Immunodeficiency 46 is a rare genetic disorder characterized by disrupted transferrin receptor 1 endocytosis, leading to defective iron transport and impaired T and B cell function. Patients experience early-onset chronic diarrhea, severe recurrent infections, and failure to thrive. Laboratory findings include hypo- or agammaglobulinemia, normal lymphocyte counts, decreased memory B cells, intermittent neutropenia and thrombocytopenia, and mild anemia that is resistant to iron supplementation. This autosomal recessive disorder is caused by mutations in the TFRC gene on chromosome 3q29.