For the past few months the Centers for Disease Control and the Food and Drug Administration have been developing assays to test blood samples donated by each of the 20 or so people who reported allergic reactions after eating corn products. The tests are designed to detect antibodies in the samples that bind to the Cry9C protein.

The blood tests are the key pieces of information needed by the EPA and its advisory panel to decide on the petition from Aventis, the maker of StarLink, for an exemption from tolerance, an exemption that would allow corn stocks containing StarLink corn to be used as human food.

If the tests are negative in all cases, such a result would support Aventis' arguments for an exemption from tolerance for StarLink. Negative results would be consistent with the possibility that the Cry9c protein was not involved in any reaction experienced by the people being tested. However, the potential for allergenicity still remains an open question, although somewhat less open than before. That's because there still is no way to prove a negative. There is no way to prove that the Cry9C protein (or any protein) will never, ever cause an allergic response.

If any of the tests show that some people have developed antibodies that specifically bind to the Cry9C protein, then it's likely the protein will be considered a known allergen. Then the EPA's scientific advisory panel will have to assess the threat to public health of a known allergen, weighing other factors including the concentration of the protein detectable in various foods after various processing steps, such as milling or baking.

The Issue of Cross-Reactivity

Being allergic to a food can be looked at as a two step process: sensitization and triggering.

There are at least two explanations for an allergic reaction to a food.

One is that the person has previously eaten the food, and something in the food sensitized the person's immune system but without causing an allergic response. Then when the sensitized person eats the food again, the same thing in the food that first sensitized the person's immune system also triggers an allergic response.

Another explanation is that something in a food sensitizes the immune system of a person. But when the sensitized person eats another food, that second food may have something in it that is similar enough to trigger an allergic response. This is called a "cross-reaction".

In other words, if any of the blood tests come back positive, researchers will still face at least these two questions. Did some other protein in the diet originally trigger the antibodies that by chance also stick to Cry9C protein? Or did Cry9C in the diet serve to both sensitize an individual in the first exposure and then trigger an allergic response in a second exposure several weeks or months later?

Cry9C is a protein encoded by a gene engineered into in StarLink corn to make the corn resist insect pests. The EPA approved Cry9C protein and its gene in StarLink for animal feed but not for human food because of unresolved concerns about the possibility that the protein, new to the food supply of humans, could trigger allergic reactions. However, on September 18 several groups opposed to biotechnology announced they had detected the Cry9C gene in human food, leading to a series of food recalls.