輸血是通過靜脈將血液或血液制品轉移到血液循環中的過程。輸血用于各種醫療條件，以替換血液中丟失的成分。早期輸血使用全血，但現代醫學實踐通常只使用血液成分，如紅細胞、白細胞、血漿、凝血因子和血小板。紅細胞含有血紅蛋白，通過提高體內的氧含量來增加鐵的含量。輸血時不常用白細胞，但白細胞是免疫系統的一部分，可以抵抗感染。血漿是血液的液態部分，起到緩沖作用，含有蛋白質和身體整體健康所需的重要物質。血小板參與血液凝結，防止身體出血。在這些成分被發現之前，醫生認為血液是同質的。因此，許多患者死于血液不相容的轉移。

Blood transfusion is the process of transferring blood or blood products into one's circulation intravenously. Transfusions are used for various medical conditions to replace lost components of the blood. Early transfusions used whole blood, but modern medical practice commonly uses only components of the blood, such as red blood cells, white blood cells, plasma, clotting factors, and platelets.Red blood cells contain hemoglobin, and increase iron levels by improving the amount of oxygen found in the body. White blood cells are not commonly used during transfusion, but are part of the immune system, and fight infections. Plasma is the liquid part of the blood, which acts as a buffer, and contains proteins and important substances needed for the body’s overall health. Platelets are involved in blood clotting, preventing the body from bleeding. Before these components were known, doctors believed that blood was homogenous. Because of this, many patients died due to the transfer of incompatible blood.