Courtesy of Mark Jenkins
 SLICED MICE: Single-cell-thick sections of adult mice expressing CD45.1 stained with nuclear dye (blue) and a monoclonal antibody specific for CD45.2 (red). The left panel shows a background level of CD45.2 staining. The center panel shows a mouse that received several million Salmonella peptide-specific CD4 T cells from a transgenic mouse expressing CD45.2. Transferred naïve T cells were found only in secondary lymphoid organs. At right, when also injected with Salmonella peptide three days prior to staining, CD4 T cells proliferated and migrated into nonlymphoid organs.

Scientists have long believed that immunological memory--the record of infections past--is maintained by highly differentiated T cells and B cells that were generally thought to reside in the spleen and lymph nodes, the secondary lymphoid organs. Thus, the lymph nodes and blood (considered a surrogate for the spleen) are typically used to probe the long-term effects of immunological challenge. But...

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