes → monocytes Answer: c) Neutrophils (6-24h) → monocytes (24-48h) → lymphocytes (later stages) Explanation: The typical sequence in acute inflammation shows neutrophils predominating in the first 6-24 hours, then being replaced by monocytes at 24-48 hours. Neutrophils arrive first because they're more numerous in blood, respond faster to chemokines, and attach more firmly to early adhesion molecules like P-selectin and E-selectin. Neutrophils are short-lived in tissues (undergo apoptosis within days), while monocytes survive longer and may proliferate, becoming the dominant population in prolonged reactions. Lymphocytes typically appear in later stages or in chronic inflammation. Eosinophils are seen mainly in allergic reactions and parasitic infections, not typical bacterial infections. 4. A patient with hereditary angioedema experiences recurrent episodes of severe swelling. This condition results from deficiency of C1 inhibitor. Which of the following best explains the pathophysiology? a) Uncontrolled activation of the membrane attack complex causes cell lysisb) Excessive C3a and C5a production leads to mast cell degranulation and increased vascular permeabilityc) Deficient opsonization results in persistent bacterial infections triggering inflammationd) Impaired phagocytosis allows accumulation of immune complexese) Loss of selectin-mediated leukocyte rolling causes defective neutrophil recruitment Answer: b) Excessive C3a and C5a production leads to mast cell degranulation and increased vascular permeability Explanation: C1 inhibitor (C1 INH) is a key regulatory protein that blocks activation of C1, the first protein of the classical complement pathway. Without it, there's uncontrolled complement activation producing excessive amounts of C3a and C5a - the anaphylatoxins. These fragments stimulate mast cell degranulation and histamine release, causing the vasodilation and increased vascular permeability that leads to angioedema (swelling). The membrane attack complex (MAC) is formed later in the cascade and isn't the primary issue here. Opsonization defects would cause infections, not swelling. This isn't about phagocytosis or leukocyte adhesion - it's specifically about uncontrolled complement activation causing vascular leakage. 5. In a patient with chronic granulomatous disease (CGD), neutrophils fail to kill ingested bacteria. Which enzyme complex is deficient? a) Myeloperoxidaseb) NADPH oxidasec) Inducible nitric oxide synthased) Cyclooxygenase-2e) 5-lipoxygenase Answer: b) NADPH oxidase Explanation: CGD results from inherited deficiencies in components of NADPH oxidase (phagocyte oxidase), which is essential for the respiratory burst that generates superoxide (O2•) and subsequent reactive oxygen species. This enzyme complex normally assembles rapidly when neutrophils phagocytose bacteria - cytosolic components translocate to the phagosomal membrane to form the active enzyme. Without functional NADPH oxidase, neutrophils can phagocytose bacteria but cannot kill them effectively. Myeloperoxidase deficiency causes a milder problem because other microbicidal mechanisms compensate. iNOS is more important in macrophages. COX-2 and 5-lipoxygenase produce inflammatory mediators, not microbicidal substances. 6. A 55-year-old diabetic develops a foot ulcer that fails to heal despite antibiotics. Which factor is LEAST likely contributing to impaired wound healing? a) Reduced oxygen delivery due to peripheral vascular diseaseb) Neuropathy preventing protective sensationsc) Systemic metabolic abnormalities affecting cellular functiond) Excessive neutrophil extracellular trap formatione) Secondary bacterial infection Answer: d) Excessive neutrophil extracellular trap formation Explanation: Diabetic ulcers result from multiple factors: ischemia from vascular disease (atherosclerosis) reduces oxygen delivery needed for healing; neuropathy removes protective pain sensation allowing continued trauma; metabolic abnormalities (hyperglycemia, insulin resistance) impair cellular functions; and the wounds are prone to infection. These are all documented contributors. NET formation, while it can occur in infections, is not a major mechanism of impaired healing in diabetes - in fact, impaired immune function in diabetes might reduce NET formation. The question asks for the LEAST likely contributor, and excessive NET formation doesn't fit the pathophysiology of diabetic wound healing failure the way the other factors do. 7. A researcher studying leukocyte recruitment observes that blocking E-selectin on endothelial cells reduces but doesn't eliminate neutrophil rolling. Which molecule likely compensates? a) ICAM-1b) VCAM-1c) P-selectind) LFA-1e) MAC-1 Answer: c) P-selectin Explanation: Leukocyte rolling is mediated specific