Phagocytes

Phagocytes are a group of immune cells specialized in detecting, engulfing, and destroying foreign particles, pathogens, and cellular debris. They form a key component of the innate immune system and also help initiate adaptive immune responses. The main phagocytes in mammals are neutrophils, monocyte-derived macrophages, and dendritic cells.

Neutrophils are typically the first responders to infection and are short-lived, rapidly recruited to sites of inflammation where they perform phagocytosis and release antimicrobial compounds. Macrophages, which differentiate from monocytes in tissues, are long-lived and versatile, clearing debris and chronic pathogens, producing cytokines, and presenting antigens. Dendritic cells patrol tissues and, after taking up material, migrate to lymph nodes to activate T cells by presenting antigens.

Phagocytosis begins with recognition of targets via receptors for opsonins such as antibodies and complement or direct microbial patterns. The particle is engulfed into a phagosome, which then fuses with lysosomes to form a phagolysosome. Hydrolytic enzymes, reactive oxygen species produced by the respiratory burst, and nitric oxide digest and kill the contents. Some material is expelled or stored as residual bodies; degradation products can be loaded onto MHC molecules for presentation.

Antigen presentation is a key link to adaptive immunity. Macrophages and dendritic cells present processed peptides on MHC class II to CD4+ T cells, while dendritic cells can also cross-present antigens to CD8+ T cells. Phagocytes recruit and activate other immune cells through cytokine and chemokine signaling and contribute to tissue repair and resolution of inflammation via efferocytosis (engulfment of apoptotic cells).

Pathogens have evolved strategies to evade phagocytosis, such as capsule formation, resistance to oxidative killing, or inhibiting phagosome maturation. Defects in phagocyte function underlie diseases such as chronic granulomatous disease, which impairs the respiratory burst and increases susceptibility to infections.