Cytotoxic T lymphocytes and cross-priming

Professional antigen-presenting cells (APCs) can capture and present exogen ous antigen on major histocompatibility class (MHC) I molecules to cytotoxi c T cells (CTLs), by the poorly understood process of cross-presentation. H ere,William R. Heath and colleagues show that APCs scan the periphery and p resent class I-restricted tissue derived antigens to naive CTLs in the lymp h nodes. They outline a working model of the process and outline outstandin g questions.