Form of agammaglobulinemia that differs from the less severe Bruton type by a complete absence of lymphocytes that results in a greater vulnerability to infections and a consequently earlier age at death. Intravenous Ig administration is not beneficial; hence the designation severe. Much of the phenotypic constellation (e.g. absence of thymus tissue) of XSCID is due to infection-induced tissuedegeneration prior to the time of presentation. The affected gene is the interleukin-2 gammareceptor chain (IL2RG); about 150 different mutations have been identified. About 50% of all SCIDcases in the US are XSCID.