 Mature lymphocytes are a critical part of the immune system that (with the exception of memory B and T cells) have short lives measured in days or weeks and must be continuously generated throughout life by cell division and differentiation from cells such as common lymphoid progenitors (CLPs) in mice.Were this system to fail, the body would be largely undefended from infection.The set comprising CLP cells and similar progenitors are themselves descendants of the pluripotential hemopoietic stem cell (pHSC) which is capable of generating all of the cell types of the complete blood cell system.