I’m not a doctor, a physiologist or an immunologist, so I may have some of this wrong, but this is what I understand about the medical use of antihistamines:
Allergies occur when the immune system overreacts to innocent foreign bodies and substances. Part of this response is the release of histamines. Antihistamine drugs are used to combat allergies by reducing the level of histamines (whether they stop the production, disable the receptors or destroy the chemical, I don’t know). This means that you have less of an immune reaction.
However, if you have a genuine infection, won’t antihistamines also interfere with the operation of the immune system? Do antihistamines lessen the body’s ability to fight infections, or is their action restricted to the “false alarms” of allergies?
Histamine response is only the initial response to an antigen and is only part of the inflammation cascade. Humoral and cellular immune response are largely independent of histamine.
The immune system is pretty complicated. Phrases like “weaken the immune system” remind me of get-rick-quick scams.
Histamine does not play an important role in fighting off the baddies. It is a short-lived, IgE inflammatory mediator from mast cells. Taking an anti-histamine would provide some symptomatic relief from an allergy, but would not really prevent you from fighting an infection. It weakens the immune system only in a very weak sense.
Folks who say things like antihistamines weaken the immune system never seem to consider that a bad allergic response, where the body over-reacts to a normally harmless substance, will leave you in a seriously weakened state and thus more vulnerable to real threats.
As a long-term allergy sufferer, I’d say this idea is bogus, but that’s purely anecdoctal. More anecdotal experience also tells me that the folks spouting these theories usualy do not suffer from allergies themselves and have little to no medical training.
Now, if someone’s allergies are so severe they require steroids to control the reaction then you could argue the medication is suppressing the immune system, but steroids are not antihistamines and work by a different mechanism.