Generally speaking, children's immune system is not developed, but because IgG can be transmitted to the fetus through the placenta, children have IgG from their mothers and IgG produced by themselves within 6 months after birth. During this period, there are fewer infectious diseases such as measles, mainly passive immunity. (1) Nonspecific immune system 1. Monocyte-phagocyte system: The phagocytic cells in blood, mainly neutrophils and monocytes, begin to develop in the fetal period and can reach 8 ~ 13× 109 cells /L (8000 ~ 13000 cells /mm after birth. The function of phagocytic factor in children's serum is lower than that of adults, which makes neutrophils have poor wandering ability and phagocytic ability, but its direct bactericidal effect is similar to that of adults. In addition, monocyte-phagocyte system can also remove microthrombosis in blood. 2. Barrier function: The barrier function of children's skin and mucosa is poor, especially in the neonatal period, which is prone to septicemia due to skin and mucosa infection. The blood-brain barrier is immature and prone to intracranial infection. Others, such as the placental barrier, are also stunted, especially in the first three months. At this time, if pregnant women suffer from viral infection, they can all cause fetal congenital viral infection through the placenta. Common are rubella, herpes and cytomegalovirus. 3. Humoral factors: There are many nonspecific antibacterial substances in normal body fluids, such as complement, lysozyme, lysin B, properdin and interferon, which are at a low level, so their disease resistance is poor. (2) Specific immune system 1. Humoral immunity: immunoglobulin is the material basis of humoral immunity. ⑴ IgG: IgG is the highest immunoglobulin content and the only immunoglobulin that can be transmitted to the fetus through the placenta. 10 ~ 12 gestational weeks can synthesize IgG by itself, and the content is very small, but the maternal IgG can be transmitted to the fetus through the placenta, and its content increases with the increase of gestational weeks. The gestational age is 56% of that of adults at 8 months and 88% at 9 months. The content of IgG in umbilical cord blood of full-term newborns can exceed that of mothers, while that of premature infants is much lower than that of full-term infants. After birth, IgG is gradually consumed, but its self-synthesis ability is still insufficient. By 1 ~ 3 years old, it is equivalent to 60% of adults, and after 10 ~ 12 years old, it basically reaches the human level. ⑵IgA: A very small amount of IgA was synthesized around 30 weeks of pregnancy, and IgA could not pass through the placenta. IgA of newborn comes from mother's colostrum. One month after birth, the content is only about 2.6% of that of adults, reaching the human level at the age of 10. Secretory IgA can approach the adult level at the latest 6 months in newborns. It plays a certain role in protecting the baby from harm. ⑶IgM: The fetus began to synthesize IgM at 10 ~ 12 weeks, which was about 10% of that of adults at birth, and then gradually increased, reaching the human level at 1 ~ 2 years old, and IgM could not pass through the placenta, and the IgM content increased during intrauterine infection. Therefore, the increase of IgM in cord blood suggests intrauterine infection. ⑷IgD: The gestational age is 3 1 week, and there is little self-synthesis. After birth, the umbilical cord blood content is only 1% of that of adults, and 1 0% at the age of1year, reaching the human level at the age of 2 ~ 3. ⑸IgE: gestational age 1 1 Zhou Hecheng, reaching human level around 7 years old. The increase of IgE in children with syncytial virus infection and asthma may be related to its pathogenesis. 2. Cellular immunity: At gestational age 15 weeks, T cells migrate from thymus to lymphoid tissues around the whole body with blood flow and participate in cellular immune response, but their functions are not yet mature. At birth, their functions are almost perfect, but they have never been exposed to antigens, so they need strong antigen stimulation to react. The function of T helper lymphocytes in neonatal period is immature, so the ability of B helper lymphocytes to synthesize antibodies is poor.

Immune system G3g new thinking education G3g new thinking education monocyte/macrophage G3g new thinking education neonatal monocyte development has been perfect, but due to lack of auxiliary factors, its chemotaxis, adhesion, phagocytosis, oxidative sterilization, G ~ CSF, IL ~ 8, IL ~ 6, IFN ~ γ, IL ~ 12 and antigen presentation ability are worse than adults. Exposure of newborns to different kinds and doses of antigens or allergens will directly affect the immune regulation function of monocytes/macrophages, especially DC, which will affect the immune status of newborns in the future. Stimulated by childbirth, the number of neutrophils in peripheral blood is high 12 hours after birth, and then gradually decreases and then gradually rises to human level. Because the storage is empty, severe neonatal sepsis is prone to neutropenia. Low complement level, lack of chemokines and opsonin are also important reasons for neutropenia in infants. G3g New Thinking Education of T Lymphocytes and Cytokines (1) Value G3g New Thinking Education Mature T cells account for 80% of peripheral blood lymphocytes, so peripheral blood lymphocyte count can reflect the number of T cells. At birth, the number of lymphocytes is small, and the percentage of neutrophils exceeds 6-7 months, which is the same at 6-7 years old. Then with the growth of age, it gradually drops to the low level of G3g new thinking education for the elderly (2) T cell phenotype and function G3g new thinking education. The vast majority of umbilical cord blood T cells (97%) are CD45RA+ "initial" T cells (50% in adult peripheral blood), while CD45RO+ memory T cells are very few. The ability of newborn T cells to express CD25 and CD40 ligands is weaker than that of adults, and the ability of assisting B cells to synthesize and transform Ig and promoting phagocytes and CTL is poor. G3 new thinking education ③ Th subgroup G3g new thinking education neonatal TH2 cells have advantages over TH 1 cells, which is beneficial to avoid maternal and infant immune rejection. ④ The new thinking of cytokine G3g educates newborn T cells to produce only 50% of TNF and GM ~ CSF, as well as IFN ~ γ, IL ~ 10 and IL ~ 4. With the repeated stimulation of antigen, the levels of various cytokines gradually increased (5) NK and antibody-dependent cytotoxic cells (ADCC). CD56, the surface marker of NK, is almost not expressed at birth, and the activity of NK reaches human level at 1-5 months after birth. The function of ADCC is only 50% of that of adults, 1 year can reach the level of adults. G3g New Ideas Educate B Lymphocytes and IgG3g New Ideas Educate (1) Phenotype and Function of B Cells G3g New Ideas Educate Fetuses and Newborns that B cells produce IgM, but not IgG and IgA. At the age of 2, IgG-secreting B cells and IgA-secreting B cells reached the human level at the age of 5. Due to the insufficient function of TH cells, B cells can't produce polysaccharide vaccine and capsular polysaccharide bacterial antibody G3g new thinking education ⑵IgGG3g new thinking education is the only Ig category that can pass through the placenta, and its transport process is active. A large amount of IgG is produced through the placenta in the third trimester of pregnancy. The serum IgG level of full-term newborns is even 5 ~ 10% higher than that of their mothers, but their ability to synthesize antibodies is very poor at this time. The serum IgG concentration of fetus or immature infant with gestational age less than 32 weeks is lower than that of mother. Although the self-synthesis ability was improved at 3 months after birth, the IgG from the mother decreased greatly and disappeared at 6 months, so the serum IgG of infants aged 3 ~ 6 months dropped to the lowest point. After 1 year, it is only 60% of that of adults, and it reaches the human level when it is 8 ~1year. IgG subclasses gradually increase with age. IgG2 is mainly an antibody against bacterial polysaccharides, and the increase rate is very slow within 2 years old. At this age, it is easy to be infected by capsular bacteria. 3. (3) IgM G3 new concept education. IgM has been produced in the fetus, but due to the lack of antigen stimulation, the amount of IgM synthesized by the fetus itself is very small, and the mother's IgM cannot pass through the placenta. If the level of IgM in umbilical cord blood rises, it indicates intrauterine infection. IgM reached adult serum level when boys were 3 years old and girls were 6 years old. G3g new thinking education (4) (4) IgAg3G new thinking education developed at the latest, and it didn't reach human level until late adolescence or adulthood. Secretory IgA could not be detected in newborn, but could be detected in saliva at 2 months. At the age of 2 ~ 4, it can reach the level of G3g new thinking education of immune molecules such as complement. (1) The mother's new thinking education of complement G3g is not transferred to the fetus, and the activity of classical complement pathways (CH50, C3, C4, C5) of newborns is 50-60% of that of their mothers, reaching the adult level in 3-6 months after birth. The development of the components of the bypass pathway is more backward, and the B factor and the leptin are only 35 ~ 60% and 35 ~ 70% of that of adults, respectively. The classical pathway and bypass pathway of complement in immature infants are lower than those in mature infants. ⑵ The plasma fibronectin concentration of newborns is only 65,438+0/3 ~ 65,438+0/2 of that of adults, and it is even lower in immature infants. The mannose-binding hemagglutinin (MBL) of immature infants is lower than that of adults, and it reaches the new thinking education level of full-term newborns at 10 ~ 20 weeks after birth.