Taken together, changes in NGF, BDNF, and NO levels in blood and ocular fluids have been reported during the onset and progression of many illnesses, including ophthalmological, neurological, psychiatric, endocrine, and immune diseases and other physio-pathological disorders, such as cardiometabolic states, stressful events, alcoholism, and aging [56]. Here, BDNF is linked to immune system disorder.