Phagocytosis

🦠 Phagocytosis: Steps in Innate Immunity

1. Chemotaxis

• Definition: Movement of phagocytes toward the site of infection due to chemical signals.

• Chemotactic signals include:

  • Microbial products

  • Damaged tissue cell components

  • Cytokines from white blood cells

  • Complement-derived peptides (e.g., C5a)

2. Adherence

• Definition: Attachment of the phagocyte to the microbe.

• How it works:

  • Microbes have PAMPs (Pathogen-Associated Molecular Patterns).

  • Phagocytes have TLRs (Toll-Like Receptors).

  • TLRs bind to PAMPs → helps the phagocyte recognize the invader.

3. Ingestion

• How it happens:

  • Phagocyte forms pseudopods (extensions of its membrane).

  • These pseudopods engulf the microbe.

  • The engulfed microbe is enclosed in a sac called a phagosome.

  • Inside the phagosome, H⁺ ions are pumped in, lowering the pH to ~4 → activates hydrolytic enzymes.

4. Digestion

• Fusion:

  • Phagosome + lysosome = phagolysosome.

• Digestion:

  • Enzymes break down the microbe inside the phagolysosome.

5. Residual Body Formation & Exocytosis

• After digestion:

  • Indigestible waste remains in a residual body.

  • This residual body moves to the cell membrane and releases waste by exocytosis.

🔁 Summary Table