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Cellular inflammation involves recruitment and activation of innate immune cells—macrophages, neutrophils, dendritic cells—through chemotactic signals and adhesion molecule expression. These cells produce additional cytokines and reactive oxygen species, amplifying the response. Endothelial cells increase permeability, allowing leukocyte extravasation into tissues.
You already know that cytokines and chemokines serve as the signaling molecules of inflammation, and that toll-like receptors detect pathogen-associated molecular patterns to initiate the innate immune response. The cellular inflammatory response is the physical process by which these molecular signals translate into an army of immune cells arriving at the site of infection or injury. The sequence follows a precise choreography: detection, alarm, recruitment, and amplification.
The process begins when tissue-resident macrophages and mast cells detect a pathogen through their toll-like receptors and other pattern recognition receptors. These sentinel cells release the first wave of pro-inflammatory cytokines — TNF-α, IL-1, and IL-6 — along with chemokines and histamine. These mediators act on the local blood vessel endothelium, triggering two critical changes: vasodilation (widening of blood vessels, increasing blood flow to the area) and increased vascular permeability (gaps open between endothelial cells, allowing fluid and proteins to leak into the tissue). This produces the classical signs of inflammation you may have learned about: redness, heat, swelling, and pain. The swelling is not mere collateral damage — the leaked plasma carries complement proteins and antibodies into the tissue, providing additional antimicrobial defense.
Leukocyte extravasation — the migration of white blood cells from the bloodstream into infected tissue — is the centerpiece of cellular inflammation. It proceeds through a multi-step adhesion cascade. First, cytokine-activated endothelial cells upregulate selectins (P-selectin and E-selectin), adhesion molecules that loosely grab passing neutrophils and cause them to roll slowly along the vessel wall. Rolling neutrophils then encounter chemokines displayed on the endothelial surface, which activate integrins on the neutrophil surface — these switch from a low-affinity to a high-affinity conformation, causing the neutrophil to firmly arrest on the endothelium. Finally, the neutrophil squeezes between endothelial cells (a process called diapedesis) and follows the chemokine gradient into the tissue. Neutrophils arrive first, within minutes to hours, followed by monocytes that differentiate into macrophages over the next day or two.
Once in the tissue, recruited neutrophils and macrophages destroy pathogens through phagocytosis (engulfment and digestion), the release of reactive oxygen species (superoxide, hydrogen peroxide) that are directly toxic to microbes, and the secretion of antimicrobial peptides and proteases from their granules. These activated cells also produce additional cytokines, creating a positive feedback loop that recruits more immune cells and amplifies the response. Dendritic cells at the site capture antigen and migrate to draining lymph nodes, where they present processed peptides to T cells — bridging the innate inflammatory response to the adaptive immune response. The inflammatory response is self-limiting: as the pathogen is cleared, anti-inflammatory cytokines like IL-10 and TGF-β shift the balance toward resolution, macrophages switch from pro-inflammatory to tissue-repair phenotypes, and the inflammation subsides.