Phagocytosis (from the Ancient Greek (phagein) ‘to eat’ and o, (kytos) ‘cell’) is the process by which a cell uses its plasma membrane to ingest a big particle (0.5 m), resulting in the formation of an internal compartment known as the phagosome. It’s a specific sort of endocytosis. A phagocyte is a cell that performs phagocytosis.

The act of a phagocyte absorbing a pathogen.
Phagocytosis is a major mechanism employed by a multicellular organism’s immune system to eliminate infections and cell debris. The phagosome then digests the ingested substance. Objects that can be phagocytized include bacteria, dead tissue cells, and microscopic mineral particles. Phagocytosis is a type of phagocytosis used by some protozoa.

Professional phagocytic cells

Light microscopic video sequence of a neutrophil from human blood phagocytosing a bacterium

Neutrophilsmacrophagesmonocytesdendritic cellsosteoclasts and eosinophils can be classified as professional phagocytes. The first three have the greatest role in immune response to most infections.

The role of neutrophils is patrolling the bloodstream and rapid migration to the tissues in large numbers only in case of infection. There they have direct microbicidal effect by phagocytosis. After ingestion, neutrophils are efficient in intracellular killing of pathogens. Neutrophils phagocytose mainly via the Fcγ receptors and complement receptors 1 and 3. The microbicidal effect of neutrophils is due to a large repertoire of molecules present in pre-formed granules. Enzymes and other molecules prepared in these granules are proteases, such as collagenasegelatinase or serine proteasesmyeloperoxidaselactoferrin and antibiotic proteins. Degranulation of these into the phagosome, accompanied by high reactive oxygen species production (oxidative burst) is highly microbicidal.

Monocytes, and the macrophages that mature from them, leave blood circulation to migrate through tissues. There they are resident cells and form a resting barrier.Macrophages initiate phagocytosis by mannose receptorsscavenger receptorsFcγ receptors and complement receptors 1, 3 and 4. Macrophages are long-lived and can continue phagocytosis by forming new lysosomes.

Dendritic cells also reside in tissues and ingest pathogens by phagocytosis. Their role is not killing or clearance of microbes, but rather breaking them down for antigen presentation to the cells of the adaptive immune system.

Initiating receptors

Receptors for phagocytosis can be divided into two categories by recognised molecules. The first, opsonic receptors, are dependent on opsonins. Among these are receptors that recognise the Fc part of bound IgG antibodies, deposited complement or receptors, that recognise other opsonins of cell or plasma origin. Non-opsonic receptors include lectin-type receptors, Dectin receptor, or scavenger receptors. Some phagocytic pathways require a second signal from pattern recognition receptors (PRRs) activated by attachment to pathogen-associated molecular patterns (PAMPS), which leads to NF-κB activation.

Fcγ receptors

Fcγ receptors recognise IgG coated targets. The main recognised part is the Fc fragment. The molecule of the receptor contain an intracellular ITAM domain or associates with an ITAM-containing adaptor molecule. ITAM domains transduce the signal from the surface of the phagocyte to the nucleus. For example, activating receptors of human macrophages are FcγRIFcγRIIA, and FcγRIII. Fcγ receptor mediated phagocytosis includes formation of protrusions of the cell called a ‘phagocytic cup’ and activates an oxidative burst in neutrophils.

Complement receptors

These receptors recognise targets coated in C3bC4b and C3bi from plasma complement. The extracellular domain of the receptors contains a lectin-like complement-binding domain. Recognition by complement receptors is not enough to cause internalisation without additional signals. In macrophages, the CR1CR3 and CR4 are responsible for recognition of targets. Complement coated targets are internalised by ‘sinking’ into the phagocyte membrane, without any protrusion

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