Here we are going to see the four types of hypersensitivity reaction with some examples of diseases that occur in horses (anaphylaxis, allergic dermatitis, food allergies, reaction to transfusion, neonatal isoerythrolysis, reaction to drugs, intravascular haemolysis, glomerulonephritis, purpura haemorrhagica, equine recurrent uveitis, equine asthma, graft rejection).