Antibody responses to bacteriophage ◊X174. Bacteriophage ◊X174 was injected twice (↓), 6 weeks apart and antibody titers determined in 12 normal male controls (•, geometric mean; the hatched area, ± 2 SD), and in nine XHIM patients. Neutralizing antibody is expressed as rate of phage inactivation or K value (Kv) on a log scale. The mean percentage of phage-specific IgG antibody in serum collected 2 weeks after secondary immunization is shown on the right. Antibody responses to bacteriophage ◊X174 were quantitatively and qualitatively abnormal in all XHIM patients studied. They characteristically failed to amplify and to switch from IgM to IgG. The three patients with a mild phenotype, PS, DJ, and TA (open symbols), had the highest titers, and one (DJ) was able to produce phage-specific IgG antibody, although at a low concentration (10%).