The Anti‐Rho(D) Responses of Immunized Volunteers following Spaced Antigenic Stimuli

Summary. Twenty‐six Rho(D) negative male volunteers previously immunized with Rho(D) positive red cells have received spaced antigenic stimuli in an attempt to produce high serum anti‐D activity during the secondary responses. Although there is a considerable individual variation in magnitude, there is, in general, either a rapid or slow fall from the maximum anti‐D concentration achieved to some steady state of anti‐D concentration which persists with little change for several months. A case is made for the acceptance of an anti‐D concentration of 20 International Units (i.u.) per cm3 (about 7μ g/cm3) as the minimum requirement for accepting plasma donations for immunoglobulin preparations. On this basis 19 of the 26 volunteers could contribute regularly, although in only 10 volunteers was a concentration of 100 i.u./cm3 or more maintained for periods of several months following an antigenic stimulus. No correlation was observed between the maximum concentration achieved in the primary response and that at the steady state following further antigenic stimulation. The potential of a volunteer cannot, therefore, be predicted from the early immune response. Indeed with certain volunteers after regular spaced antigenic stimuli, markedly reduced antibody responses have been observed. The implications of these findings are discussed with respect to the collection of suitable donations of anti‐D plasma.