**6. Conclusions**

This review first analyzed current evidence of a potential relationship between EGIDs and IEI. According to recent data, EGIDs seem more common in IEI patients than was already reported in the literature. It is reasonable to speculate that EGID can worsen the course of IEI, and vice versa. For this reason, early diagnosis is crucial to prevent complications and define the best personalized treatment. In this context, several unmet needs are still to be clarified. The literature data are still limited, and more research is needed to understand the pathogenetic relationship between these two chronic and invalidating clinical entities. Multicentric prospective studies should be performed to establish the real epidemiology of EGID in IEI patients, the disease-course phenotype, and the response to available treatments.

**Author Contributions:** Conceptualization, M.V. and R.C.; methodology, M.V. and R.C.; writing original draft preparation, M.V. and M.N.; writing—review and editing, M.V., M.N., I.B. and R.C.; supervision, I.B., M.D.F., S.C., A.L., G.L.M. and R.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Daniela Montagna and Lorenza Montagna (Pediatric Unit, Department of Clinical, Surgical, Diagnostic, and Pediatric Sciences, University of Pavia, Pavia, Italy) for their invaluable support to the research in the field of inborn errors of immunity.

**Conflicts of Interest:** The authors declare no conflict of interest.
