Aulus Cornelius Celsus described heat, redness, swelling, and pain as the four physical signs of inflammation in the year 38 AD. These four signs remain the cornerstone of medical diagnosis today and form the Latin triad of calor, rubor, tumor, and dolor.
Resident immune cells like macrophages and mast cells activate immediately upon contact with foreign invaders or damaged tissue. These cells possess surface receptors known as pattern recognition receptors that bind to pathogen-associated molecular patterns found on bacteria or damage-associated molecular patterns released by injured cells.
Atherosclerosis is now understood as a chronic inflammatory condition involving the arterial walls where inflammation mediates all stages from initiation through progression to thrombotic complications. Low-grade chronic inflammation indicated by levels of the inflammatory marker C-reactive protein prospectively defines the risk of atherosclerotic complications and adds to prognostic information provided by traditional risk factors like LDL levels.
As of 2012, chronic inflammation was estimated to contribute to approximately 15% to 25% of human cancers. Inflammatory mediators such as prostaglandins, cytokines, and chemokines create an environment that fosters cancer development while reactive oxygen species and reactive nitrogen species cause DNA damage that can lead to mutations and epigenetic alterations.
A 2019 meta-analysis found that chronic inflammation is associated with a 30% increased risk of developing major depressive disorder. The brain enters a sickness mode when cytokines increase causing symptoms that overlap with depression such as lethargy and mood disorders.
In extreme cases of systemic inflammatory response syndrome the number of leukocytes in the blood can increase to between 15,000 and 20,000 cells per microliter with extreme cases approaching 100,000 cells per microliter. This condition is known as leukocytosis and occurs when the infection escapes the confines of the immediate tissue via the circulatory system or lymphatic system.