Phagocytosis involves three sequential steps: (1) recognition and attachment of the particle to be ingested by the leukocyte; (2) engulfment, with subsequent formation of a phagocytic vacuole; and (3) killing or degradation of the ingested material.

Phagocytic Receptors. Mannose receptors, scavenger receptors, and receptors for various opsonins bind and ingest microbes. The macrophage mannose receptor is a lectin that binds terminal mannose and fucose residues of glycoproteins and glycolipids. These sugars are typically part of molecules found on microbial cell walls, whereas mammalian glycoproteins and glycolipids contain terminal sialic acid or N-acetylgalactosamine. Therefore, the mannose receptor recognizes microbes and not host cells. Scavenger receptors were originally defined as molecules that bind and mediate endocytosis of oxidized or acetylated low-density lipoprotein (LDL) particles that can no longer interact with the conventional LDL receptor. Macrophage scavenger receptors bind a variety of microbes in addition to modified LDL particles. Macrophage integrins, notably Mac-1 (CD11b/CD18), may also bind microbes for phagocytosis. The efficiency of phagocytosis is greatly enhanced when microbes are opsonized by specific proteins (opsonins) for which the phagocytes express high affinity receptors. The major opsonins are IgG antibodies, the C3b breakdown product of complement, and certain plasma lectins, notably mannose-binding lectin, all of which are recognized by specific receptors on leukocytes.

Engulfment. After a particle is bound to phagocyte receptors, extensions of the cytoplasm (pseudopods) flow around it, and the plasma membrane pinches off to form a vesicle (phagosome) that encloses the particle. The phagosome then fuses with a lysosomal granule, resulting in discharge of the granule’s contents into the phagolysosome. During this process the phagocyte may also release granule contents into the extracellular space.

The process of phagocytosis is complex and involves the integration of many receptor-initiated signals that lead to membrane remodeling and cytoskeletal changes. Phagocytosis is dependent on polymerization of actin filaments; it is, therefore, not surprising that the signals that trigger phagocytosis are many of the same that are involved in chemotaxis.

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