I am a pharmacist, but usually hesitate to provide medical advice on forums such as this . . . still . . . a couple important points (especially since I get the horrible-unable-to-sleep itch after some bites as well :shock:).
And remember, it is not entirely a specific immune response (certainly it is in part an immune response), but in part it is a response to a specific direct toxin that also includes the release of other inflammatory mediators than histamine, so actually building an immunity may well not happen for many.
Antihistimines at any point after a bite can be quite helpful. They block histamine at receptors, not the release of histamine, but they don't provide a complete block, so when your body is overwhelmed by histamine release after a bite they will never be completely effective. But they will always block part of the histimine part of the response (again here, there are other inflammatory mediators at play that the antihistamine does not help control). You need to take them regularly (daily for loratadine) because histamine will be released for some time after a bite (different for different people).
That being said, there is no reason not to use the nonsedating antihistamines such as Claritin type products (I just use generic loratadine, just as good, cheaper) over benadryl/diphenhydramine. Benadryl can be very sedating for some people and cause other problematic side effects, especially in the first few days of taking it, which is when you need it most. Using more than the recommended dose of loratadine can bring about sedation and probably will not help more - so stick with recommended dose. Zyrtec (cetirizine) is another reasonable choice, possibly a little more sedating than loratadinine, but still much less than benadryl.
For topical agents to help control itch, the 2 mainstays will be a hydrocortisone cream or ointment and a benzocaine (anesthetic) cream or spray (I like the sprays).
Vagisil is simply either a hydrocortisone or benzocaine product (it depends on which vagisil product you use there are a bunch of them - not always the cheapest way to go, but not an unreasonable choice - just no better than other hydrocortisone or benzocaine products).
Hydrocortisone cream or ointment is available generically and cheaply over the counter. It won't get rid of all the itch because it can only penetrate so far and the histamine and inflammatory mediators surround the entire bite area and can be quite deep.
benzocaine or lidocaine products such as Americaine spray (or lanacaine or solarcaine - or even vagisil version that that contains benzocaine, or the "sting-kill" swabs or wipes) can provide very nice short term itch/pain relief. Don't over do it though as too frequent use or putting on too much of these anesthetics can be dangerous. Read the directions for the benzocaine products - be careful near eyes or mucous membranes.
So, quick and regular use of non-sedating antihistamine like loratadine, some benzocaine spray when the itch/pain is really bad, and some hydrocortisone cream are the mainstays. They won't stop the whole shebang, but its the best we have. :(
Only other option is oral doses of steroids, but that really needs to be reserved for very very serious reactions (and is only available by prescription for good reason) - not just horrible itching :'(.
Here was a bad one I got a couple weeks ago. Just after I took off my bee suit by my truck away from hive I got a sneak attack. (before and after - I don't always swell up bad and this one actually didn't itch much, but it was not great to get stung right between the eyes :shock:). face protection is good!