Azathioprine is a prodrug that after ingestion metabolizes into the active drug 6-MP.  Azathioprine has an advantage over 6-MP by itself in that Azathioprine is less toxic.  Azathioprine is an immunosuppressant and increases the risk of infection for anyone taking the drug.

6-MP

6-MP (mercaptopurine) is a thiopurine that inhibits the immune response by suppressing the production of white and red blood cells.  Mercaptopurine reduces the human body's ability to fight off infection.  This drug is also listed as a carcinogen by the U.S. Department of Health and Human Services.

Methotrexate

Methotrexate is another anti-metabolite that reduces normal cell function by inhibiting the metabolism of folic acid.  As with the other two immunosuppressants mentioned, Methotrexate can increase the risk of infection.