Phagocytes: Macrophages and Neutrophils

Phagocytes are cells that engulf and destroy pathogens, cellular debris, and foreign substances. They are a crucial component of the innate immune system. Two main types of phagocytes are macrophages and neutrophils, each with distinct characteristics and modes of action.

Macrophages

Macrophages are large, long-lived phagocytic cells derived from monocytes. They are found in various tissues throughout the body, including the lungs (alveolar macrophages), liver (Kupffer cells), and spleen.

Mode of Action:

1. Chemotaxis: Macrophages are attracted to sites of infection or inflammation by chemical signals, such as chemokines and complement factors.
2. Adherence: They adhere to the surface of the pathogen or damaged cells via specific receptors. These receptors recognize pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).
3. Ingestion (Phagocytosis): The macrophage extends its cell membrane around the pathogen, forming a phagosome.
4. Phagosome-Lysosome Fusion: The phagosome fuses with lysosomes, forming a phagolysosome.
5. Digestion: Lysosomes contain hydrolytic enzymes (e.g., lysozyme, proteases) that digest the pathogen.
6. Antigen Presentation: Macrophages can process pathogens into antigens and present them on their surface via MHC class II molecules to activate T helper cells, linking the innate and adaptive immune responses.

Neutrophils

Neutrophils are the most abundant type of white blood cell in the blood. They are short-lived phagocytes that are rapidly recruited to sites of infection.

Mode of Action:

1. Chemotaxis: Neutrophils are strongly attracted to sites of infection by chemokines, complement factors (e.g., C5a), and bacterial products.
2. Extravasation: Neutrophils leave the bloodstream and enter the infected tissue by squeezing between endothelial cells (diapedesis).
3. Phagocytosis: Similar to macrophages, neutrophils engulf pathogens via phagocytosis, forming a phagosome.
4. Respiratory Burst: Upon engulfment, neutrophils undergo a "respiratory burst," increasing oxygen consumption and producing reactive oxygen species (ROS) like superoxide radicals and hydrogen peroxide. These ROS damage the pathogen.
5. Degranulation: Neutrophils contain granules filled with antimicrobial substances, such as lysozyme, defensins, and proteases. These substances are released onto the pathogen.
6. Neutrophil Extracellular Traps (NETs): Neutrophils can release their DNA into the extracellular space, forming NETs. NETs trap and kill pathogens.

Suggested diagram: A diagram illustrating a macrophage engulfing a bacterium and a neutrophil engulfing a bacterium, highlighting the phagosome-lysosome fusion and respiratory burst respectively.