someone who is related to you by birth rather than through marriage or another relationship such as adoption (= becoming the legal parent of a child who is not your own)

I'd always known Sam was not a blood relative, but still regarded him as a brother. 我一直就知道山姆不是我的血亲，但还是把他当作兄弟。

He tested 23 of his patients and 87 of their blood relatives for antibodies. 他测试了自己的二十三个病人和他们的八十七位血亲以寻找抗体。