Covering each cell in your body are small markers called antigens. Some of these markers can be found across all humans, however, others are specific to you (self-antigens). Antigens act as little flags, allowing your immune system to know what the cell is and if it should be there.

Self antigens are products of a group of linked genes, know as the Major Histocompatibility complex (MHC).

They are found on almost ever nucleated cell in your body, however they are particularly seen on antigen-presenting cells. Such as Dendritic cells and Phagocytes.   

MHC molecules are small glycoproteins that are presented on a cell surface. They are the only molecule capable of presenting an antigen to a T cell.

Observe the video to the left. The cell enulfs the pathogen and presents the foreign antigens on its MHC molecules (shown in red).

The correct helper T cell (yellow), attaches to the MHC molecules and identifies the foreign antigen, causing an immune response.

MHC molecules are the antigens which play a main role in transplant rejection. Since MHC molecules are specific to the individual, when we transplant organs, the immune system sees this antigen and recognises it as non-self.

The image to the right is the rejected transplanted kidney, of a 32 year old male. He received a kidney from his mother, however the differences between the markers where to great and the immune system attacked the kidney.
Cytotoxic T cells, NC, phagocytes and other immune cells attacked the kidney, requiring it to be removed.

Only identical twins have the same MHC markers, making the success rate of transplants between them, extremely high.

In all other combinations, the tissue between recipient and donor must be analysed. Often members of the same familly will have similar antigen markers, making the chances for success slightly higher.

Red blood cells however do not have MHC molecules or have significantly less (non-nucleated). This is why blood transfusions are more successful than organ transplants.

However even when success rates are high and an excellent match has been found, it is still important to use immunosuppressive drugs.

These drugs suppress and reduce the strength of the immune system. However while on these the individual is a high risk for infections and developing cancers.

The incidences of cancer is higher in immuno suppressed patients.

Cyclosporine is one such drug. Cyclosporine inhibits the proliferation of activated T cells, and interferes with cellular signalling.

Methotrexate is used in treatment of rheumatoid arthritis and alleviate joint pain and swelling of poly arthritis. It works by interfering with the production of folic acid (a building block of growing cells), hindering growth of certain cells, including those of the immune system.