Many parents will find that from six months after birth to three years old, children are most likely to catch a cold. Some children have a cold for just a few days, and then for no apparent reason, they get a cold and runny nose, and they get injections and medication, so parents are anxious and children suffer. Parents are anxious and helpless as they see their children gradually losing weight. Could it be an immunodeficiency disease? In fact, most children who often catch colds are physiologically immunocompromised. The so-called physiological immune deficiency is actually a process of human survival and is not a pathology. Similar to human intellectual development, no one would or should expect a child to have the same intelligence as an adult, and similarly, we cannot expect a child to have the same immunity as an adult. The manifestations of physiological immune deficiency are usually mild, with infections mainly caused by the upper respiratory tract, such as colds, rather than serious infections such as pneumonia, meningitis, or sepsis. So what causes a child to have physiological immune deficiency? This is determined by the developmental characteristics of the human immune system. The immune system of children is generally not well developed, but because IgG can be transmitted to the fetus through the placenta, so within 6 months after birth, children have IgG from the mother and their own IgG, etc. During this period of time, less infectious diseases, mainly passive immunity is at work. 6 months later, the maternal source of immunoglobulin depletion, its own developmental imperfections will become apparent. 1. We know that immunoglobulins play an important role in fighting against foreign bacterial invasion. Immunoglobulins mainly include 5 kinds: IgG, IgM, IgD, IgA and IgE, while IgG is only equivalent to 60% of the adult level at the age of 1 to 3 years, and basically reaches the adult level only after the age of 10 to 12 years; IgM is only 10% of the adult level at birth, and gradually increases afterwards, and reaches the adult level only after the age of 1 to 2 years; IgD is only 1% of the adult level in the cord blood after birth, 10% at the age of 1 year, and reaches the adult level at the age of 2 to 3 years; IgE IgD is only 1% of the adult level after birth in umbilical blood, 10% at 1 year old, and reaches the human level at 2-3 years old; IgE reaches the human level at about 7 years old; IgA is the most delayed, and reaches the human level only in late adolescence or adulthood; secretory IgA can be detected in saliva at 2 months old, and reaches the human level at 2-4 years old. After 6 months of life, immunoglobulins of maternal origin are gradually consumed, while the ability of self-synthesis is still insufficient, which is one of the causes of low immunity. 2. T cells are important cells for cellular immunity, which not only assist B cells to produce antibodies, but also have the function of killing viruses and fighting against tumors. At birth, T-cell function is nearly perfect, but because they have never been exposed to antigens, they need strong antigenic stimulation to respond. In addition, T helper lymphocyte function is not mature in pediatric patients, so the ability of auxiliary B lymphocytes to synthesize antibodies is poor. 3. The function of pro-phagocytic factors in the serum of pediatric patients is lower than that of adults, which makes the ability of neutrophils to travel and their phagocytic function poor. 4. There are a variety of non-specific anti-microbial substances in normal body fluids, such as complement, lysozyme, beta-lysozyme, preparatostatin and interferon, etc. All of these substances are at a low level in pediatric patients, so the ability to resist disease is poor. Based on the above-mentioned characteristics of pediatric immune system development, it takes 2 to 3 years for a child’s immune system to be truly activated and to function after birth because the development of each system is not yet complete, so during this time, the child’s own immune function is quite low and it is easy for pathogens to take advantage of the situation. As the child grows older, the immune system becomes more functional as growth and development continue to improve. Therefore, parents need not worry too much about recurrent respiratory infections caused by physiological reasons. As long as you arrange proper care and exercise for your child, the frequency of respiratory infections will gradually decrease as your child grows older. If parents are really unsure, they can go to the hospital to have their immune function checked, and nowadays medical technology can distinguish between physiological and pathological immune deficiency.