Description:
<jats:p><jats:bold>Summary: </jats:bold> The outcome of positive selection of T lymphocytes is that there is a close match between the lineage adopted by a particular cell (CD4<jats:sup>+</jats:sup> or CD8<jats:sup>+</jats:sup>) and the specificity of the T‐cell receptor for the class of Major Histocompatibility Complex molecule recognized. How this match is obtained has been a matter of debate. We review the evidence, from recent and older experiments, that indicates that the process follows a selective logic, rather than an instructive one.</jats:p>