Fig. 1.
The Classical Monolithic View of Helper T Cell Differentiation: Lineages and Master Regulators. Initial studies arising from in vitro cultured Thelper1 (Th1) and Th2 cells led to the idea that these subsets behaved like lineages, meaning their phenotype (i.e. selective cytokine production) was inflexible. Accordingly, these subsets expressed lineage-defining transcription factors that were sufficient to impart this selective cytokine production. As newer subsets of cytokine producing cells were identified, they too were viewed as stable lineages.