CD4 and infection: It was reported that recipients after solid organ transplantation or allo-HSCT with significantly lower counts of CD3+, CD4+, and CD8+ T cell were prone to PvB19 infection, and about 99% of immunocompromised patients with PvB19 infection would develop PRCA due to reduction of antiviral T cells and lack of diversity of T cell receptor [6, 27].