Page 122 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

D. Phagocytosis

Macrophages, neutrophils and dendritic cells

are called professional phagocytes.

- Thay have specific receptors on their

surfaces (Pattern Recognition receptors,

PRRs) that recognize pathogen associated molecular pattern (PAMPs)

- Once recognized, the pathogen is phagocytosed (destructed)

Stages of Phagocytosis

1. Chemotaxis: Phagocytes are chemically attracted to the site of infection.

2. Recognition and attachment (Adherence): Phagocytic cells use various

receptors to bind invading microbes either directly or indirectly.

• Direct binding occurs through receptors that recognize compounds found on

microbes. For example, one type of receptor on phagocytic cells binds

mannose, a sugar found on the surface of some bacteria

• Indirect binding occurs when a particle has first been opsonized

• Opsonin includes the complement component C3b and certain classes of

antibody molecules.

3. Ingestion: Plasma membrane of phagocytes extends projections (pseudopods)

which engulf the microbe. The microbe is enclosed in a sac called phagosome.

4. Fusion of the phagosome with the lysosome: Within the phagocyte, the

phagosome is transported along the cytoskeleton to a point where it can fuse

with lysosomes (filled with various digestive enzymes, including lysozyme and

proteases). The fusion results in the formation of a phagolysosome.

General Microbiology & Immunology (PM 401) theoretical handbook Page 111