Ramit Mehr The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel Elucidating lymphocyte development using BrdU labeling and mathematical models Wednesday 15th October 14:05-14:55pm, Eastern Avenue Lecture Theatre. The immune response involves cells of various types, including B, T and NK lymphocytes expressing a large diversity of receptors which recognize foreign antigens and self-molecules. The various cell types interact through a complicated network of communication, regulation and control mechanisms. This is what enables the immune system to perform the functions of danger recognition, decision, action, memory and learning. As a result, the dynamics of immune cell repertoires, in particular their development, are highly complex and non-linear. Understanding the population dynamics which underlie lymphocyte development is essential for elucidating the causes of various immune dysfunctions and cancers. We have addressed this issue by combining mathematical modeling of cell population dynamics with experimental data, including BrdU labeling. These studies revealed unexpected feedback mechanisms in T cell development (Immunol. Today 1997, 18:581-585), phenotypic reflux in B cell development (Mehr et al, Int'l Immunol. 2003, 15:301-312; Gorfine et al, Bull. Math. Biol. 2003, 65:1131-1139), selection checkpoints in transitional B cells (Shahaf et al, Int'l Immunol. 2004, 16:1081-1090 and work in progress) and the reasons for decreased production of T cells (Mehr et al, Mech. Age. Develop., 1993, 67:159-172; AGING: Immunology and Infectious Disease, 1996, 6:133-140) and B cells (Shahaf et al, Int'l Immunol. 2006, 18:31-39) in aging. We found that in aging animals, the carrying capacity of the Pre-B compartment, and the probability of cell cycle reentry in this compartment, are reduced, while the numbers of static B cells in the immature compartment increase.