(Also called T-helper cell.) Antibody-triggered immune cells that seek and attack invading microorganisms. Macrophages summon T4 cells to the infection site. There, the T4 cell reproduces and secretes potent chemicals that stimulate B cells to produce antibodies, signal natural killer or cytotoxic (cell-killing) T cells, and summon other macrophages to the infection site. In healthy immune systems, T4 cells are twice as common as T8 cells.