This course starts with a brief summary on the basic mechanisms of immunepathologic reactions. Subsequently it focuses on the semiology of allergic diseases in children and adults with particular emphasis on respiratory symtoms, urticaria, angio-edema and anaphylaxis. Particular interest is paid to allergies to inhalant allergens (such as house dust mite, pollen pets, mould spores), food, drugs and hymenoptera venom (wasps and bees). Other conditions that are addressed are mastocytosis and C1-esterase inhibitor deficiency. Where appropriate, the differential diagnosis between allergic and non-allergic reactions will be elaborated. Cross-immunity and cross-allergy will be explored.