Immunodeficiency-61 (IMD61) is an X-linked recessive primary immunodeficiency characterized by onset of recurrent infections in early childhood due to impaired antibody production. Affected individuals have normal numbers of circulating B and T cells, but B cells have an intrinsic defect in antibody production (summary by Keller et al., 2018). For a general phenotypic description of X-linked agammaglobulinemia, see 300755.