Autacoids, also known as local hormones, are naturally occurring substances that have different structures and pharmacological actions. They include decarboxylated amino acids, polypeptides, and eicosanoids. Histamine is one of the most important autacoids and is stored in mast cells. It is released through immunologic or non-immunologic mechanisms to modulate inflammatory and immune responses. Antihistamines work by blocking the H1 receptor and are used to treat various allergic conditions by reducing the effects of histamine. First generation antihistamines are less selective and have more side effects while second generation antihistamines have fewer side effects.