Abstract: CD20 is a membrane-spanning hosphoprotein strongly expressed on the cell surface of B lineage cells and is widely regarded as a B cell specific marker. However, CD20 (Figure 1) is also expressed at a low level on a small subset of CD3+ T cells which therefore are sometimes referred to as CD20dimCD3+ T cells in contrast to CD20brightCD19+ B cells which represent the majority of cells expressing CD20 (Hultin et al., 1993). The amount of the CD20 antigen has been assessed to be 25 to 50 times higher on CD20+CD19+ B cells compared to CD20+CD3+ T cells (Hultin et al., 1993). At first description of this cell population the frequency of these CD20+ T cells has been described to represent an average of 2.4% of all peripheral blood lymphocytes (50 healthy controls) (Hultin et al., 1993). In the largest characterized cohort with 142 healthy individuals CD20+CD3+ T cells constituted a mean proportion of 1.6% (range from 0.1–6.8%) of all circulating CD3+ T cells and in absolute numbers accounted for approximately 28 cells/μL (Wilk et al., 2009).