F2 and tetanus: A role of memory B cells in absence of circulating Abs was also suggested in human studies: children that received a diphtheria, tetanus wP prime and a booster vaccination with diphtheria, tetanus aP vaccine at the age of 4 had stable levels of anti-PT, FHA, and PRN memory B cells and low/no correlation was determined with anti-PT, FHA, and PRN IgG Abs in the serum 5 years later (19).