The following information was obtained from a great book called "The Wheatgrass Book"
This book was written by Ann Wigmore, the woman who introduced wheatgrass juice to America.

The many active compounds found in grass juice can cleanse the blood and neutralize and digest toxins in our cells.
Think of your body as a gigantic biologically active chemical factory. Under the skin, millions of chemical actions and reactions take place all the time. The force behind many of these delicate metabolic operations is enzymes. Long considered catalysts (substances that affect the rate of chemical reactions, but are not changed by them), enzymes are now understood by many scientists to be more than mere supplementary material; they are life energy itself. Whether you want a cut on your finger to heal or you desire to lose weight, enzymes must be called upon to do the actual work. In fact, every action from thinking to digesting food or moving your legs requires the activity of thousands of enzymes. Enzymes are especially vital to the process of blood cleansing. We can emply two basic types of enzymes in the process of cleansing and rebuilding our blood, and ultimately, our bodies. The first group is indogenous enzymes, which are found inside us. While this group is powerful and capable of great detoxification, indogenours enzymes do wear out with age. Their lifespan and abilities can be extended if we help them from the outside by adding exogenous enzymes, like the ones found in wheatgrass juice, to our diet.
We can only get the benefits of the many enzymes found in grass by eating it uncooked. Cooking destroys 100 percent of the enzymes in food. A certain amount of enzyme activity is also lost by prolonged exposure to air, processing, or drying. This is why it is recommended that you grow your own wheatgrass and drink it within minutes of juicing.
In addition to enzymes, wheatgrass also contains amino acid chains (polypeptides) and bioflavonoids (compounds related to vitamins) that also help cleanse the blood and other tissues. The amino acids are absorbed directly into the blood. As well as neutralizing toxic substances like cadmium, nicotine, strontium, mercury, and polyvinyl chloride, by changing them into insoluble salts which the body can eliminate more easily, they stimulate cellular metabolism. The flavonoids act to detoxify cells and prevent their deterioration.