Sensitisation

Sensitisation, or sensitization, is a process by which a biological system becomes more responsive to a stimulus after prior exposure, often with repeated or stronger exposure. It can occur in immune, nervous, and behavioral systems, leading to a heightened reaction to the same or related stimuli compared with earlier encounters.

In immunology, sensitisation refers to the development of an immune response to a normally harmless substance, such as an allergen. Repeated exposure can result in the production of specific antibodies—often IgE—and priming of mast cells and basophils. Upon subsequent contact, these mediators are released, causing allergic symptoms ranging from itching and runny nose to bronchoconstriction or anaphylaxis. Sensitisation can arise from skin contact, inhalation, or ingestion and may persist for years.

In neuroscience and psychology, sensitisation describes an increased behavioral or physiological response to a stimulus following repeated exposure or certain conditioning protocols. This contrasts with habituation, where responses diminish. Examples include drug sensitisation, where repeated use of stimulants enhances locomotor activity or craving, and central or nociceptive sensitisation, where the nervous system amplifies pain signals. Mechanistic explanations involve changes in neurotransmitter release, receptor expression, and synaptic plasticity, with NMDA receptor activity often implicated.

Desensitization or tolerance is the opposite process, in which responses diminish with continued exposure or adaptive changes. Sensitisation can be short-lived or long-lasting, and its dynamics are influenced by dose, timing, genetic factors, and the nature of the stimulus. Understanding sensitisation informs areas such as allergy management, addiction research, pain therapy, and vaccine or adjuvant design.