Phagocytosis

Q: What is the exact function of the Fc-gamma receptor on the leukocyte membrane?

The Fcγ-receptor on the phagocyte membrane provides contact with antibodies on the object's surface and triggers intracellular signaling cascades. Main functions: Recognition and capture — phagocytosis of microorganisms opsonized with IgG class antibodies (e.g., via FcγRI);Signal transduction — receptor clustering activates tyrosine kinases (Src and Syk), leading to the formation of the phagocytic cup;Induction of cytolysis — expression of FcγRIII (CD16) ensures antibody-dependent cellular cytotoxicity.

For medical students2 min readUpdated 2026-10-10

Phagocytosis is an active, high-energy biological process in which specialized phagocytes engulf and intracellularly destroy foreign material. This phenomenon forms the basis of inflammatory reactions.

DiscovererI.I. Mechnikov, 1882

Number of stages4 main phases

Destruction pathwaysOxygen-dependent and independent

PathologyIncomplete phagocytosis

Main Stages of the Process

The dynamics of this defense mechanism include 4 sequential stages:

Approached/Chemotaxis — active movement of the phagocyte toward the foreign object.
Recognition and Adhesion — identification of the structure via receptors and firm attachment to its surface.
Ingestion — engulfment of the material into the cell forming a phagosome, which approaches and fuses with lysosomes to form a phagolysosome.
Destruction — intracellular digestion of the engulfed object.

Mechanisms of Intracellular Destruction

To destroy the engulfed material, the cell utilizes two major mechanisms:

Oxygen-dependent mechanism (or respiratory burst) — plays a key role in destruction, accompanied by a sharp metabolic spike with the active participation of oxygen.
Oxygen-independent cytotoxicity — triggered at the moment of contact between the opsonized foreign agent and the phagocyte membrane.

Recognition Stage, Adhesion, and Opsonization

The interaction between a leukocyte and its target follows a precise sequence of events:

Recognition of the object by receptors.
Opsonization — a special "tagging" of the object that facilitates its subsequent engulfment.
Direct contact involving the leukocyte's $Fc\gamma$-receptor.
Activation of intracellular processes: a metabolic spike, degranulation, as well as enhanced expression of adhesion molecules and major histocompatibility complex (MHC) proteins.

Incomplete Phagocytosis and Its Causes

Sometimes engulfed bacteria are not destroyed by lysosomal enzymes, but continue to survive inside the defense cell. Microorganisms protect themselves using dense hydrophobic cell walls, capsules, enzymatic resistance, or the ability to block phagosome-lysosome fusion.

Main etiological causes of impairment:

Lysosomal pathology: membranopathies or enzymopathies of organelles.
Defective opsonization and low tagging efficiency.
Enzyme deficiencies (e.g., myeloperoxidase deficiency reduces bactericidal potential).
Impaired adhesive properties and a deficit of adhesion molecules.
Hormonal imbalance disrupting process regulation.

Mnemonic

Stage formula: Approach $\rightarrow$ Recognition and adhesion $\rightarrow$ Ingestion with phagolysosome $\rightarrow$ Destruction.

Frequently asked questions

What is a "respiratory burst" and what active substances are produced during it?

A "respiratory burst" is an oxygen-dependent mechanism of intracellular destruction in which oxidative processes and oxygen metabolism are sharply activated in the phagocyte.

As a result, reactive oxygen species are formed:

hydrogen peroxide ($H_2O_2$);
superoxide;
hydroxyl radical;
reactive oxygen species of phagocytes also include $O_2^-$, $H_2O_2$, $\cdot OH$, $OH^-$, $OCl^-$, $^1O_2$, $O_3$.

Hydrogen peroxide, superoxide, and hydroxyl radicals accumulate inside the phagolysosome.

What specific substances function as opsonins during recognition?

Opsonins are molecules that coat the object of phagocytosis, preparing it and facilitating recognition by the leukocyte's membrane receptors.

Main opsonization factors include:

Antibodies — IgG class immunoglobulins (bind to Fcγ-receptors);
Complement components — the iC3b fragment, as well as C3b molecules.

What enzymes and proteins provide oxygen-independent cytotoxicity?

Oxygen-independent bactericidal activity of phagocytes is provided by factors whose main site of action is the phagolysosomes. Enzymes and peptides come from pre-formed lysosomes and can act outside the cell during degranulation.

These include:

Cationic proteins: serprocidins (cathepsin G, elastase, azurocidin, proteinase-3), lysozyme, lactoferrin, BPI proteins;
Acid hydrolases: myeloperoxidase, 5'-nucleotidase, $\beta$-arylsulfatase, $\beta$-glucuronidase, acid glycerophosphatase;
Bactericidal peptides: $\alpha$- and $\beta$-defensins, cathelicidins;
Enzymatic acidification factor: V-ATPase, whose activity leads to a pH of 4.5–5.0.

What is the exact function of the Fc-gamma receptor on the leukocyte membrane?

The Fcγ-receptor on the phagocyte membrane provides contact with antibodies on the object's surface and triggers intracellular signaling cascades.

Main functions:

Recognition and capture — phagocytosis of microorganisms opsonized with IgG class antibodies (e.g., via FcγRI);
Signal transduction — receptor clustering activates tyrosine kinases (Src and Syk), leading to the formation of the phagocytic cup;
Induction of cytolysis — expression of FcγRIII (CD16) ensures antibody-dependent cellular cytotoxicity.

Who and when discovered the role of phagocytosis in inflammation?

The phenomenon was discovered by I.I. Mechnikov in 1882, proving its pivotal role in the mechanism of the inflammatory response.

What happens during the engulfment stage?

The object is internalized into the phagocyte, forming a phagosome. Lysosomes then approach it, their membranes fuse to form a phagolysosome, and digestion begins.

Which destruction mechanism plays the leading role?

The oxygen-dependent mechanism, known as the "respiratory burst," plays the leading role and is coupled with a sharp increase in oxygen metabolism.

What is incomplete phagocytosis?

This is a situation where engulfed microorganisms are not destroyed by enzymes, but survive inside the phagocyte due to microbial defense factors or cellular pathologies.

Go deeper

The role of leukocyte cell membrane receptors in the immune response
Biochemical pathways of the oxygen-dependent respiratory burst
Lysosomal membranopathies and enzymopathies as systemic pathology
Significance of opsonins and complement system proteins in particle engulfment
Molecular mechanisms of MHC protein and adhesion molecule expression