The immune system is an important system for the execution of body's immune response and has the role of immune surveillance, defence and regulation. It is mainly composed of immune organs, immune cells and immune active substances. Immunologically active substances are the signalling links of immune system and are mainly involved in the regulation of immune response to maintain the balance and stability of immune system, including membrane molecules (TCR, BCR, CD molecules, adhesion molecules, MHC molecules,etc.) and secretory molecules (lgA, lgD, lgM, etc.).