While interviewing a patient with a complaint of an allergic reaction, your partner asks, "What is new and different?" Most patients with an allergy must be exposed to the allergen at some point. What is the process called?

Sensitization is the process where an antigen enters the body and there is an immune response where antibodies are produced to protect the organism from an invader.  The antibodies help protect the patient from developing an overreaction towards the antigen which will then become an anaphylactic reaction.  Anaphylactic reaction is such a massive response against a first time exposure to an antigen this reaction looks like a full on anaphylaxis, the differential is no prior sensitization.