How is histamine useful against allergies?
Histamine is produced in allergic reactions. - True

Histamine is produced against allergic reactions - False

Okay. Lets get a few things right here first. An allergy is an inappropriate immune hypersensitive response. It is something, we are not supposed to have. A small harmless pollen grain, in normal people shouldn't initiate an allergic reaction.

IgE antibodies are produced against an allergen. These attach their Fc ends to the mast cell. When these antibodies are cross linked by the antigen, degranulation of the mast cell ensues which release various mediators one of which is histamine. So there is a lot of histamine produced in allergic reactions and common antihistaminics like cetirizine and terbinafine act by blocking histamine receptors thereby reducing its biological effects.

So, allergy is a bad thing. The whole IgE, cross linking, degranulation and histamine release wouldn't have happened in a normal person. Histamine isn't going to protect from allergies. It is the cause of allergic effects. Hence the antihistaminics work. You might know that if you have taken a cetirizine during a rash or severe sneezing.

So if allergy is a bad thing, what is the normal physiological role of IgE?

IgE has a role to play in the immune response against parasites where a similar mast cell degranulation process happens.

And if histamines are causing the allergic effects and allergy is bad, what is the normal role of histamine?

Histamine has several roles to play. One is that it is produces as a part of immune response by basophils and mast cells. It also is a neurotransmitter and also is important is gastric acid secretion physiology. The various functions of histamine have important pharmacological considerations while talking of antihistaminics.

Reference : Kuby, Immunology 6th Ed.