CD4 and lymphopenia: Within the adaptive immune compartment, global T and B cell lymphopenia predominated in critically ill patients, with the remaining T cell pool enriched with PD-1+ CD4+ and CD8+ T cell clusters (CD4-2, CD4-3 and CD8-6) and regulatory T (Treg) cell (CD4+CD25+FoxP3+, CD4-4) clusters (Extended Data Fig. 6a,b and Supplementary Table 8).