When the body is deficient in vitamin B-12, immature cells cannot form enough new DNA to divide properly, so the cells keep their nuclei, and they're fewer and larger than normal. The resulting production of the abnormal cells decreases the blood's ability to carry oxygen. This condition is a type of anemia. Over time, this causes the folate-deficiency condition called megaloblastic anemia. A deficiency of vitamin B-12 also causes the formation of enlarged immature red blood cells in the bloodstream that cannot function as well as their normal counterparts. When the vitamin B-12 deficiency develops because of the lack of intrinsic factor in the stomach, it's called pernicious anemia.