How plant foods boost your immune system
by Mike Johnston, summarized from a report by PCRM

Unfortunately, our air, water, soil and much of our food is full of chemicals that can cause health problems ranging from asthma to cancer. The good news, however, is that a natural chemical removal system is built into our basic biochemistry. It is powerful enough to eliminate thousands of different chemicals. It just needs to be turned on.
This chemical rejection process is based on enzymes that can take the most dangerous chemicals and, in an instant, render them harmless.
These enzymes arrest toxins and send them away, so they can, in fact, save your life but they need to be woken up to do their thing.
This involves a two-phase process. In Phase I, an enzyme in the liver cell grabs hold of the toxic molecule and attaches oxygen to it. In Phase II, a second enzyme hooks the culprit molecule onto a large carrier molecule that drags it away. These are then sent out of your body in your urine or feces.
As this process unfolds however, there is one very dangerous point. When a toxic chemical has had oxygen attached by the Phase I enzyme, it can be even more dangerous than when it entered your body. The key is to have plenty of Phase II enzymes around to deal with these activated toxins and kick them out of your body.
This phenomenon depends on the right kinds of food in your diet. Some foods contain powerful enzyme inducers. They cause the body to make more Phase II enzymes. Some of these foods are broccoli, cabbage, Brussels sprouts and cauliflower. These vegetables also work well at detoxifying cancer-causing chemicals in beef. Note however, that none of the vegetables reduces the risk to zero, so avoiding beef is still the healthiest strategy.
Soybean products, such as tofu, tempeh and soy milk, also contain natural compounds that stimulate your body to make more of these helpful Phase II enzymes, which may be part of the reason why Asian countries have especially low rates of most cancers. Green onions are also potent enzyme boosters, as are many other vegetables. Cooking does not destroy them.
We are confronted by new chemicals every day and our natural enzyme systems are busy trying to disarm them. They even try to destroy medications. Researchers at Rockefeller University in New York gave Brussels sprouts and cabbage to research subjects over the course of several days. They then tested their response to phenacetin and antipyrine, two analgesic and anti-inflammatory drugs. They found that when subjects were on the vegetable-rich diet, they tended to eliminate each medicine more rapidly. Tests with many other medications have shown the same result, although the response varies from one person to the next.
Don't let this scare you. It does not mean that medicines do not work on vegetarians. It simply means that plant-based diets make your body more ready and able to knock out chemicals of many types. Overall, that is a great plus, although it may mean that some medicines are eliminated a bit faster than they might be otherwise.
Source: Good Medicine, Autumn 1995, The newsletter of the Physicians Committee for Responsible Medicine (PCRM).