Main stages of the process
The dynamics of this defense mechanism comprise 4 sequential stages:
- Approaches — 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 with the formation of a phagosome, its approach to lysosomes, and membrane fusion to form a phagolysosome.
- Destruction — intracellular digestion of the engulfed object.
Mechanisms of intracellular destruction
The cell employs two primary mechanisms to destroy the engulfed material:
- 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, adhesion, and opsonization stage
The interaction between a leukocyte and its target follows a clear sequence of events:
- Recognition of the object by receptors.
- Opsonization — a special "tag" on the object that facilitates its subsequent engulfment.
- Direct contact involving the leukocyte $Fc\gamma$ 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 live inside the defensive cell. Microorganisms protect themselves using dense hydrophobic cell walls, capsules, enzymatic resistance, or the ability to block the fusion of the phagosome with the lysosome.
Main etiological causes of impairment:
- Lysosomal pathology: membranopathies or enzymopathies of organelles.
- Opsonization defects and low efficiency of tag binding.
- Enzyme deficiency (e.g., myeloperoxidase deficiency reduces bactericidal potential).
- Impaired adhesive properties and deficiency of adhesion molecules.
- Hormonal imbalance disrupting the regulation of the process.
Briefly: answers to topical questions
What is a "respiratory burst" and what active substances are formed during it?
The "respiratory burst" is an oxygen-dependent mechanism of intracellular destruction in which oxidative processes are sharply activated in the phagocyte and oxygen-dependent metabolism increases.
As a result, reactive oxygen species are formed:
- hydrogen peroxide (H2O2);
- superoxide;
- hydroxyl radical;
- reactive oxygen species of phagocytes also include O2-, H2O2, *OH, OH-, OCl-, 'O2, O3.
Hydrogen peroxide, superoxide, and the hydroxyl radical accumulate inside the phagolysosome.
Which specific substances act as opsonins during recognition?
Opsonins are molecules that coat the phagocytosis target, preparing it and facilitating recognition by leukocyte membrane receptors.
Main opsonization factors include:
- Antibodies — IgG class immunoglobulins (bind to Fcγ receptors);
- Complement components — the iC3b fragment, as well as C3b molecules.
Which specific 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 preformed lysosomes, and during degranulation, they can act outside the cell.
These include:
- cationic proteins: serprocidins (cathepsin G, elastase, azurocidin, proteinase-3), lysozyme, lactoferrin, BPI proteins;
- acid hydrolases: myeloperoxidase, 5'-nucleotidase, β-arylsulfatase, β-glucuronidase, acid glycerophosphatase;
- bactericidal peptides: α- and β-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 surface of the object and triggers intracellular signaling cascades.
Main functions:
- Recognition and engulfment — 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.
Formula of stages: Approach $\rightarrow$ Recognition and adhesion $\rightarrow$ Ingestion with phagolysosome formation $\rightarrow$ Destruction. (ARID).
Class and exam questions
Who discovered the role of phagocytosis in inflammation and when?
The phenomenon was discovered by I.I. Mechnikov in 1882, proving its key role in the mechanism of inflammatory response development.
What happens during the object engulfment stage?
The object is engulfed into the phagocyte with the formation of a phagosome. Then lysosomes approach it, their membranes fuse, forming a phagolysosome, where digestion begins.
Which destruction mechanism plays a leading role?
The leading role is played by the oxygen-dependent mechanism known as the "respiratory burst," which 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.
What else to study on this topic
- Role of leukocyte cell membrane receptors in the immune response
- Biochemical pathways of the oxygen-dependent respiratory burst
- Membranopathies and lysosomal enzymopathies as a systemic pathology
- Significance of opsonins and complement system proteins in particle capture
- Molecular mechanisms of MHC protein and adhesion molecule expression
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