**1. Introduction**

Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive disorder that is defined in three categories consist of severe immunodeficiency, thrombocytopenia, and eczema. Moreover, high frequency of lymphomas (mainly B cell origin), susceptibility to autoimmune diseases, recurrent infections, variable defects in B and T-lymphocyte function ,and bloody diarrhea are classified as other symptoms [1]. A genetic mutation in the WAS gene encoding Wiskott-Aldrich syndrome protein (WASp) affecting the immune system and leads to immunodeficiency. There is broad-spectrum depending on gene mutations ranging from severe phenotype (Classic WAS) to mild ones which consist of X-linked thrombocytopenia (XLT) and X-linked neutropenia (XLN) [2, 3]. The purpose of this chapter is to discuss WASp function, clinical and pathological manifestations associated with mutations of the WAS gene as well as a comprehensive review of WAS clinical diagnosis and treatment methods and their complications. In addition, a range of therapeutic approaches will be discussed, including the use of allogeneic hematopoietic stem cell (HCT) transplantation and gene therapy.
