The recipients of hematopoietic stem cell transplants are characterized by an immunodeficiency of varying severity and duration. Their immunoincompetence is due in part to: 1) a lack of sustained transfer of donor immunity, 2) a recapitulation of lymphoid ontogeny, 3) the effects of graft‐versus‐host disease and its therapy, and 4) a reduction in thymic function. Recipients can have delays in the production of naive T lymphocytes following transplantation which result in defects in the production of new antigen‐specific T lymphocytes and an inability to produce antibodies, especially to carbohydrate antigens.