All of our blood cells are made inside of our bones. In our bone marrow is a single type of stem cell that can give rise to all of the blood cell types. Each time one of these stem cells divides, one cell remains a stem cell. The other matures and moves into the blood, sometimes dividing multiple times along the way.

As a newly born blood cell matures, different sets of genes are turned on and off. Active genes code for proteins that help the cell do its job. Each cell type has a different set of active genes related to its unique function. For example, red blood cells fill themselves with hemoglobin protein, which carries oxygen. B cells make antibody proteins that recognize invaders like bacteria and viruses. Megakaryocytes, and the platelets that break off of them, have proteins that help to form blood clots.

Every day, our bone marrow makes many millions of new blood cells. This process is tightly controlled to make sure that we always have the right number and proportion of blood cells.