Making blood cells Before birth, foetuses produce red and white blood cells in their spleens. Shortly before birth the spleen loses its ability to make red blood cells and bone marrow takes over this job.hen blood flows through your spleen, white blood cells attack and remove any foreign invaders. This keeps your blood clean and helps protect you against infection. Destroying old red-blood cells Red blood cells have a lifespan of around 120 days, after which your spleen breaks them down.