describe the mode of action of phagocytes (macrophages and neutrophils)

The Immune System – Phagocytes

Objective

Describe the mode of action of phagocytes (macrophages and neutrophils).

Overview of Phagocytosis

Phagocytosis is a form of cellular immunity in which specialised white blood cells engulf and destroy invading microorganisms, debris, and dead cells.

Key Steps in Phagocytosis

1. Chemotaxis – movement towards a chemical attractant (e.g., formyl‑methionyl peptides, C5a, IL‑8).
2. Adherence – binding of the pathogen to receptors on the phagocyte surface (e.g., Fc receptors, complement receptors).
3. Engulfment – extension of pseudopodia to surround the particle, forming a phagosome.
4. Phagosome‑lysosome fusion – the phagosome merges with lysosomes to become a phagolysosome.
5. Destruction – killing of the pathogen by reactive oxygen species (ROS), nitric oxide, and hydrolytic enzymes.
6. Exocytosis – expulsion of indigestible debris from the cell.

Neutrophils (Polymorphonuclear Leukocytes)

• Most abundant circulating leukocyte; first responders to acute infection.
• Short‑lived (≈6–8 hours in tissue).
• Contain abundant granules rich in myeloperoxidase, defensins, and proteases.
• Primarily use the oxidative burst to generate ROS:

\$\text{O}2 \xrightarrow{\text{NADPH oxidase}} \text{O}2^- \rightarrow \text{H}2\text{O}2 \rightarrow \text{HOCl}\$

These reactive species damage bacterial membranes, proteins and DNA.

Macrophages (Mononuclear Phagocytes)

• Derived from monocytes that leave the bloodstream and differentiate in tissues.
• Long‑lived (weeks to months) and reside in connective tissue, liver (Kupffer cells), lungs (alveolar macrophages), etc.
• Play a dual role: innate defence and activation of the adaptive immune system.
• Present antigen to T‑cells via MHC II after processing in phagolysosomes.

Comparison of Neutrophils and Macrophages

Integration with the Immune Response

After engulfing a pathogen, macrophages process antigenic peptides and display them on MHC II molecules. This presentation is essential for the activation of helper T‑cells, which in turn stimulate B‑cells to produce antibodies and cytotoxic T‑cells to kill infected cells.

Key Points to Remember

• Both neutrophils and macrophages use receptor‑mediated adherence to recognise pathogens.
• The oxidative burst generates ROS that are lethal to microbes.
• Macrophages have a crucial role in linking innate and adaptive immunity through antigen presentation.
• Neutrophils act quickly but are short‑lived; macrophages provide sustained defence and tissue repair.