Potential mechanisms of aging of the immune system were studied in experimental heterochronic parabiosis of CBA mice. 
Vascular anastomoses were formed between parabionts thus providing an exchange of the humoral and cell factors and repopulation of lymphoid organs by the partner cells.
Investigation into the primary immune response to sheep red blood cells has shown the suppression of antibody production in young partners to be the result of the old body influence.
The effect of stimulation of the old immune system by the young animal, however, was only slightly pronounced.
It is suggested that the factors actively suppressing the immune response of the young animal may exist in the old body.
Another group  of experiments with CBA mice aged 4-5 and 24-27 months under 2-months heterochronic parabiosis to determine the primary immune response to administration of sheep red blood cells have shown that one and three months after parabionts separation the magnitude of the immune response did not significantly differ from that of non-separated parabionts and approximated the immune response in old mice.
Splenectomy in old partners did not remove the inhibitory effect on young parabionts.
X-Ray irradiation of old parabionts 3-5 days after coupling with young ones caused an appreciable recovery of immune function both in old and young partners.