Inflammation usually shows four classic signs—redness, heat, swelling and pain—each linked to distinct physiological events.
Redness (rubor)
When tissue is injured, mast cells and other sentinels release histamine and prostaglandins. These mediators cause arterioles and capillaries to dilate, allowing a larger volume of blood to flow into the area. The extra blood gives the skin a pink or red hue, a process known as hyperemia.
Heat (calor)
The same vasodilation that creates redness also brings warm blood from the body’s core to the surface. The influx of heat raises the local temperature, which can hinder temperature‑sensitive microbes and speed up enzymatic reactions that support immune activity.
Swelling (tumor)
Inflammatory mediators make the walls of post‑capillary venules more permeable. Small gaps appear between endothelial cells, allowing plasma proteins and fluid to escape into the interstitial space. The resulting edema expands the tissue and supplies antibodies, clotting factors and white blood cells to the site.
Pain (dolor)
Swelling stretches surrounding tissues, putting pressure on nociceptors, pain‑sensing nerves. At the same time, chemicals such as bradykinin, prostaglandins and serotonin bind to receptors on these nerves, lowering their activation threshold. The combined mechanical and chemical stimulation produces the characteristic ache or throbbing sensation.
Why the signs matter
Each sign reflects a step in the body’s defense strategy. Redness and heat deliver immune cells and create an unfavorable environment for pathogens. Swelling provides a protein‑rich fluid that carries defensive molecules, while pain encourages the individual to protect the injured area. Recognising these signs helps clinicians assess the severity and stage of an inflammatory response.