MHCⅱ class ⅱ molecules are expressed on the surface of full-time APC, which is responsible for presenting exogenous antigens to CD4+T cells. There are two types of MHC. The important physiological function of class I molecules is to restrict the antigen recognition function of CD8+T cells, that is, to participate in the process of presenting antigens to CD8+T cells. CD8+T cells can only recognize antigens that bind to the same class I molecule (mostly endogenous cellular antigens, such as virus-infected cells and tumor cells), which is called MHC restriction. The function of class II molecules is to present the processed antigen fragments to CD4+T cells at the initial stage of immune response. Just as CD8+T cells can only recognize antigen fragments bound to MHC molecules, CD4+T cells can only recognize antigen fragments bound to MHC molecules. Class ⅱ molecules are mainly involved in the presentation of exogenous antigens, and can also deliver endogenous antigens under some conditions. In the process of tissue or organ transplantation, class II molecules are important target antigens that cause transplant rejection. In the immune response, class ⅱ antigen mainly coordinates the interaction between immune cells and regulates humoral immunity and cellular immune response.