Mannose-binding lectin (or mannan-binding lectin, MBL) may have an influence on susceptibility to infection in patients given chemotherapy to induce remission or as conditioning before stem cell transplantation. The most surprising finding reported from an inconsistent literature was the observation that mbl-2 gene mutations in donors could influence the risk of serious infections in recipients of allogeneic stem cell transplants. This could be explained if leukocytes in the stem cell preparations (or their derivatives) were able to synthesize and secrete MBL, but the available evidence seems to exclude that possibility. An alternative mechanism could involve MBL binding to autologous cells and inducing immunological maturation of those cells. MBL can certainly bind to various cell types via surface glycoconjugates and the possible significance of this for MBL replacement therapy will be discussed.