CD4 and influenza: Previous studies have demonstrated associations between the expression of the costimulatory receptor CD28 on CD8+ T cells (CD8+T cells with CD28- (null) expression/% of CD8+ T cells), but not on CD4+ T cells, with the defective humoral immune response (HAI titers) after influenza vaccination, Th1/Th2 cytokine disbalance and the development of immune deficiency in the elderly [6,7,8].