Recent reports suggest a quorum of T cells is required to activate T lymphocytes and that this requirement may help explain why scarce lymphocytes, specific for peripheral self-antigen, are rarely activated by Ag. This proposal runs counter to the commonly held framework that the Ag-dependent, but CD4 T lymphocyte–independent, activation of CD8 T lymphocytes, and the activation of CD4 T lymphocytes themselves, can occur when a single CD8 or CD4 T lymphocyte encounters Ag under appropriately dangerous circumstances. We argue that a review of older literature often ignored, as well as of contemporary studies, supports the quorum concept and is difficult to reconcile with the Danger Model.
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