An inflammatory response is a localized, nonspecific immune reaction that occurs when a tissue is injured. It is characterized by redness, swelling, heat, and pain, which are commonly called the cardinal signs and symptoms of inflammation. Inflammation can sometimes result in a loss of function.

Inflammation can be triggered by various stimuli, such as impact, abrasion, chemical irritation, infections, and extreme hot or cold temperatures. These can damage cells and connective tissue fibers, changing the chemical composition of the interstitial fluid. These damaged tissues and resident immune cells release a cascade of inflammatory chemicals, triggering inflammation. For example, mast cells in the tissue release histamine and prostaglandins into the extracellular fluid.

Vasodilation

Histamine increases the dilation (vasodilation) and permeability of nearby blood vessels. This vasodilation results in an increased blood flow to the affected area, causing the characteristic redness and heat associated with inflammation. Increased blood flow also enhances enzymatic reactions and facilitates the entry of clotting factors and complement proteins into the injured or infected area. Additionally, the increased vessel permeability allows plasma fluid to leak into the surrounding tissues, causing swelling or edema. This swelling creates pressure within the tissue, which, along with the action of prostaglandins, stimulates nerve endings and induces pain.

Phagocyte Recruitment

One of the critical aspects of the inflammatory response is the recruitment of phagocytes, such as macrophages and neutrophils, to the site of injury. The inflammatory chemicals act as chemotactic agents, creating a chemical gradient that attracts these phagocytes. Neutrophils, being the first responders, arrive rapidly at the site, followed by macrophages. These phagocytes migrate through the more permeable blood vessel walls via a process called diapedesis or extravasation.

Once they reach the injury site, neutrophils and macrophages engulf and digest pathogens, debris, and dead cells through phagocytosis. This cleanup process not only helps to eliminate infectious agents but also prevents the spread of infection to adjacent tissues. In addition to their phagocytic activity, macrophages release cytokines, which are signaling molecules that modulate the immune response and promote tissue repair.

Regulation of Inflammatory Response

Although the inflammatory response is beneficial, it must be tightly regulated to prevent excessive tissue damage and chronic inflammation. Anti-inflammatory cytokines are released to counteract the pro-inflammatory signals and promote the resolution of inflammation. Additionally, regulatory T cells help modulate the immune response and prevent autoimmunity. The resolution phase also involves the clearance of apoptotic cells and debris by phagocytes, promoting tissue remodeling and restoration of function. This finely tuned regulation is essential for maintaining homeostasis and preventing chronic inflammatory diseases.