Immunity is our ability to fight against pathogens and avoid diseases, just like an army stationed in the human body, and it plays an important defensive role when fighting against external pathogens and toxins. Under normal circumstances, it may not be felt, but the body will fight back when it is not hit, such as a cold or a small wound, which can be healed without injections or medication, which is related to the repair function of the immune system. The immune system peaks around 25 years old and declines slowly after 30 years old; good memory is one of the characteristics of the immune system, which is an important principle for vaccines to prevent diseases; endocrine affects the immune system, so women are not as stable as men, and autoimmune diseases can occur; different age groups face different immune crises and have different ways to deal with them. Middle-aged people are the most dangerous group, and they should give more care and rest to their immune system. However, due to economic and mental pressure, many people still squander their health like young people do, with irregular work and rest, and unbalanced nutrition, which are most likely to cause problems with the immune system and make cancer more frequent.