Reader Cathy wrote in wondering what becomes of a donor’s DNA once it gets inside another person during a blood transfusion.
Most of the time, the answer is nothing, because the donor’s DNA never makes the trip to a new home. Modern transfusions typically use only certain components of blood (i.e. red and white blood cells, plasma or platelets), and most of the transfusions done these days use what's called packed red blood cells (erythrocytes). These are red blood cells that have been separated from “whole blood.” They transport oxygen and carbon dioxide to and from cells throughout the body and are typically used to restore oxygen carrying capacities to the blood of anemics. The cells don’t contain any DNA, though, because they lose their nuclei when they mature.
Some blood transfusions do involve the “donation” of DNA, though. Certain transfusions use white blood cells (leukocytes), which are involved in defending the body against infections, diseases and foreign materials—and do contain DNA. Others use whole blood, and every component is transfused. Typically, transfused nucleated cells reach the end of their lifespan in about three to four weeks and are then removed from circulation by the spleen, taking the donor’s DNA with them. In some of these cases involving large transfusions, though, the donor’s DNA has been detected in recipients for up to a year and a half.
For more on blood transfusions, check out Holly Tucker’s Blood Work, about the history and science of the earliest blood transfusions and the murder, politics and intrigue that surrounded them.