**1. Introduction**

Aplastic anemia (AA) is a rare disease, caused by bone marrow (BM) aggression resulting in hypo or aplastic BM with precocious fat replacement and consequently to peripheral blood pancytopenia [1, 2]. The autoimmunity process in AA occurs due to the activation of the oligoclonal cytotoxic T cells that will lead the hematopoietic cells to apoptosis. Its triggering occurs by the imbalance between CD8 +, CD4 +, T-Helper (Th), Th type 1 (Th1), Th type 2 (Th2), Th17 type (Th17), Natural Killer (NK) and T-regulatory cells (Treg). Besides, there is also an abnormal production of proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and transformed growth factor (TGF) [3–7].

For severe cases, immunosuppressive therapy is accepted as the first-line treatment option and the allogeneic transplantation of BM and hematopoietic stem cells (HSCs). However, 30–40% of patients with severe aplastic anemia (SAA) remain pancytopenia following the treatment. The transplant option still has a restricted number of compatibility between suitable donors. Additionally, patients aged >50 years are not eligible for transplant [8].

A new viable alternative for the treatment of AA has been sought and the use of mesenchymal stem (MSCs) therapy may be a promising therapeutic candidate mainly because of their hypoimmunogenicity and the lack of rejection after transplants and immunomodulatory effects, which may promote decreasing the symptoms of the disease [9, 10]. These benefits are attributed to the paracrine effects, above all by its ability to regulate the immune system [11].

Actually, is known that MSCs have wide therapeutically potential attributed by paracrine effects and the past decades have seen explosion research directed to understand better these MSCs mechanism and function [12]. One of the main and most important features of MSCs is the low expression of human leukocyte antigen (HLA) class I, with no expression of HLA class II. This feature allows the cell to be characterized as hypoimmunogenic, since it does not stimulate the patient's immune system and can be used safely in transplants [13]. More recently, the studies showed that the main cause of AA is autoimmunity. Through the secretion of bioactive molecules, MSCs have the capacity of regulating immune responses. The mechanism of MSCs may decrease secretion of proinflammatory cytokines such as transforming growth factor (TGF), IFN-γ TNF-α, interleukin (IL)-17 and increase secretion of many soluble mediators, including anti-inflammatory cytokines stimulation that inhibit antigen-presenting cells (APCs) functions, which are capable to decrease proliferation of dendritic cells (DCs) and regulate macrophage activity by polarizing proinflammatory phenotype (M1) to anti-inflammatory phenotype (M2) [14, 15]. Therefore, the decrease of B cells proliferation and antibodies production and adjustment of T cells activities as well as inhibit the proliferation of cytotoxic T cells and stimulate Treg activity [16].

MSCs therapy has gained space due to its vast therapeutic potentials such as immunomodulation mechanisms and main safety as bioproduct. Thus, this chapter will discuss the challenges of allogeneic MSCs as an alternative for an efficient therapeutic in AA immune-mediated treatment.
