Exogen

Exogen is a term used in several scientific contexts to denote something that originates from outside a given system. In practice, the more common form is exogenous, but exogen appears in older or discipline-specific texts as a noun or adjective referring to an external origin. The underlying idea across fields is a contrast with endogenous factors, which arise from within the system.

In biology and medicine, exogenous refers to substances or influences that come from outside the organism. Examples include administered drugs, environmental toxins, or external pathogens. Exogenous hormones or drugs are used therapeutically to modify physiological processes, in contrast to endogenous hormones produced by the body's own glands. Vaccines introduce exogenous antigens to provoke an immune response.

In pharmacology and toxicology, exogenous compounds are those introduced into the body by ingestion, inhalation, injection, or topical exposure. These compounds are subject to absorption, distribution, metabolism, and excretion (ADME), and their external origin is a key consideration for dosing, safety, and potential interactions with endogenous processes.

In ecology and systems modeling, exogenous factors are external drivers that influence a system but are not produced by it. Examples include climate variables, pollution inputs, or immigration in population models. In statistical and econometric contexts, exogenous variables are inputs determined outside the model and treated as given when assessing system behavior.

Overall, exogen (or exogenous) serves as a concise way to describe external origin relative to the studied entity. The term’s precise meaning depends on context, but the common theme is external origin compared with internal, endogenous sources.