*Review* **The Importance of the Transcription Factor Foxp3 in the Development of Primary Immunodeficiencies**

**Paulina Mertowska, Sebastian Mertowski \*, Martyna Podgajna and Ewelina Grywalska**

Department of Experimental Immunology, Medical University of Lublin, Chod ´zki 4a St., 20-093 Lublin, Poland; paulinamertowska@umlub.pl (P.M.); marpodgajna@gmail.com (M.P.); ewelina.grywalska@umlub.pl (E.G.) **\*** Correspondence: sebastianmertowski@umlub.pl

**Abstract:** Transcription factors are an extremely important group of proteins that are responsible for the process of selective activation or deactivation of other cellular proteins, usually at the last stage of signal transmission in the cell. An important family of transcription factors that regulate the body's response is the FOX family which plays an important role in regulating the expression of genes involved in cell growth, proliferation, and differentiation. The members of this family include the intracellular protein Foxp3, which regulates the process of differentiation of the T lymphocyte subpopulation, and more precisely, is responsible for the development of regulatory T lymphocytes. This protein influences several cellular processes both directly and indirectly. In the process of cytokine production regulation, the Foxp3 protein interacts with numerous proteins and transcription factors such as NFAT, nuclear factor kappa B, and Runx1/AML1 and is involved in the process of histone acetylation in condensed chromatin. Malfunctioning of transcription factor Foxp3 caused by the mutagenesis process affects the development of disorders of the immune response and autoimmune diseases. This applies to the impairment or inability of the immune system to fight infections due to a disruption of the mechanisms supporting immune homeostasis which in turn leads to the development of a special group of disorders called primary immunodeficiencies (PID). The aim of this review is to provide information on the role of the Foxp3 protein in the human body and its involvement in the development of two types of primary immunodeficiency diseases: IPEX (Immunodysregulation Polyendocrinopathy Enteropathy X-linked syndrome) and CVID (Common Variable Immunodeficiency).

**Keywords:** Foxp3; IPEX; CVID; primary immunodeficiencies
