Effector/ɪˈfektər/ Definition 1 of 2Pro Tip 1/3Biology Usage In biology, 'effector' often refers to muscles or glands triggered by nerves. Certain glands act as effectors releasing hormones. Pro Tip 2/3Immune Response 'Effector' also relates to immune cells that respond to pathogens. Effector cells help the body defend against infections. Pro Tip 3/3Technical Term 'Effector' is technical and used mostly in scientific contexts. In a reflex action, the effector muscle contracts.