Allergies occur when the immune system overreacts to a stimulus. There are 4 types of hypersensitivity reactions. Type I is immediate and severe. This is usually the reaction dogs get to bee stings, drugs, inhaled antigens like pollens and flea bites. In Type II hypersensitivity, antibodies form to the body's own proteins and occurs with many autoimmune diseases. Type III occurs when so many antibody-antigen immune complexes are formed they get lodged in certain organs like the kidneys. Type IV is called delayed hypersensitivity and occurs more than 24 hours after exposure to the antigen. This is the type of reaction that occurs with contact allergies to things like dyes and chemicals.
The most common reaction an owner will see is type I hypersensitivity. Signs may be mild moderate or severe. Mild reactions may be characterised by fever, loss of appetite and lethargy. Moderate reactions can manifest as urticaria which is seen as hives, or wheals in the skin, rapid facial swelling, and extreme itchiness. Urticaria can progress to anaphylaxis which is the most severe reaction. This can produce vomiting, diarrhoea, swelling of the larynx resulting in breathing difficulties, drop in blood pressure, seizures, collapse and possibly death. When anaphylaxis occurs, immediate veterinary care is necessary to support life.