A person with no history of allergies can indeed experience sudden onset of a respiratory or severe food allergy.
More usually, allergic patients will come with past history of eczema (atopic dermatitis), food allergy at a young age, asthma or perhaps a positive familial history of allergies. Yet more and more commonly are we seeing patients developing allergies “out of the blue”. Unfortunately, science does not yet fully understand the mechanisms involved to explain why this happens. Several theories attempt to explain the sudden development of a specific immunologic reaction to a normally harmless substance. A recent and controversial theory proposed by Polly Matzinger (www.medscape.com) explains that immune responses are triggered by ‘danger signals’ released by the body’s own cells. In other words, traumatic events or even psychological factors could contribute to the development of an allergic response to a substance the immune system had formerly reacted ‘normally’ to.