The Inflammatory Response

Inflammation is a complex immune response to harmful stimuli, often recognized by five classical signs: pain (dolor), redness (rubor), swelling (tumor), heat (calor), and loss of function. While chronic inflammation is detrimental, acute inflammation is an essential evolutionary adaptation that helps contain infectious microbes by destroying invaders, inducing blood clotting to prevent spread, and repairing injured tissue.

Acute inflammation involves vascular and cellular components. The process begins with tissue-resident immune cells sensing damage or microbial infection. Activated macrophages release inflammatory mediators like leukotrienes, prostaglandins, and nitric oxide, which cause vasodilation (widening of blood vessels), leading to redness and heat. Vasodilation slows blood flow, allowing immune cells to interact with the vasculature. Mediators also increase vascular permeability, causing fluid leakage (swelling) and concentrating immune proteins like complement and antibodies in the affected tissue. Other protein cascades like the kinin system (producing bradykinin, which increases permeability and pain) and the coagulation system (forming fibrin clots to contain pathogens) are also activated.

Inflammation activates endothelial cells, increasing the expression of cell adhesion molecules like P-selectin and E-selectin on their surface. As blood flow slows, immune cells (neutrophils and monocytes) attach to these selectins and roll along the blood vessel walls. They then express adhesion molecules like LFA-1 and CR3, interacting with ICAM-1 and ICAM-2 on endothelial cells, strengthened by chemokines like CXCL8, eventually stopping and adhering to the endothelium. The immune cells then extravasate, or squeeze between endothelial cells into the tissue, guided by chemotaxis to the site of infection.

Once in the tissue, immune cells engage in the cellular component of inflammation, including phagocytosis (enhanced by opsonization) and the release of pro-killing effectors like lysozyme, granzyme B, and nitric oxide. They also propagate the immune response by releasing cytokines and chemokines. Many of these cells are also crucial for subsequent tissue repair.

The acute inflammatory response is powerful and designed for short-term action. Resolution is an active process involving regulatory molecules to decrease inflammation and promote healing. Neutrophils, abundant during infection, have a short half-life and undergo apoptosis, then are phagocytosed by macrophages (efferocytosis). This reprograms macrophages to be less pro-inflammatory and more pro-resolving, producing cytokines that promote wound healing. Lipid mediators like resolvins, protectins, lipoxins, and maresins, derived from polyunsaturated fatty acids, also contribute to resolution by producing anti-inflammatory cytokines, stimulating efferocytosis, and decreasing neutrophil chemotaxis, adhesion molecule expression, and vascular permeability.

The resolution phase includes tissue repair processes such as angiogenesis (new blood vessel growth), epithelial cell proliferation, and deposition of extracellular matrix proteins to restore tissue function. Failure to undergo this resolution phase, due to persistent pathogens or tissue damage, leads to chronic inflammation. Chronic inflammation can have severe consequences, including scarring, non-healing wounds, ulcers, and can even promote cancer. A balanced inflammatory response is crucial for eliminating pathogens and allowing proper tissue healing and return to homeostasis.