CD19 and cancer: Immunohistochemistry staining for CD19 and CD68 (a marker of macrophages and mononuclear phagocytes23) also revealed a small number of CD19+ cells within CD68+ myeloid cells and clusters of CD19+ B cells in the primary tumors of WT mice with 4T1.2 and MC38 cancers (Fig. 1d and Supplementary Fig. 1g, h).