The Ig receptor that binds to the FCR fragment on the mast cell membrane is the IgE receptor, and can therefore mediate a type I hypersensitivity reaction. There are five types of human Ig receptors, and the one that binds to FCR fragments on mast cell membranes is the IgE receptor, which has a molecular weight of 160 KD and is the least abundant Ig in total normal human serum. The important feature of IgE is that it is a class of cytophilic antibody, and its CH2 and CH3 structural domains are able to bind to the high-affinity FCRⅠ on mast cells and basophils and sensitize them. If the same allergen re-enters the body and binds to the specific IgE on the surface of the sensitized target cells, it will cause a type I hypersensitivity reaction. IgE is one of the five types of Ig in the human body, has a special role, and can bind to the FCR fragment on the membrane of mast cells in the body and cause type I hypersensitivity reaction.