Immune responses are largely memorized by terminally differentiated memory T cells, but the precise mechanism by which a relatively small number of cells can memorize a vast number of antigens has been unclear. In the Early Edition of the Proceedings of the National Academy of Sciences, Mojgan Ahmadzadeh and Donna Farber from University of Maryland School of Medicine, Baltimore, show that the antigen-specific memory CD4 T cell population has substantial functional plasticity and can provide responses against antigens met in different immunological contexts (Proc Natl Acad Sci USA 2002, DOI:10.1073.192263099).

Ahmadzadeh & Farber used influenza hemagglutinin (HA)-specific memory CD4 T cells recovered from adoptive hosts that received in vitro-activated HA-specific T cell receptor-transgenic CD4 T cells 2-12 months previously. They observed that this HA-specific memory T cell population produced predominantly T helper 1 or T helper 2 effector cytokines depending on the nature of the recall...

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