In physiology, a stimulus (plural stimuli) is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity.When a stimulus is applied to a sensory receptor, it normally elicits or influences a reflex via stimulus transduction.he endocrine system is affected largely by many internal and external stimuli. One internal stimulus that causes hormone release is blood pressure. Hypotension, or low blood pressure, is a large driving force for the release of vasopressin, a hormone which causes the retention of water in the kidneys.
Macrophages can be activated by cytokines such as interferon-gamma (IFN-gamma) and bacterial endotoxins, such as lipopolysaccharide (LPS). Activated macrophages undergo many changes which allow them to kill invading bacteria or infected cells.
In acute virus infection, macrophage is recruited from the circulation in stimulation of chemokines and serves as a powerful killer of invading pathogens with the secretion of inflammatory mediators, such as TNF-Î± and inducible nitric oxide synthase [30].