The immune system protects the body from external invaders such as viruses and bacteria. Transplant rejection happens when the body’s immune system assumes the transplanted organ is a potentially harmful external invader and attacks it - in the same way that the immune system would attack a virus.
Usually, when you have a viral or bacterial infection the immune system responds to destroy the invader, and it is this immune response that can cause the symptoms associated with infection, such as a fever with influenza, or redness and swelling at the site of an infected wound.
The immune system mainly attacks invaders with two types of cells, known as B cells and T cells, which circulate in the blood.
B cells – When B cells recognise an external invader, they produce large proteins called antibodies. These then attach themselves specifically to that invader. This has the effect of marking the invader for destruction by other cells of the immune system. Antibody-mediated rejection happens when the cells of the transplanted organ are recognised as external invaders.
T cells – This video explains how T cells recognise the transplanted organ as ‘foreign’ or ‘non-self’ (as they would recognise an external invader) and launch an attack, destroying the cells of the transplanted organ. This is known as T cell-mediated rejection.
In the healthy body, the immune system produces millions of T cells, which constantly circulate through the bloodstream, and police for foreign invaders. When a virus, for example, infects cells in the body, T cells travel to the infected area. Next they multiply and send out chemical messages to attract more immune cells and trigger them to destroy the foreign invaders. When you receive a new organ through transplantation, your immune system will recognise it as foreign. T cells will travel to the new tissue, multiply, recruit more immune cells to the area and destroy your transplant.
Types of transplant rejection
Transplant rejection can be classified in three ways, depending on how soon after transplantation it occurs:
Acute rejection – ‘Acute’ means short-term, and refers to an illness with a rapid onset. Acute rejection generally occurs during the first few months after transplantation. Acute rejection can be either T cell mediated or antibody mediated.
Hyperacute rejection – Occurs within a few minutes after the transplant. This kind of rejection can happen when antibodies against the transplanted organ are already present in the body (a type of antibody-mediated rejection).
Chronic rejection – ‘Chronic’ means long-term and refers to an illness that develops slowly, over time. Chronic rejection happens over a span of months or years and normally happens more than a year after the transplant. Chronic rejection is often antibody mediated.
How does rejection affect the transplanted organ?
In cases of hyperacute rejection, large numbers of immune cells move into the transplanted organ, stick in the blood vessels and form blood clots, meaning the transplant organ must be removed immediately. This only occurs when the donor and recipient are not compatible.
In cases of acute rejection, large numbers of immune cells move into the transplanted organ meaning it becomes swollen and cannot function correctly. The body can react as if the immune system is fighting a large infection meaning you may experience a fever, but there can be no symptoms at all.
In cases of chronic rejection, damage is done gradually to the transplanted organ resulting in scarring and too much tissue being formed (known as fibrosis). The excess scar tissue can block important vessels and ultimately stop the organ from working.
However, damage to the transplanted organ may also be caused by other factors such as hypertension. For this reason, chronic rejection is now called chronic allograft dysfunction or, in kidney transplant patients, chronic allograft nephropathy.
To avoid transplant rejection, anti-rejection drugs are prescribed for you by the transplant team. As every transplant is unique, the anti-rejection drugs you take may be different to someone else’s, and may also change in type, number or strength depending on how your body responds.