An immunomodulator is a drug used for its effect on the immune system. There are two types of such drugs based on their effects:
Immunosuppression involves an act that reduces the activation or efficacy of the immune system. Some portions of the immune system itself have immuno-suppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reaction to treatment of other conditions. But deliberately induced immunosuppression is generally done to prevent the body from rejecting an organ transplant or for the treatment of auto-immune diseases such as rheumatoid arthritis or Crohn's disease. This is typically done using drugs, but may involve surgery (splenectomy), plasmapharesis, or radiation.
A person who is undergoing immunosuppression, or whose immune system is weak for other reasons (for example, chemotherapy and HIV) is said to be immunocompromised. When an organ is transplanted, the immune system of the recipient will most likely recognize it as foreign tissue and attack it. The destruction of the organ will, if untreated, end in the death of the recipient.
In the past, radiation therapy was used to decrease the strength of the immune system, but now immunosuppressant drugs are used to inhibit the reaction of the immune system. The downside is that with such a deactivated immune system, the body is very vulnerable to opportunistic infections, even those usually considered harmless. Also, prolonged use of immunosuppressants increases the risk of cancer.
Cortisone was the first immunosuppressant identified, but its wide range of side effects limited its use. The more specific azathioprine was identified in 1959, but it was the discovery of cyclosporine in 1970 that allowed for significant expansion of kidney transplantation to less well-matched donor-recipient pairs as well as broad application of liver transplantation, lung transplantation, pancreas transplantation, and heart transplantation.
A list of immunosuppressant drugs is given on the transplant rejection page.
Immunostimulators are the drugs which stimulate the immune system by inducing activation or increasing activity of any of its components.
There are two main categories of immunostimulators:
Specific immunostimulators are those which provide antigenic specificity in immune response, such as vaccines or any antigen.
Non-specific immunostimulators are those which act irrespective of antigenic specificity to augment immune response of other antigen or stimulate components of the immune system without antigenic specificity, such as adjuvants and non-specific immunostimulators.
re Tysabri, i don't know and wikipedia doesn't either:
The mechanism of action of natalizumab is believed to involve the inhibition of immune cells from crossing blood vessel walls to reach various tissues, including the brain.