CD28 and tetanus: Compared with healthy controls (HC), both siblings’ T cells demonstrated decreased thymidine uptake in response to the T cell mitogen phytohemagglutinin (PHA) (P1: 40% of control; P2: 34% of control), TCR/CD28 costimulation (anti-CD3/CD28; P1: 52% of control; P2: 54% of control), and no response to tetanus and candida antigens (Table 1).