- Phagocytosis is a fundamental cellular process where specialized cells engulf and destroy particles, pathogens, and dead cells. This process is essential for both immune defense and tissue homeostasis, representing a key mechanism of innate immunity.
- Professional phagocytes include neutrophils, macrophages, and dendritic cells. Each type has specific roles: neutrophils provide rapid response to infection, macrophages perform sustained surveillance and cleanup, and dendritic cells link innate and adaptive immunity through antigen presentation. These cells express various surface receptors that recognize pathogen-associated molecular patterns (PAMPs) or opsonins that mark targets for phagocytosis.
- The process occurs in several stages. Initial recognition involves surface receptors binding to their targets, triggering actin-dependent membrane extensions called pseudopods that surround the particle. The membrane then closes around the target, forming a phagosome that internalizes into the cell. This phagosome fuses with lysosomes to form a phagolysosome, where the internalized material is degraded by hydrolytic enzymes, reactive oxygen species, and antimicrobial peptides.
- Enhanced phagocytosis occurs through opsonization, where particles are coated with molecules like antibodies, complement proteins, or collectins. These opsonins are recognized by specific receptors on phagocytes, making the process more efficient. This mechanism is particularly important in clearing pathogens that have evolved to resist direct recognition.
- Beyond pathogen clearance, phagocytosis serves crucial functions in tissue maintenance by removing dead cells and debris. This process, particularly in the clearance of apoptotic cells, is essential for preventing inflammation and autoimmune responses. Defects in phagocytosis can lead to various pathological conditions, including increased susceptibility to infections and autoimmune disorders.
