Pioneering work was done with chemically linked immunotoxins by the group of Stirpe in Bologna and Flavell in Southampton, directing Saporin to human leukemia and lymphoma cells expressing differentiation antigens such as CD2, CD7, CD19 and CD22 on their plasma membrane surface [27,28,29,30,31,32,33,34,35,36]. Here, CD19 is linked to lymphoma.