An infant with transcobalamin II deficiency had the unusual feature of a low serum cobalamin level. Moreover, the level did not rise greatly with cobalamin therapy. Clinically, the features of the infant's illness were unexceptional other than the somewhat delayed onset of symptoms at 3? months of age. In addition, the patient's impaired antibody response to specific antigen challenge was not completely corrected following cobalamin therapy, and this result contrasts with the findings in the only previous examination of this feature of the disorder. As in previous cases, the transcobalamin II deficiency was accompanied by findings in the serum of complexing of the patient's R binder and the relative prominence of a cobalamin-binding peak eluting at a molecular weight of approximately 70,000 on Sephadex G-200 gel chromatography. In addition, the total serum R binder (free and complexed) level was low. This case demonstrates that transcobalamin II deficiency should be considered in any infant with a low serum cobalamin level in the first few months of life.
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