The defense system inside the human body is the immune system. Simply speaking,it is the function of immune system that removes all the foreign agents from human body, including external pathogenic microorganisms, implants, foreign bodies and toxins from wounds, and internal senile cells and malignant cells. Through a long-term evolution, the immune system with both inside and outside functions has developed perfect tissue structures and exceptional functions. The immune system can be divided into innate immune system and adaptive immune system. The innate immunity,also called congenital immunity, is the oldest existing functionality in the biological evolution, which eliminates the encountered foreign bodies immediately through its components, macrophages, granulocytes, natural killer cells and complement system.However, the adaptive immunity, also called specific immunity, is acquired after birth with a characteristic of having memories, which can specifically attack the same invasive foreign agents.

The process of immune balance is a complicated process. When the functions of immune cells blancing collapse, the human body will lose the defensive functions of immune system, external pathogenic microorganisms or tumor cells will intrude into the circulating system, and then the alternative defensive barrier is activated inevitably. It is a basic principle of releasing compensatory reserved functions. The formation of intravenous biological filter indicates the existence of external pathogenic microorganisms or tumor cells in the veins and the supplement of the lost immune functions.