Pernicious anemia occurs when the body loses the ability to absorb sufficient quantities of a micro-nutrient, vitamin B-12. The cause is presently believed to be autoimmune attack on the parietal cells of the stomach, which destroys them. These cells secrete intrinsic factor, a chemical that binds to B-12 in the stomach. The intestine cannot absorb B-12 directly, but can absorb the B-12/IF complex. (This is also why patients who have had Roux-en-Y gastric bypass must take B-12 supplements for the remainder of their lives - the surgery sacrifices the part of the stomach that hosts these cells.) The body requires B-12 for nerve and brain health, and to help erythrocytes (red blood cells) mature properly; this anemia is characterized by excessive circulating immature erythrocytes, among other problems. Treatment involves regular injections of B-12, or more recently, oral supplements. In some cases, the autoimmune attack is against the intrinsic factor itself, making replacement of this factor with an oral supplement an unreliable approach.