By "Allergic Meningitis" he probably meant "Aseptic Meningitis," which is a form of meningitis that is not caused by bacteria or viruses. As far as I know it is far less dangerous, although the symptoms can be just as nasty. Here's the Wikipedia write up on it:
http://en.wikipedia.org/wiki/Aseptic_meningitis
It is a known, but infrequent side effect of some medications, including IVIG. When I first started IVIG a year and a half ago I had a reaction that as far as I'm concerned was aseptic meningitis, although my neuro didn't seem to think much of my auto-diagnosis.
In my case it went down like this:
Loading dose of IVIG on Tuesday, Wednesday, and Thursday. Felt like a million bucks by the end of the day Thursday.
On Friday I was feeling a bit odd and restless by end of day. Headache started early evening and by 10:00 PM it was excruciating. (I was literally squirming in my seat.) Ibuprofen did nothing. Went to bed and slept badly.
Next day (Saturday) I could barely function. Spent most of the day in bed. Couldn't eat. Very stiff neck, and felt like my brain was in a clamp. No appetite at all. Food was repulsive. I felt like I wanted to vomit, but my stomach was empty. Started taking a cocktail of Tylenol, Naproxen, and Benadryl, which helped a bit (the Naproxen was the most effective.) I could hardly move all day. Couldn't stand any sound.
Slept reasonably well Saturday night, under the circumstances. Felt a bit better Sunday, and continued taking the 3-drug cocktail. Appetite returned, and neck stiffness was greatly reduced. By Sunday evening I felt a lot better (5/10, whereas Saturday was 9/10 on the pain scale). Stopped taking the Benadryl, but took two Tylenol PMs before bed. Unfortunately the headache and stiff neck got a bit worse and I slept very badly.
Monday: got up early feeling rotten (stiff neck and headache). Naproxen helped. By mid-morning the pain was down to about 3/10, but I felt dizzy (the same dizzy you get from taking Benadryl -- except I wasn't taking Benadryl). By Tuesday I was able to function pretty normally (went to work, etc.) and by Wednesday all symptoms were gone. Fortunately they never came back.