If innate immune cells (dendritic cells) decide that the material is dangerous (part of a virus or bacteria), then they stimulate a specialized group of white blood cells causes CD4 + helper T cells to become activated. CD4 + refers to a surface protein on this class of T cells. Helper T cells can stimulate another group of white blood cells called B cells to produce antibodies that bind that specific antigen and immobilize it, preventing it from causing infection. Antibodies are specific for only one antigen.