Allergy causes can be classified by their origin:
External (environmental factor):
- Inhaled factors: small particles that may cause allergy when they enter the body through the airways or after contact with the eyes. Among these inhaled factors are the pollen of trees, weeds, grasses, house dust mites, molds, animal skin particles and secretions.
- Food: there are several foods to which persons may develop allergy.
- Contact allergens: several materials such as latex (rubber) that may elicit a reaction in some persons after contact with them.
- Insect stings: allergic reaction to the injected substance, for example to the injected substance by a honey bee sting.
- Drugs.
Internal (the cause is in the body itself):
- Immune diseases: allergic reactions may occur as a manifestation of activation of the immune system (which is the system responsible for allergic reactions, among other functions). This may happen during a viral disease or in the context of autoimmune diseases (in which the body attacks itself).
- Deregulation of the immune system activity: in these cases the immune system has a very low threshold for activation of the allergic response and allergic reactions to certain unspecific stimuli such as warming the body may erupt.
- Idiopathic problems: This category contains different causes which are not understood or only partially understood.