The cells that carry oxygen through the blood are the erythrocytes or red blood cells. Typically, red blood cells form in the bone marrow. However, up to the fifth month in the womb, a child’s spleen is also creating red blood cells. This process, hematopoiesis, ceases in the spleen after birth. However, the spleen retains its ability to create lymphocytes, making it an integral part of the immune system. In some rare cases, a disorder will enable the spleen to continue producing the red blood cells after birth.

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