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Cells of the Inflammatory Infiltrate

For medical students2 min readUpdated 2026-10-10

An inflammatory infiltrate is an accumulation of cells involved in defending the organism against damaging factors. The dynamics of the process are strictly regulated: resident cells and neutrophils take the first hit, followed by macrophages migrating into the focus to assume the role of master coordinators of the immune response and tissue repair.

Main taskThe primary function of macrophages is not bacterial destruction, but antigen presentation to the immune system.
First lineNeutrophils are the first to attack the pathogen, block it, and undergo massive cell death within the inflammatory focus.
Focus topographyMacrophages concentrate at the periphery, forming a protective barrier around the zone of necrosis.
Infiltration timingMonocytic and macrophage infiltration always follows neutrophil infiltration.

Initiation of Inflammation and the First Line of Defense

The earliest responders to tissue damage are mediator-producing cells already residing within the tissue (resident cells): macrophages, mast cells, eosinophils, and natural killer (NK) cells. They trigger the vascular response.

Following this, polymorphonuclear leukocytes (predominantly neutrophils) massively migrate from the microvasculature into the lesion. These are the frontline cells of nonspecific resistance. They are the first to contact the pathogen, block its entry into the body, and destroy it via phagocytosis and exocytosis. Neutrophils provide the primary defense (especially in purulent infections), but inevitably perish in the process.

Macrophages: Conductors of Inflammation

The macrophage is a key cell linking the local inflammatory focus with systemic body reactions. Contrary to common misconceptions, direct destruction of bacteria is not their main task (their bactericidal activity is weaker than that of neutrophils).

Their primary function is antigen presentation. They phagocytose damaging factors, identify antigenic determinants, and transmit this information to the immune system to launch a specific defense.

Additionally, macrophages perform several crucial tasks:

Spatial Distribution Within the Focus

At the height of the exudative reaction, the center of the lesion is oversaturated with dead leukocytes and aggressive hydrolytic enzymes. If macrophages were to move to the center, they would perish uselessly due to the high toxicity of the environment.

Therefore, they concentrate at the periphery, forming a cellular wall—the "second barrier." This ensures the demarcation (isolation) of the inflamed zone from healthy tissues. The outcome directly depends on macrophage activity: their activation leads to the rapid formation of a robust connective tissue capsule, while their suppression leads to an enlarged zone of necrosis and suppuration.

Cellular Cooperation

Macrophages actively interact with other participants of inflammation:

  1. With lymphocytes: This is clearly manifested in delayed-type hypersensitivity (DTH). The result can be complete immune cytolysis of the pathogen or granulomatosis. For example, in tuberculosis, incomplete phagocytosis occurs: mycobacteria are isolated inside epithelioid cells, providing the host with non-sterile immunity and preventing generalized infection.
  2. With fibroblasts: During the repair phase, monocytes affect collagen- and elastin-synthesizing cells, triggering fibrillogenesis.

Mnemonic

To remember the roles of the main cells, imagine a fortress under siege: Neutrophils are the infantry (go into battle first, kill the enemy, and die en masse). Macrophages are the generals and scouts (stand at the periphery, study enemy antigens, call for reinforcements, and direct the reconstruction of the walls).

Frequently asked questions

What inflammatory mediators are released by mast cells?

Upon degranulation, mast cells release primary and secondary inflammatory mediators. The main ones include:

  • Histamine — a biogenic amine causing mucosal edema, hypersecretion of mucus, and smooth muscle spasm.
  • Leukotrienes — a complex forming the slow-reacting substance of anaphylaxis.
  • Chemotactic factors — eosinophil and neutrophil factors that attract inflammatory cells to the lesion.
Which cells form a tuberculosis granuloma?

A tuberculosis granuloma is epithelioid-cellular and forms from several cell types arranged in layers around a central zone of caseous necrosis. Its cellular composition includes:

  • Epithelioid cells — surround the zone of necrosis in a palisade arrangement.
  • Langhans giant cells — located between epithelioid and lymphoid cells.
  • Macrophages and sensitized T-lymphocytes — form the cellular wall at the periphery.
  • Fibroblasts — form the outer boundary of the granuloma.
What enzymes are contained in neutrophil granules?

Neutrophil granules contain aggressive hydrolytic enzymes involved in microorganism destruction and tissue damage. These include:

  • Proteinase 3 — localized in primary (azurophilic) neutrophil granules.
  • Myeloperoxidase — an enzyme of azurophilic granules that generates strong oxidants upon interaction with hydrogen peroxide.
  • Lysosomal hydrolases and proteases — released during exocytosis and phagocytosis, causing barrier destruction and tissue alteration.
Which specific growth factors do macrophages produce in the repair phase?

During the repair phase, macrophages produce several growth factors that stimulate cellular proliferation and angiogenesis. These include:

  • Fibroblast growth factor — stimulates proliferation of fibroblasts, endothelium, and smooth muscle cells.
  • Epidermal growth factor — participates in cellular proliferation processes.
  • Transforming growth factor-alpha (TGF-α) — ensures cellular proliferation directly within granulation tissue.
  • Transforming growth factor (TGF) — secreted by macrophages alongside other cytokines.
Who arrives first at the site of inflammation?

Local (resident) tissue cells react first, while neutrophils are the first to migrate from the bloodstream, providing early defense and blocking the pathogen.

Why do macrophages accumulate at the edges of the focus rather than the center?

The center of the focus has a high concentration of aggressive hydrolytic enzymes and dead leukocytes. Macrophages would simply perish there, which is why they form a protective demarcation wall at the periphery.

What is the significance of incomplete phagocytosis using tuberculosis as an example?

In tuberculosis, mycobacteria are not completely destroyed but persist inside macrophages (epithelioid cells). This isolates the pathogen, prevents its spread throughout the body, and provides non-sterile immunity.

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