Primary immunodeficiency disease: is a genetic disease. More than 150 types of primary immunodeficiency diseases have been identified to date and are developing at a rate of one to two new primary immunodeficiency diseases per year. They usually develop in infancy and childhood, but some types of primary immunodeficiency diseases can develop at a later age or even in adulthood, with approximately 40% of cases developing within 1 year of age, 40% within 5 years of age, 15% within 16 years of age, and only 5% in adulthood. When should parents suspect primary immunodeficiency disease and seek prompt medical attention when their child presents? The first is the child’s medical history, with more than 8 otitis media in a year, or more than 2 severe sinus infections, or more than 2 pneumonias, or more than 2 deep infections at uncommon sites, or repeated deep skin or organ infections, or infections that require intravenous antibiotics to clear, or infections by uncommon or conditional pathogens, or children with a family history of primary immunodeficiency disease; the second is the child’s symptoms, the child presents with growth arrest, lack of lymph nodes and tonsils, seen with skin lesions such as dilated capillaries, bleeding spots, etc., fungal skin infections, lupus erythematosus-like rash, ataxia (which is the easiest sign for parents to detect, such as the child walking uncoordinated and writing unstable), and also thrush and mouth ulcers after the age of one. In both of these cases, primary immunodeficiency is suspected. Once parents suspect that their child has a primary immunodeficiency disorder, they should bring the child to a hospital immunology specialist promptly. The doctor may choose to test several immunological indicators, such as serum immunoglobulin levels, serum IgG subclass levels, serum complement levels, T-cell subsets, B-cell counts, NK-cell counts, phagocytosis, cell surface molecules, and chest radiographs for the presence of thymus. Cytokines can be tested if necessary, and even genetic testing can be taken to clarify the genetic localization of the disease. After the diagnosis is confirmed, treatment can be started accordingly. Parents should be patient and cooperate with the doctor to help the child fight the disease.