Fig. 1

Timeline of key discoveries in CD4⁺ T-cell biology. This timeline highlights major milestones in the identification and characterization of CD4⁺ T-cell subsets. Mouse CD4⁺ T cells (Ly-1⁺, lacking Ly-2 and Ly-3) and human CD4⁺ T cells (OKT4⁺) were identified in 1975 and 1979, respectively. The Th1 and Th2 subsets were first distinguished in 1986, followed by the discovery of regulatory T (Treg) cells in 1995 and their transcriptional regulator Foxp3 in 2003. Follicular helper T (Tfh) cells were described in 2000, with BCL6 later identified as the defining transcription factor in 2009. Th17 cells were characterized in 2005, with RORγt identified as a key transcriptional regulator in 2006. Additional subsets, including Th9 and Th22 cells, were reported in 2008 and 2009, respectively. Created with BioRender.com