Abstract
Conventional dendritic cells (cDCs) can integrate multiple stimuli from the environment and provide three separate outputs in terms of antigen presentation, costimulation, and cytokine production; this guides the activation, expansion, and differentiation of distinct functional T helper subsets. Accordingly, the current dogma posits that T helper cell specification requires these three signals in sequence. Data show that T helper 2 (Th2) cell differentiation requires antigen presentation and costimulation from cDCs but does not require polarizing cytokines. In this opinion article, we propose that the 'third signal' driving Th2 cell responses is, in fact, the absence of polarizing cytokines; indeed, the secretion of the latter is actively suppressed in cDCs, concomitant with acquired pro-Th2 functions.