CD19 (Cluster of differentiation 19), also known as B-lymphocyte antigen CD19, is a 95 kDa transmembrane glycoprotein on the surface of the immunoglobulin superfamily (IgSF) and classified as a type I one-pass transmembrane protein. It maps to the short arm of chromosome 16p11.2 encoded by the 7.41 kilobite CD19 gene that produces a 556-amino acid protein with two extracellular C-type IgSF domains together with a 242-residue cytoplasmic tail that exhibits highly conservation between mouse and human. The CD19 molecules are expressed from the earliest stage of B cells development until plasma cells terminal differentiation. It can associate with the B cell antigen receptor (BCR), CD81 (TAPA-1), CD38, CD21, CD22 and CD225 to establish intrinsic B cell signaling and reduce the thresholds. FcγRI, synonymously also referred to CD64, is a membrane spanning protein of 72 kDa and belongs to the immunoglobulin (Ig) supergene family. It has three extracellular Ig-like domains and is the only receptor that can efficiently bind monomeric IgG and aggregated IgG17. Hence, FcγRI is known to functions as a high-affinity Fcγ receptor where exists other two members FcγRII (CD32) and FcγRIII (CD16), but with low affinity. In humans, FcγRI is constitutively expressed on monocytes/macrophages, myeloid precursors, as well as follicular dendritic cells and its expression can also be induced on polymorphonuclear neutrophils via IFN-γ or G-CSF. Now, FcγRI is considered a great monocyte/macrophage-associated marker, normally used in flow cytometry (FCM) to identify leukemias with myelomonocytic differentiation.