The best way to understand a person's immune system is to find out how sick they are. On the one hand, the less and the more days you get sick , on the other hand, the faster you recover from that illness, the better your immunity. Even then, some blood tests can show the state of immunity. Such as overall picture of blood or CBC , immunoglobulin status of blood etc. To increase immunity, you should eat high quality meat (fish , eggs , milk , pulses , meat, etc.) and antioxidant rich foods (lemon , orange , mango , papaya , all kinds of purple , red , yellow , orange, vegetables, fruits, etc.) every day. . Whether a person's immunity will increase rapidly or not depends on his or her body's needs , needs , and certain things (food-nutrient interactions) , such as a small lemon (vitamin-C) with a meal that triples the amount of hemoglobin in the blood . Drinking tea or coffee immediately reduces the amount of hemoglobin.