allergies

Allergies are abnormal immune responses to foreign substances (allergens) introduced into the body. The symptoms of allergies are caused when certain hormones, chief among them histamine, are released by trigger cells which have previously been sensitized to a foreign substance or one that is sufficiently similar in structure to fool the triggering cell into reacting to it. Allergies are treated by

Avoidance of the allegen, if known
Drugs that block the effects of the hormones (antihistamines being most common)
Drugs that reduce the trigger cell response to the allergen (sodium cromolyn, nedocromil)
Immunotherapy (allergy shots) - where an attempt is made to reduce the immune response to an allergen by desensitizing the immune system response.

Patients (and astonishingly, on occasion even medical personnel) forget a fundamental fact about allergy in general: the allergic response can only be developed to something to which one has already been exposed. So the statement, "I don't understand, she hasn't had anything new," displays a lack of knowledge of just how allergy works. The body must have been exposed at least once before to the sensitizing agent (or something very, very close in chemical structure) before the triggering antibodies for the allegic response can be formed.