CD19 and CD20 are common humanCD markers used to differentiate B cells by flow cytometry. B cells are a vital and complex cell type. They play key roles in autoimmunity, humoral immunity against infections, and transplantation. CD19 regulates intracellular B cell signaling by amplifying the activity of Src-family kinase. CD20 functions as a membrane embedded Ca2+ channel. CD45R/B220, CD19, CD22 are common mouse CD markers used to differentiate B cells by flow cytometry.