Question

Phagocytosis is the killing of bacteria by the neutrophils or macrophage and it is considered as the first line of defence against an invading pathogen. Stages of phagocytosis are given below. Arrange them in the order it occur.
(A). Ingestion of particle by the neutrophil and formation of phagosome
(B). Recognition of particle by the neutrophil followed by chemotaxis
(C). Fusion of phagosome with lysosome and formation of phagolysosome
(D). Pathogen killing
Choose the correct answer from the options given below:

• A. (B), (C), (A), (D).
• B. (D), (C), (A), (B).
• C. (B), (A), (C), (D).
• D. (C), (B), (D), (A).

Hint:

To remember the sequence easily, follow the physical path of the pathogen:
1. Approach/Detect (Chemotaxis/Recognition)
2. Take inside (Ingestion into Phagosome)
3. Mix with chemicals (Phagolysosome)
4. Destroy (Killing)

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Phagocytosis is a specialized cellular process performed by professional phagocytes (such as neutrophils and macrophages) to engulf, neutralize, and digest foreign cellular debris and invading pathogens.
This biological process occurs in a highly regulated sequence of distinct steps.

Step 2: Detailed Explanation:
Let us analyze the chronological order of the stages of phagocytosis:
-

Step 1: Chemotaxis and Recognition (B):
Before engulfment can occur, the phagocyte must locate the foreign invader.
Chemotaxis is the migration of phagocytes toward the site of infection in response to chemical signals (chemokines, complement factors, bacterial byproducts).
Once at the site, receptors on the phagocyte's membrane recognize specific pathogen-associated molecular patterns (PAMPs) or opsonins on the target particle.
- Ingestion and Phagosome Formation (A):
Upon receptor binding, the phagocyte extends pseudopodia around the target particle.
These pseudopodia fuse, fully enclosing the particle in a membrane-bound intracellular vacuole known as a phagosome.
-

Step 2: Fusion and Phagolysosome Formation (C):
The newly formed phagosome moves deeper into the cytoplasm.
It then fuses with cytoplasmic organelles called lysosomes (which contain hydrolytic enzymes, antimicrobial peptides, and acidic environments) to form a unified vesicle called a phagolysosome.
-

Step 3: Pathogen Killing and Digestion (D):
Inside the acidic environment of the phagolysosome, active digestive enzymes and reactive oxygen/nitrogen species destroy the pathogen.
The structural components of the pathogen are broken down, and any indigestible waste is subsequently expelled via exocytosis.
Therefore, the correct chronological sequence of events is:
\[ \text{(B) Chemotaxis/Recognition} \rightarrow \text{(A) Ingestion/Phagosome} \rightarrow \text{(C) Fusion/Phagolysosome} \rightarrow \text{(D) Killing} \] This matches the sequence (B), (A), (C), (D), which corresponds to Option (C).

Step 3: Final Answer:
The correct order of stages in phagocytosis is (B), (A), (C), (D).
Therefore, the correct choice is Option (C).