Atopy:

Atopy is a personal and/or familial tendency, usually in childhood or adolescence, to become sensitised and produce IgE antibodies in response to ordinary exposure to allergens, usually proteins.
Atopy is a personal or familial tendency to produce IgE antibodies in response to low doses of allergens, usually proteins, and to develop typical symptoms such as asthma, rhinoconjunctivitis, or eczema/dermatitis.

Sensitisation:

Being sensitised means that the immune system has come into contact with an allergen in first time; committed it to its memory; synthesised IgE antibodies ((a process that may take up to 6 weeks to develop)  against it and is ready to launch a defence reaction when the allergen reappears. In initial sensitisation phase there is no adverse reaction is taken place.