You're right, a body is less likely to reject an allograft (transplanted tissue or organ) if it comes from a relative, especially a close relative. This is due to the concept of histocompatibility, meaning the similarity of the tissue types between the donor and recipient.

Here's why:

* Genetic Similarity: Relatives share a portion of their genes, making their tissue types more similar. The more closely related individuals are, the more similar their Human Leukocyte Antigen (HLA) genes are. HLA genes are responsible for determining the body's immune system recognition of self versus non-self.

* Lower Chance of Immune Rejection: A lower degree of HLA mismatch between donor and recipient means the recipient's immune system is less likely to recognize the allograft as foreign and mount an attack against it.

Important Points:

* Not Guaranteed Success: Even with close relatives, rejection can still occur. The closer the relationship, the lower the chance of rejection, but it's never a certainty.

* Other Factors: The age, health, and general immune status of both donor and recipient can also influence the outcome of an allograft.

* Matching: Even with relatives, careful HLA typing is still necessary to identify the best match for the recipient.

In Summary: The genetic similarity between relatives makes them better candidates for allograft donation, leading to a lower chance of rejection. However, other factors play a role, and thorough testing is always necessary to ensure the best possible outcome for the recipient.