骨髓（英语：bone marrow）位于较大骨骼的腔中，占人体体重的4-6%，含有造血干细胞以及多种其他的干细胞，他们可以分化产生不同的组织。骨髓是重要的造血及免疫器官。血液的所有细胞成分都来源于造血干细胞，其中髓系细胞（红细胞系、粒细胞系、单核细胞系与巨核细胞-血小板系）是完全在骨髓内分化生成的；淋巴系细胞（T细胞与B细胞）的发育前期是在骨髓内完成；另外B细胞分化为浆细胞后，也回到骨髓，并在这里大量产生抗体。通常人体在稳定状况下，每小时约有10个红细胞与10-10个白细胞生成，以维持外周血循环中血细胞的组成与数量。

Bone marrow is the flexible tissue in the interior of bones. In humans, red blood cells are produced by cores of bone marrow in the heads of long bones in a process known as hematopoiesis. On average, bone marrow constitutes 4% of the total body mass of humans; in an adult having 65 kilograms of mass (143 lbs), bone marrow typically accounts for approximately 2.6 kilograms (5.7 lb). The hematopoietic component of bone marrow produces approximately 500 billion blood cells per day, which use the bone marrow vasculature as a conduit to the body's systemic circulation. Bone marrow is also a key component of the lymphatic system, producing the lymphocytes that support the body's immune system.