Lysosomal storage diseases are a class of inborn errors of metabolism that lead to widespread disease in multiple tissues. The murine model of mucopolysaccharidosis type VII (MPS VII) closely parallels the human syndrome and has been extensively used to investigate the natural history and therapeutic strategies for lysosomal storage diseases in general. Here we demonstrate a previously undescribed immune defect in the MPS VII mouse. Although the normal populations of cells are present in lymph nodes of these mice, MPS VII mice show a blunted T cell proliferative response and decreased antibody production after immunization with antigens. One mechanism of this defect is ineffective processing of protein antigens, as responses to peptide antigens are normal. This phenotype is presumably caused by the lysosomal disorder, as the defect can be corrected in vivo by direct enzyme replacement therapy. These findings have implications for the use of this animal model, and may have clinical significance for other, more-common lysosomal storage diseases.