**2. Prevalence**

Individual type of PIDs is considered to be rare in the population; however, recent studies have shown that PIDs may be more common than previously estimated 1% of the population when all varieties are combined [5]. The prevalence of PIDs varies depending on the type of immunodeficiencies and is difficult to be precisely calculated as the number of diagnosed cases is rapidly increasing. A 2018 global survey from the Jeffrey Modell Centers Network (JMCN) reported the case of PID patients followed in the JMCN increased by 35.4% to 102,097, while the case of patients identified with a specific gene defect increased 21.8% to 67,308 during the same period [5]. Up to 2018, 354 distinct disorders with 344 different gene defects were recognized [6]. Of note, most of the cases reported are from developed countries. It is estimated that 70–90% of individuals living with a PID are undiagnosed [7], particularly in the area with poor medical condition and lacking laboratory resources. With the extensive application of exome or whole genome sequencing, it was predicted that the associated PID genetic defects would reach 1000 under current trend in next decade [5]. **Table 1** listed the reported number of 18 most common PID defects among 354 inborn errors of immunity [5]. As shown, antibody deficiencies have much higher occurrence rate against other types of the disorders. Studies also showed that the selective IgA deficiency has the highest prevalence worldwide with a range from 1 in


**Table 1.** *Global prevalence of PIDs reported by Jeffrey Modell Centers Network [5].* 223 to 1 in 1000 depending on ethnic background [8], while severe combined immunodeficiency (SCID), although fatal, is much rarer (1 in 100,000) [9, 10].
