The immune response to thetaX 174 in man. I. Primary and secondary antibody production in normal adults.
The bacteriophage [unk]X 174 has been administered intravenously in doses within the range 108–1010 PFU to twenty-six normal human adult volunteers. Antibody was measured as a 50% bacteriophage neutralization titre (SD50). Pre-immunization antibody was found in only one subject. All subjects showed a primary (IgM) response. Peak titres of antibody of 18–8000 occurred between days 6 and 9 in a group of early responders, and between days 14 and 24 in a late group. The secondary response was tested in twenty subjects and peak titres of 1400–50,000 were found in a single group, between days 6 and 13. Thirteen out of fourteen subjects receiving a secondary dose after 6 weeks showed a peak antibody titre between 11,000 and 50,000. 
 
The uniformity of the secondary response indicates that it will be a valuable clinical test of antibody-producing capacity.