Dynamics and conditions of degranulation
The process of granule content release is tightly linked to the stage of the inflammatory reaction and the pH level in the lesion site.
- Specific granules react first. In the early stages of inflammation, a neutral pH is maintained in the extracellular environment, which is the optimal condition for the function of their enzymes (specifically, lysozyme and alkaline phosphatase).
- Azurophilic granules come into play later. They contain acidic hydrolases whose pH optimum is shifted toward the acidic range. The corresponding acidic environment is formed exclusively at the peak of the inflammatory reaction.
Thus, as inflammation progresses, the composition of the surrounding environment changes: acid hydrolases and reactive oxygen species (ROS) join lysozyme.
Efficiency and destructive consequences
Factors secreted by neutrophils have a potent antimicrobial effect, but their potential is limited in the extracellular space.
The main active substances include:
- Bactericidal factors: ROS and halogen-containing compounds. They act for a very short time, but with maximum efficiency.
- Anti-pathogen factors: lysozyme, lactoferrin, and cationic proteases (elastase, azurocidin, cathepsin G).
The problem of extracellular action: Outside the cell, the concentration of enzymes drops sharply, and tight contact with the pathogen (as occurs inside the phagosome) is absent. Therefore, degranulation cannot be considered a full-fledged supplement to intracellular cytolysis.
Moreover, the released substances exhibit pronounced cytotoxicity. They do not distinguish between "self" and "non-self," massively damaging the body's own cells. A mixture of degranulation products and necrotically dead neutrophils leads to massive tissue "digestion," which visually and clinically manifests as pus formation.
Impact on vascular responses
In addition to directly destroying bacteria, neutrophil granule enzymes (especially proteases) play a critical role in regulating vascular tone during inflammation. They participate in the generation of vasoactive peptides, some of which are already stored in granules in ready-made form.
- Acidic and neutral proteases stimulate the production of kinins. These molecules locally alter the permeability and contractility of small blood vessels.
- Serine protease acts as a systemic factor: it catalyzes the conversion of angiotensinogen into angiotensin II. This leads to the constriction of large vessels and an increase in blood pressure.
- Cationic proteins cause mast cells and platelets to release histamine and other vasoactive peptides, while simultaneously promoting platelet aggregation.
- Complement cleavage: specific neutrophil proteases are capable of cleaving C3 and C5 factors. As a result, potent anaphylatoxins C3a and C5a are formed, amplifying the inflammatory response.
Tissue death and clearance
Having fulfilled their function, tissue neutrophils inevitably die. The primary and safest form of cell death for the organism is apoptosis. It is worth noting that neutrophils in tissues undergo apoptosis even without participating in phagocytosis, although the latter significantly accelerates this process.
Mechanism of neutrophil apoptosis:
- Mitochondrial permeability to cytochrome c and Apaf-1 (Apo-1) factor increases sharply.
- A protein complex—the apoptosome—forms in the cytoplasm.
- The effector enzyme—caspase-9—is activated.
During this process, the cell membrane changes its structure: phosphatidylserine and other signaling markers begin to be expressed on its outer surface. Macrophage membrane receptors read these molecules as an "eat me" signal. Phagocytosis of apoptotic bodies occurs, allowing macrophages to "clean up the area" without triggering additional inflammation.
Briefly: answers to topical questions
Which enzymes make up neutrophil specific granules?
Specific (secondary) neutrophil granules contain enzymes active in neutral and slightly alkaline environments. These include:
- Alkaline phosphatase—an enzyme active at neutral pH values.
- Lysozyme—an enzyme with muramidase activity belonging to the group of cationic proteins.
- NADPH oxidase—a key enzyme catalyzing the "respiratory burst" and the generation of reactive oxygen species.
Which enzymes are contained in neutrophil azurophilic granules?
Azurophilic (primary) neutrophil granules contain a wide range of acid hydrolases and other enzymes active at acidic pH. Their composition includes:
- Myeloperoxidase—a marker enzyme of azurophilic granules.
- Lysozyme (muramidase)—an enzyme that degrades the glycan component of bacteria.
- Acid hydrolases and glycosidases—α-fucosidase, 5'-nucleotidase, β-galactosidase, arylsulfatase, α-mannosidase, N-acetylglucosaminidase, β-glucuronidase, acid glycerophosphatase.
- Neutral proteases (serprocidins) — cathepsin G, elastase, collagenase, azurocidin.
What oxygen-dependent bactericidal factors do neutrophils produce?
Neutrophils produce reactive oxygen species (ROS) and halogen-containing compounds, which are the main bactericidal factors. Oxygen-dependent factors include:
- Hydrogen peroxide ($H_2O_2$)—a component of the myeloperoxidase microbicidal system.
- Hypochlorous acid (HOCl)—a strong microbicidal agent formed by the interaction of chloride with hydrogen peroxide.
- Chloramines—bactericidal compounds resulting from the interaction of HOCl with amino acids.
- Hydroxyl radical (OH)—a powerful oxidizing agent causing profound chemical alterations in macromolecules.
- Singlet oxygen ($^1O_2$)—formed by the oxidation of HOCl by superoxide.
What mediators do mast cells secrete under the action of neutrophil cationic proteins?
Under the action of neutrophil cationic proteins, mast cells release vasoactive peptides and histamine. These mediators are critical for the vascular response during inflammation:
- Histamine—a vasoactive amine that causes vasodilation, increased vascular permeability, and smooth muscle spasm.
- Vasoactive peptides—affect the contractility and permeability of blood vessels in the inflammation site.
SNAP: Specific granules — Neutral environment, Azurophilic granules — Acidic environment. This will help prevent confusion regarding the sequence and operating conditions of the enzymes.
Class and exam questions
Why is degranulation viewed as a side effect rather than the main defense?
Outside the cell, enzymes lose the required concentration and lack close contact with bacteria. Instead, they aggressively damage the body's own tissues, causing necrosis and suppurative inflammation.
How can neutrophils increase systemic blood pressure?
Their granules contain serine protease, which converts inactive angiotensinogen into angiotensin II, causing spasms of large blood vessels.
How do macrophages distinguish a dead neutrophil from a living cell?
During apoptosis, neutrophils display phosphatidylserine on the outer surface of the membrane. Macrophages recognize this marker via their receptors and engulf the cell remnants.
What else to study on this topic
- Mechanisms of intracellular cytolysis within the phagosome
- Detailed composition of specific and azurophilic granules
- Biochemistry of reactive oxygen species (ROS) generation
- Complement system: the role of C3a and C5a anaphylatoxins
- Caspase cascade in immune cell apoptosis
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