The term 'Cause of a Rash Response' can be answered by two crossword solutions: ALLERGY (7 letters) and SUMAC (5 letters). ALLERGY is an exaggerated immune response to a normally harmless substance, such as pollen, pet dander, or certain foods. SUMAC is a shrub or small tree that produces clusters of red or yellow flowers and is known to cause allergic reactions in some people. Both allergies and SUMAC can cause a rash response when exposed to the allergen.