It increases IQ, histidine, a precursor for histamine, doubled the IQ of people with downs syndrome in an experiment over several months, but histamine also increases aggression which probably explains why you dont see many aggressive people with Down Syndrome.

Aged food & drink is higher in histamine. Red wine more than white wine. Dark spirits more than clear spirits. Red meat more than white meat. It can cause spontaneous erections, 60mg of histamine can be used for erectile dysfunction. Histamine can increase blood saturation levels and help reduce the symptoms of COPD and other respiratory problems.

If you consumed alot of histadine, like several heaped table spoons with meals throughout the day, depending on your age and health you will notice some or all of the above. Its one of the fastest amino acids into muscle, its untouched by the liver.

I dont know why you are taking anti-histamines, histamine helps the white immune cells move through tissue, it causes inflammation, there is also a histamine feedback loop in the brain which affects sleep patterns, which is why anti-histamines can help people get to sleep, but lots of histamine can improve sleep conversely.

Loads of studies exist that can be found on Google Scholar, but multiple chemicals affect peoples personality, and their health.

If you want to read up on this, also look at the pathways and other chemicals which work with histidine (or any other amino acid) for that matter.