Differentiation of CD4⁺ T-cell subsets (in mice). Upon activation, naïve CD4⁺ T cells can differentiate into different subsets depending on the surrounding cytokine milieu. The different subpopulations show distinct expression patterns of transcription factors and can be characterized by secretion of signature cytokines that are unique for each subset. Each subset takes part in different kinds of immune responses against various pathogens or in mediating autoimmunity.