How can I be sensitive to wheat but not to cous cous?

This is a sensitivity to the concentrated antigen/protein versus testing the whole food. In this case pure wheat is the concentrated antigen/protein and couscous is a mixture of semolina, wheat flour, water. Consider this analogy. When testing a pizza, you may not be sensitive to the pizza. However, you may be very sensitivity to oregano and basil. In other words, you can be sensitive to individual, concentrated ingredients tested in their purest forms, but not as a small percentage of the whole food.