1. Chemotaxis: Pathogens or cellular debris release chemical signals called chemokines and cytokines. These signals attract macrophages to the site of infection or injury.
2. Adhesion: Once macrophages arrive at the infection site, they adhere to the surface of the pathogen or damaged tissue through specific receptors on their cell membrane.
3. Phagocytosis: The macrophage extends finger-like projections of its cell membrane called pseudopodia, which encircle the pathogen. The pseudopodia then fuse, engulfing the pathogen within a membrane-bound compartment known as a phagosome.
4. Phagosome–Lysosome Fusion: The phagosome containing the pathogen fuses with a lysosome, a membrane-bound organelle within macrophages that contains digestive enzymes and antimicrobial substances. The fusion results in the formation of a phagolysosome.
5. Acidification and Enzymatic Digestion: The environment inside the phagolysosome is highly acidic, with a pH of around 5.0. The acidic environment activates the digestive enzymes present within lysosomes, such as proteases, lipases, and nucleases. These enzymes begin to break down the ingested pathogen and its components.
6. Killing and Degradation: The acidic environment and the action of digestive enzymes lead to the killing and degradation of the pathogen. Macrophages may also produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) to further damage and destroy the pathogen.
7. Antigen Presentation: After the digestion of the pathogen, fragments or antigens from the pathogen are presented on the macrophage’s cell surface. These antigens can be recognized by specific immune cells, such as T lymphocytes, which help in the adaptive immune response against the pathogen.
8. Exocytosis and Release: Sometimes, macrophages may expel undigested material through exocytosis. This can help in the removal of cellular debris and damaged organelles, as well as the release of signaling molecules that can recruit other immune cells to the site of infection.