Hi Lori,
to be sure the anaphylactic reaction is quite rare and can be caught very early on if somebody watches. The majority of the people (>80%) on Erbitux get a rash (of varying severity).
Chemo agents enhance the effects of the radiation, this has been shown for platin compounds and I believe also for Erbitux. Both rad and chemo kill cancer cells and the idea is that the combination provides the extra punch to cause these cells to pack up an die. In addition to the local effect chemo also has some distant action.

Markus

Here is some more info on Erbitux.
http://www.fda.gov/cder/foi/label/2004/125084lbl.pdf