Protein expression analysis: Mouse CD200 protein was detected in wild-type C57BL/6 (+/+) mice, while human CD200 protein was detected in homozygous B-hCD200/hCD200R (H/H) mice. Mouse CD200R protein was detected in wild-type C57BL/6 (+/+) mice, while human CD200R protein was detected in homozygous B-hCD200/hCD200R (H/H) mice. Immune cell analysis: CD200 and CD200R humanization did not change the overall development, differentiation, or distribution of immune cell types in the spleen and lymph node.