CD4 and hereditary spastic paraplegia: In summary, the results had shown that the frequencies of circulating CXCR5+CD4+TFH cells and ICOS+CXCR5+CD4+TFH cells, expressions of TFH cell associated molecules such as IL-21, IL-6, and Bcl-6, and serum IgA and C3 concentrations were significantly increased in the children during the acute phage of HSP compared to HCs, which implied that expanding the frequencies of circulating CXCR5+CD4+TFH cells and ICOS+CXCR5+CD4+TFH cells with increased IL-21 and IL-6 production might contribute to the production of IgA.