CD79a, also known as mb-1, is a 47kDa transmembrane glycoprotein that forms a heterodimer with CD79b to constitute the B cell receptor (BCR). This receptor is responsible for signal transduction in B cells, leading to activation, apoptosis, or allergy. CD79a expression is present from the earliest B cells to plasma cells. It is expressed in B cell tumors and some non-B cell malignancies such as AML. Unlike other antibodies that can be expressed in B cell tumor cells, HM47 is a reliable antibody for recognizing CD79a, making it a good marker for B cells. Due to the presence of several variants of CD79a, the exact location of the epitope may vary slightly.