Q.29 Arrange the following steps involved in phagocytosis of bacterium by a
macrophage .
A. Phagosome fuses with lysosome
B. Bacterium is ingested, forming a phagosome
C. Bacterium is killed and then digested by lysosomal enzymes
D. Bacterium becomes attached to membrane evaginations called pseudopodia
Choose the correct answer from the options given below :
- A, C, D, B
- D, B, A, C
- D, C, A, B
- B, C, A, D
Phagocytosis by macrophages eliminates bacteria through chemotaxis, recognition, engulfment, phagolysosome formation, and digestion. For Q.29—”Arrange: A. Phagosome fuses with lysosome, B. Bacterium ingested forming phagosome, C. Bacterium killed/digested by lysosomal enzymes, D. Bacterium attaches to pseudopodia”—correct answer is option 2: D, B, A, C.​
Option Analysis
Option 1: A, C, D, B
Lysosome fusion (A) and killing (C) before attachment (D) or ingestion (B) is impossible—macrophages cannot fuse organelles with non-existent phagosomes.​
Incorrect; reverses chronological order.​Option 2: D, B, A, C
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D: Bacterium binds Fc receptors/complement via opsonins, triggering pseudopod extension.
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B: Pseudopodia enclose bacterium, pinching off phagosome (early phagosome forms).
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A: Phagosome matures, fuses with lysosome → phagolysosome (acidifies to pH 4.5).
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C: ROS, NO, hydrolases (cathepsins, lysozyme) kill/digest bacterium; antigens presented.
Correct; matches standard 4-step phagocytosis pathway.​
Option 3: D, C, A, B
Killing (C) before phagosome formation (B) or lysosome delivery (A) defies logic—extracellular bacteria cannot be digested internally.​
Wrong; digestion requires prior enclosure.​Option 4: B, C, A, D
Ingestion (B) without prior attachment (D) skips recognition; killing (C) mid-process disrupts sequence.​
Incorrect; attachment initiates entire cascade.​Phagocytosis Pathway Overview
Step Event Key Features 1 D: Pseudopodia Attachment Opsonin receptors (FcγR, CR1); actin polymerization​ 2 B: Phagosome Formation Zipper/race model; PI3K signaling​ 3 A: Lysosome Fusion Rab5→Rab7 maturation; V-ATPase acidification​ 4 C: Killing/Digestion Myeloperoxidase, defensins; residual body exocytosis​ Macrophages/neutrophils phagocytose >0.5μm particles; non-opsonized via scavenger receptors.​
Exam Relevance
CSIR NET/GATE Immunology essential: D-B-A-C sequence. Defects (CGD) impair ROS killing step C. Note: Dendritic cells present antigens post-phagocytosis.
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