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Anemia (Am.) or anaemia (Brit.) is the most common disorder of the blood. It occurs when the blood cannot transport sufficent amounts of oxygen, because the number of red blood cells is too low or something is wrong with the hemoglobin (the oxygen-carrying protein in the red blood cells). The tissues of an anemic person become, in a sense, "starved" of oxygen. Deprived of oxygen, the tissues cannot produce enough energy to function properly.

There are several kinds of anemia, produced by a variety of underlying causes. The initial classification of anemia is made on the basis of the appearance of the red blood cells on microscopic examination of a peripheral blood smear. If the cells are smaller than normal, the anemia is said to be microcytic; if they are normal size, normocytic; and if they are larger than normal, the anemia is classified as macrocytic. Other characteristics visible on the peripheral smear may provide valuable clues about a more specific diagnosis.

The most common causes of macrocytic anemia are a deficiency of vitamin B12, due either to inadequate intake or insufficient absorption, or a deficiency of folic acid. Alcoholism can cause macrocytic anemia. Lastly, sideroblastic anemia is caused by abnormal production of red blood cells and can lead to hematologic malignancies.

Normocytic anemia can be caused by acute blood loss or chronic disease. Renal failure or liver failure causes normocytic anemia. Certain hormonal deficiencies, like testosterone, can cause normocytic anemia.

The most common and most severe type of microcytic anemia, and the most common cause of anemia overall, is iron deficiency anemia. Just as the name implies, this form of anemia is caused when the dietary intake or absorption of iron is insufficient. (Haemoglobin contains iron.) In the United States, 20% of all women of childbearing age have iron deficiency anemia, compared with only 2% of adult men. The principal cause of iron deficiency anemia in premenopausal women is blood lost during menses.

Iron deficiency anemia is the final stage of iron deficiency. When the body has sufficient iron to meet its needs (functional iron), the remainder is stored for later use in the bone marrow, liver, and spleen. Iron deficiency ranges from iron depletion, which yields little physiological damage, to iron deficiency anemia, which can affect the function of numerous organ systems. Iron depletion causes the amount of stored iron to be reduced, but has no affect on the functional iron. However, a person with no stored iron has no reserves to use if the body requires more iron. In essence, the amount of iron absorbed by the body is not adequate for growth and development or to replace the amount lost.

Studies have shown that iron deficiency without anemia causes poor school performance and lower IQ in teenage girls.

The anemias above are dietary deficiency anemias. Anemia can also be caused by the inability of the bone marrow to produce blood cells. This is called aplastic anemia. In some cases, the cause of the aplastic bone marrow is an autoimmune disorder - the white blood cells attack the bone marrow and prevent it from producing new blood cells. Suppressing the immune system (or in mild cases just perturbing it a bit) can allow the bone marrow to become functional again.

A third kind of anemia is the result of a genetic defect: an example of this is sickle-cell anemia. Other genetic anemias include spherocytosis, andhemoglobin C.

Aplastic anemias are much rarer than dietary deficiency or genetic defect anemias.