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Allogeneic directly translates to the word homologous. Both are adjectives that mean refer to tissue, bone marrow, or blood, in any form of transplant or transfusion, that is from same species. These terms are contrasted to autologous, which means using one of these tissues from the same person to the same person. A person getting his or her own tissue retransplanted or replaced is having an autologous procedure. When an allogeneic transplant is performed, the donated tissue comes from the same species, as opposed to any other form of animal species (xenograft) or from themselves (autologue).
In most cases, transplant of tissues, blood, or other types of cells are allogeneic. This still means that a certain degree of matching must take place before inserting into one body what lived in another. Even a blood donation must be checked to confirm that it will not conflict with the receiver's blood type. Rejection of an autologous blood transplant can occur if the blood is not fully matched. This doesn’t always mean blood types must match exactly. O positive blood might be able to receive most other types of blood, but O negative blood usually can only receive O negative.
More precise matching may be needed for things like allogeneic bone marrow transplant or organ transplants. This is why doctors will frequently search among family members for an appropriate match for bone marrow or organs that people can donate all or portions of, such as kidneys or livers. The closer the match, the less likelihood of rejection. From time to time suitable matches are made outside close family, and for certain organ transplants matches are almost always made outside of the family, such as heart transplants.
What should be understood is that whether a transplant or transfusion originates in a person’s family or comes from without it, as long as it comes from another human it is allogeneic. If it comes specifically from the person needing the transplant/transfusion, it is described differently. Take, for example, a person who cannot have blood transplants. In this instance a surgeon might decide to collect blood lost during a surgery and retransfuse it into the person’s body. This would be autologous.
Similarly, collection of bone marrow at birth, usually through cord collection, might give an option to people who later develop cancer to use their own marrow. It would be a perfect match. This would not be allogeneic but would be autologous. Collecting eggs for later fertilization might also be considered autologous, though once fertilized, they would be allogeneic too, containing the genetic material of an additional person.
Simply put, homologous or allogeneic mean from the same species. The donor may or may not be a close relative. Close matching of genetics is still usually very important but this doesn’t imply relation. Two things a homologue cannot be are the self or a member of another species.