Boosting the immune system to fight disease isn't revolutionary. Research has shown that the chicken soup your mom gave you when you were sick actually helped your immune system rid your body of whatever was making you feel under the weather. Merck's (NYSE: MRK) PegIntron and Roche's Pegasys work by giving patients' immune systems a boost to help them fight off the hepatitis C virus.

But lately companies have taken the idea of using the immune system a bit further to include fighting cancer. The idea sounds simple enough, but since the immune system has been trained to only attack foreign cells and viruses, it can be a little tricky depending on the tumor type.

For melanoma -- aka skin cancer -- it turns out that pumping up the immune system is enough to help patients. In March, both Merck's PegIntron -- branded as Sylatron for cancer -- and Bristol-Myers Squibb's (NYSE: BMY) Yervoy were approved as a treatment for melanoma.

The two drugs work in different ways but produce the same effect. Sylatron stimulates the production of immune cells while Yervoy inhibits a protein that inhibits the immune cells; inhibit an inhibitor, and you get a positive reaction.

Getting specific
In contrast to the general immune system promoters, Dendreon's (Nasdaq: DNDN) Provenge encourages the immune system to attack a specific type of cell -- in this case prostate cancer cells. It's often called a cancer vaccine, but it's not the traditional type of vaccine that you get to keep from being sick. Instead, Provenge is used after a patient has prostate cancer to train the immune system, and it's a little more complicated than a jab in the arm.

To produce Provenge, immune cells are taken from a patient and sent to a processing plant where they're exposed to an antigen that's present on the prostate cancer cells. The immune cells are then put back into the patient where they train the rest of the immune system to attack the prostate cancer cells.

The next Provenge
There are a bunch of companies trying to develop immunotherapies.

Oncothyreon (Nasdaq: ONTY) is developing Stimuvax, which trains the immune system to target the MUC-1 protein. The drug, which is in a pivotal phase 3 trial in lung cancer, is backed by Merck KGaA.

Likewise, CEL-SCI (AMEX: CVM) has the backing of Teva Pharmaceuticals (Nasdaq: TEVA) in the development of its cancer therapy Multikine as a treatment for head and neck cancer. The pivotal study just began at the end of last year, so investors will have to wait awhile to see if the drug works.

GlaxoSmithKline has a cancer vaccine, MAGE-A3 ASCI, which homes the immune system in on the MAGE-A3 protein that's present on lung cancer cells. The company has hooked up with Abbott Labs (NYSE: ABT) to develop a companion diagnostic that determines if the tumor expresses the MAGE-A3 protein.

And Dendreon is looking to expand into other cancer types by using the same system as Provenge, but different antigens to train the immune system to attack tumors of different origins. Next up: DN24-02 in bladder cancer.

Not a sure thing
While the success of Yervoy and Provenge should give investors confidence that the approach is working, it's still far from a sure thing. There's no guarantee that an antigen will actually stimulate the immune system enough to inflict any real damage on the tumor. It's also possible for an overreaction to occur, causing the immune system to attack cells it really shouldn't. Dendreon is likely the safest bet here since its technology is already proven to work in at least one cell type.

The backing of large companies Merck KGaA, Teva, and Glaxo is certainly a plus -- at least someone with drug-development experience thinks it's worthy of an investment. But keep in mind that the immunotherapies are just one of many drugs in their pipelines. If you're going to invest in this high-risk space, I'd suggest you do the same.

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Fool contributor Brian Orelli holds no position in any company mentioned. Click here to see his holdings and a short bio. The Motley Fool owns shares of GlaxoSmithKline, Teva Pharmaceutical Industries, and Abbott Laboratories. Motley Fool newsletter services have recommended buying shares of Abbott Laboratories, GlaxoSmithKline, and Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.