ActivationInduced

Activationinduced is a term used in biology to describe processes and molecules that are triggered by cellular activation. It is most commonly encountered in immunology as part of compound terms such as activation-induced cytidine deaminase and activation-induced cell death. The phrase signals that the activity or fate of the molecule or cell depends on prior or ongoing activation signals.

Activation-induced cytidine deaminase (AID) is an enzyme expressed by activated B cells and is central to antibody diversification. AID deaminates cytosine to uracil in DNA, initiating somatic hypermutation and class switch recombination, which together generate antibody diversity and the ability to tailor antibody effector functions. AID expression is induced by signals from helper T cells, including CD40-CD40L interactions and specific cytokines. Regulation of AID is tight, because off-target or excessive activity can lead to genomic instability and has been linked to lymphomas and certain autoimmune conditions.

Activation-induced cell death (AICD) refers to apoptosis of immune cells following activation, particularly through the Fas-FasL pathway. In T cells, repeated or strong antigen receptor stimulation can upregulate Fas ligand, triggering Fas-mediated apoptosis to help maintain immune homeostasis and tolerance. AICD helps prevent autoimmunity and limits the size of immune responses, but abnormalities in this process can contribute to chronic infections, cancer, or autoimmune diseases.

In summary, activationinduced terms describe how immune cells and molecules respond to activation signals, with AID and AICD representing two well-characterized examples that influence antibody production and immune regulation, respectively.