**3. Classification**

The classification of PIDs is generally based on the defects of the major components of human immunity, such as innate/adaptive immunodeficiencies, phagocytic deficiencies, complement deficiencies, and immune dysregulation. The classification has evolved over time with more phenotypic and genetic defects identified [4, 11].


#### **Table 2.**

*The 2017 IUIS phenotypic categorization of PIDs [6, 11].*

The International Union of Immunological Societies (IUIS) expert committee, currently named as Inborn Errors of Immunity Committee, has been responsible for issuing the classification of PIDs every other year from 1970. The complete catalog of classification has now been widely used as a reference by clinicians and researchers. From 2013, IUIS published more user-friendly phenotypic classification in two formats: one is a pdf file, namely clinically oriented phenotype categorization in the Journal of Clinical Immunology, and the other is a csv file containing a comprehensive list of various disorders that can be downloaded from http://www. iuisonline.org [6, 11]. The phenotypic categorization published in the journal has been well designed for clinical use, while the online list contains the most updated information demonstrated in a digital friendly excel format that can be sorted by phenotypic and genetic features, which are very useful for designing sequencing panels, disease code lists, and diagnostic algorithms.

The major category and subcategory of PIDs from the revised 2017 IUIS phenotypic classification are summarized in **Table 2**.
