Allergen-driven lymphocyte proliferative response in the blood: Peripheral blood mononuclear cells (106 cells/ml) will be cultured with autologous plasma for 3 days (mitogen or anti-CD3/28) or 7 days (recombinant Der p1 allergen) in a humidified atmosphere at 37°C/5% CO2. Proliferation will be measured by incorporation of tritiated thymidine (0.5uCi) for 16hr before harvest onto glass fibre filters and counting in a beta counter.