The present disclosure provides methods useful for prolonging the survival of an allograft organ in a recipient mammal. The methods include administration of an anti-CD200 antibody or a CD200-binding fragment of the antibody. The disclosure also provides biomarkers, a change in one or more of which indicates that an anti-CD200 antibody has produced a desired immunomodulatory effect in a mammal. Also featured are pharmaceutical compositions, kits, and solutions that contain at least one anti-CD200 antibody and are useful in the methods described herein.