Sequence of donor platelet transfusions. Each recipient received up to 8 weekly transfusions from a donor, or until the recipient became platelet refractory. Refractoriness was defined as less than 5% of the donor's platelets circulating in a transfused recipient at 20 hours after transfusion after 2 sequential transfusions. As soon as a recipient developed refractoriness to a donor, or at the end of 8 weeks, platelet transfusions from the next donor were given. Different types of donor platelet transfusions were given in order, moving from left to right in the figure. For the tolerance evaluation studies, some recipients received only C-LR platelets from their initial and third-party donors, some recipients received only STD platelets from these donors, and some recipients received both C-LR and STD platelets from these donors. Third-party donors were donors that the recipient had not received before.