Anemia is a condition in which the concentration of hemoglobin in blood is below the level appropriate for a person’s age, sex, and physiological status. Hemoglobin is the iron-containing protein in red blood cells that transports oxygen from the lungs to tissues. Its reduction decreases the blood’s oxygen-carrying capacity. Anemia is not a single disease: it is a manifestation of underlying nutritional, inherited, inflammatory, or other conditions, with severity ranging from mild to life-threatening. (who.int)
Biological basis
Red blood cells develop in bone marrow through hematopoiesis, the production of blood cells. They normally circulate for up to approximately 120 days before being removed, mainly by the spleen. Anemia develops when production cannot keep pace with blood loss or red-cell destruction. Several mechanisms can operate simultaneously, particularly in chronic illness. (nhlbi.nih.gov)
The kidneys contribute to this balance by producing erythropoietin, a hormone that stimulates red-cell production. In chronic kidney disease, reduced erythropoietin production can cause anemia. Shortened red-cell survival, nutrient deficiencies, inflammation, and blood loss may contribute as well. Thus, kidney-associated anemia frequently has more than one cause. (niddk.nih.gov)
Causes and major forms
Blood loss may occur suddenly, as after injury or surgery, or gradually through heavy menstrual bleeding or gastrointestinal bleeding. Persistent losses can deplete iron reserves and lead to iron-deficiency anemia. Inadequate dietary intake and impaired intestinal absorption are other important causes of iron deficiency. Iron deficiency is the most common nutritional cause of anemia, but anemia and iron deficiency are not interchangeable diagnoses. (nhlbi.nih.gov)
Reduced or ineffective production occurs when the marrow lacks necessary nutrients or is affected by disease or treatment. Vitamin B12 and folate are required for healthy blood-cell production. B12 deficiency can arise from inadequate intake or impaired absorption, including a lack of intrinsic factor, a stomach-produced protein involved in absorption. Inflammation, cancer, and some autoimmune diseases can suppress red-cell production; chemotherapy may also contribute. (niddk.nih.gov)
Excessive destruction, termed hemolysis, produces hemolytic anemia when red cells are destroyed faster than they can be replaced. Causes include inherited blood disorders, immune-mediated destruction, infections, certain medicines, and transfusion complications. Sickle cell disease involves abnormal hemoglobin structure, whereas thalassemia involves impaired synthesis of hemoglobin chains. Both can cause anemia through mechanisms that differ from simple nutrient deficiency. (nhlbi.nih.gov)
Infection-related anemia varies geographically. Malaria and parasitic infections are important contributors in some settings, through red-cell destruction, nutrient loss, or other effects. An individual may therefore have nutritional deficiency together with infection or chronic disease. (who.int)
Clinical features
Common manifestations include fatigue, weakness, dizziness, headache, and shortness of breath, particularly during exertion. Pallor and a rapid heart rate may occur, especially in more severe anemia. These findings are nonspecific: symptoms alone cannot establish either anemia or its cause. Slowly developing anemia, including anemia associated with early kidney disease, may initially produce few symptoms. (who.int)
Some features reflect the underlying disorder rather than reduced oxygen transport alone. Vitamin B12 deficiency can produce tingling, walking difficulties, and cognitive changes because the vitamin is also necessary for nervous-system function. Serious anemia can contribute to cardiac complications. During pregnancy, anemia is associated with adverse maternal and birth outcomes; severe anemia in children can affect cognitive and motor development. (nhlbi.nih.gov)
Diagnosis and classification
Diagnosis relies on blood measurements interpreted alongside medical history and examination. The complete blood count measures hemoglobin and other blood components. Hematocrit expresses the proportion of blood volume occupied by red cells, while mean corpuscular volume describes their average size. Cell size helps characterize the anemia, but does not independently establish its cause. Diagnostic thresholds vary across population groups, and pregnancy and elevation of residence affect interpretation. (nhlbi.nih.gov)
Further investigations are selected according to the suspected mechanism. They may include measurements of ferritin, an iron-storage protein, and transferrin, an iron-transport protein; vitamin B12 and folate testing; and counts of immature red cells called reticulocytes. Evaluation may also investigate kidney function, gastrointestinal bleeding, or inherited abnormalities. Bone-marrow aspiration and biopsy can assess blood-cell production when marrow disease is suspected. Establishing the cause is distinct from confirming a low hemoglobin concentration. (niddk.nih.gov)
Treatment and public-health significance
Treatment depends on the cause and severity. Established approaches include replacing deficient iron or vitamins, addressing bleeding or underlying disease, and using medicines that stimulate red-cell production in selected circumstances. Blood transfusion can rapidly increase circulating red cells in serious anemia. Hematopoietic stem-cell transplantation is used for certain blood-forming disorders. Iron is not a universal treatment: excessive iron can damage organs, and repeated transfusions can produce iron overload. (nhlbi.nih.gov)
Anemia is a major public-health concern, particularly among young children and menstruating, pregnant, and postpartum women. In its February 2025 fact sheet, the World Health Organization reported estimates of anemia affecting approximately 40% of children aged 6–59 months, 37% of pregnant women, and 30% of women aged 15–49 years globally. Population-level measures address nutrition, infections, reproductive blood loss, and access to diagnosis and treatment; their relevance depends on the local pattern of causes. (who.int)