Our bodies are in a dangerous environment where pathogens are everywhere – germs, viruses, and parasites are always ready to take advantage of the situation. The skin is the body’s first line of defense and the blood is our second line of defense. In the blood there are various immune cells, immune molecules, they play different roles, there are police, there are agents, there is a standing army, there is a reserve force …… division of labor cooperation, effective defense against foreign invaders. Unfortunately, this immune army sometimes gets confused, unable to distinguish between the enemy and me, and takes out its own cells as the enemy as well, triggering autoimmune diseases. Sometimes, foreign substances that accidentally enter the body are harmless, and the immune system will try to eliminate them as if they were enemies, causing allergic reactions that can be uncomfortable in mild cases or fatal in severe ones. In the immune army, a very important detachment consists of antibodies, which are immunoglobulins (Ig for short) produced by B lymphocytes, and there are several kinds of IgG, IgA, IgM, IgD and IgE. They vary in content and have different divisions of labor. For example, the most abundant IgG can attack a variety of pathogens such as bacteria, viruses and fungi and neutralize the toxins they secrete. The least abundant IgE, on the other hand, can trigger powerful allergic reactions. Allergic reactions are harmful to the body and IgE cannot be specifically designed to cause allergies. It has a normal physiological function. In developed countries, where parasitic diseases are rare, IgE has nothing to do but attack harmless substances that enter the body.