Atopic reactions are caused by localized hypersensitivity reactions to an allergen. It is used to detect the causative allergen for an allergy. A patch test is used to identify the causative allergen in allergic conjunctivitis. Psyllium may act as a potent inhalant allergen capable of eliciting asthma symptoms. In allergic angioedema, avoidance of the allergen and use of antihistamines may prevent future attacks. It can be differentiated from allergic rhinitis because of the absence of an identifiable allergen. The humoral immune system produces antibodies against the inhaled allergen. Phototoxic dermatitis occurs when the allergen or irritant is activated by sunlight. Contact dermatitis is typically treated by avoiding the allergen or irritant. The first dose contain tiny amounts of the allergen or antigen. When the mast cells are stimulated by an allergen, histamine (and other chemicals) are released. "Acer negundo" pollen is a severe allergen. Some substances act both as allergen and irritant (wet cement, for example). Severe angioedema cases may require desensitization to the putative allergen, as mortality can occur. Chronic exposure to the allergen may result in a lichenoid lesion.