Injecting plasma (cell-free fraction of blood) from old mice into young mice's tails produces many of the debilitating effects of an out-and-out circulatory hook-up to an old mice [http://stanmed.stanford.edu/2011fall/article9.html].
Bilateral interactions during Heterochronic Parabiosis has too strong effect on immune system of young partner.
Just using young plasma, which represents young and healthy systemic environment, in one-sided influence may restore age-related changes in old-mice and have beneficial effects.
The age-related changes in old mice cannot be corrected just with injections of young blood plasma.
Organism of old mice may make some sort of reactions on the administration of the young plasma, but probably they may not represent rejuvenation.