What do allergy tablets do to stop my rashes?

Decrease inflammatio. Allergy tablets, I am guessing, are referring to antihistamines such as zyrtec (cetirizine), Claritin (loratadine), or Allegra (fexofenadine) act by blocking histamine receptors, which are responsible for the inflammation, swelling, itching, and erythema associated with allergic rashes.