1. Less than a normal response or reaction to stimuli: The doctor was trying to determine the degree of nerve damage in Helen's arm by brushing it with a feather, but the hyposensitivity she was experiencing delayed her response.
2. An overreaction of the immune system (defense against infection), to an antigen (a protein recognized as foreign to the body): Although protective immunity is meant to help people fight a disease, some hypersensitivities are related to allergies that can lead to tissue damage and disease.