Hi, interesting question with a not so easy answer.
The complex part of eosinophilic complex is there because there are three distinct syndromes we see:
1. Eosinophilic granuloma- caused by either hypersensitivity (allergy) or genetic, it is basically a red lesion on the skin, tongue or palate.
2. Eosinophilic plaque- this is usually an allergic reaction to something like fleas, food or environment allergens. These are usually red lesions on the skin.
3. Indolent Ulcer- this is an oral ulcer of the upper lip, yellow to orange and usually next to the nose.
All three may develop at the same time or independently. They are basically over-active immune responses to normal allergens, flea bites, or food proteins.
Treatment: these are often responsive to steroid therapy to suppress the immune system. Often long term or lifelong therapy is needed. If the underlying cause can be determined and treated (food allergy or flea allergy) then lifelong therapy is not needed. Often, the underlying cause is not determined.
Sorry so long winded, but good question. Hope that gives you the general picture. Let me know if you have any other questions.
Cat Veterinarian
DVM, Emergency Veterinarian; BS (Physiology) Michigan State Univ