Allotransplantation

Overview
Allotransplantation (allo- from the Greek meaning "other") is the transplantation of cells, tissues, or organs, sourced from a genetically non-identical member of the same species as the recipient. . The transplant is called an allograft or allogeneic transplant or homograft. Most human tissue and organ transplants are allografts.

In contrast, a transplant from another species is called a xenograft. A transplanted organ or tissue from a genetically identical donor, i.e., an identical twin, is termed an isograft. Finally, when a tissue is transplanted from one site to another on the same patient, it is termed an autograft.

When a host mounts an immune response against an allograft or xenograft, the process is termed rejection. An allogenic bone marrow transplant can result in an immune attack against recipient, termed Graft-versus-host disease.

Examples
A variety of tissue and organs types can be used for allografts. These include:
 * skin transplants
 * corneal transplants
 * heart transplants
 * lung transplantation
 * liver transplants
 * kidney transplants
 * bone marrow transplants
 * bone allograft
 * ligament or tendon allograft

Pregnancy
The fetus inside a mother may be viewed as an unusually successful allograft, since it genetically differs from the mother. . In the same way, many cases of spontaneous abortion may be described in the same way as maternal transplant rejection.

For this purpose, the placenta functions as a barrier, and uses several mechanisms:
 * It secretes Neurokinin B containing phosphocholine molecules. This is the same mechanism used by parasitic nematodes to avoid detection by the immune system of their host.
 * Also, there is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2.

However, the placental barrier is not the sole means to evade the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of the placental barrier.

If the mechanisms of rejection-immunity of the fetus could be elucidated, it could avail for xenopregnancy, having, for example pigs carry human fetuses to term as an alternative to a human surrogate mother, providing a sober, drug-free and nonsmoking carrier.