Histamine is an amine found in many living organisms.  We have histamine in nearly all our body tissues and it is used in many body functions, such as dilation of blood vessels and gastric acid secretion in the stomach.  This entry from the Encyclopedia Brittanica On-line has a basic overview of histamine.  This information page  by Dr. Janice Joneja, a researcher in the field of allergies and food intolerances, gives a good overview of histamine and what happens when one gets more than your body can handle.  She also wrote a book I highly recommend, Dealing With Food Allergies.

Too much histamine in the body is being linked to several disorders, including rosacea, chronic hives, eczema, interstitial cystitis, panic attacks, and irritable bowel syndrome.  There are many different reasons for having problems with histamine.  Some people are born with an inability to break down ingested histamine, damage in the intestines can damage the villi on which the enzyme that breaks histamine down is made (much like when the lactase producing villi are damaged), the body may be so overwhelmed with allergic reactions that it can't process all the histamine flooding it, ... the list goes on.  Some researchers (see bottom list) feel cutting down as much as possible on the amount of histamine one is ingesting can be helpful.

There are many different histamine-food lists - the problem is they tend to disagree on some of the medium level to low level foods.  One reason could be that histamine levels vary depending on the ripeness, freshness, and the varietal of many foods.  Histamine can't be cut out completely - one has to look at limiting it to a lower level.  This list is a good place to start:

Histamine Intolerance Organization

As food decomposes, the histamine levels rise, so look for the freshest food you can find.  Overripe fruits and vegetables can be a problem, as is meat that has been aged.  Or leftovers from the night before.  I freeze everything we're not eating immediately after cooking it.  Small glass or plastic containers work well for lunch sized portions.  Don't buy meat that is near its "best by" date.  I only buy meat if it has at least five days left - and freeze it immediately, even if we're eating it the next day.  Aged cheeses can also be a problem.

Fermented foods have higher histamine levels - sometimes very high.  Avoid vinegars or anything made with vinegar, yogurt, fermented soy products such as soy sauce and tofu, fermented vegetables such as sauerkraut, and most alcohol.  I found I can drink a glass or two of wines that have not gone through malolactic fermentation, but someone who has more trouble breaking down ingested histamine than I do may not be able to.  These wines tend to be whites such as pinot gris/grigio, pinot blanc, and riesling.  Some sauvignon blancs are okay and a few chardonnays.  Roses also do not usually go through malo.

There doesn't appear to be a lot of research currently being done in this country on problems with histamine - Europe is ahead of us on this.  Here are some research papers and other information I found useful (I've given a link when available):

Joneja, J. and Carmona-Silva, C.  "Outcome of a Histamine Restricted Diet Based on a Chart Audit."  Journal of Nutritional and Environmental Science.  2001, 11, 249 - 262.  Available here.

Maintz,  and L.Novak, N. "Histamine and Histamine Intolerance." American Journal of Clinical Nutrition. May 2007, 85(5): 1185-96.  Available here.

Wantke, F., Gotz, M. and Jarisch, R.  "Histamine Free Diet: Treatment of Choice for Histamine Induced Food Intolerance and Supporting Treatment for Chronic Headaches."  Clinical and Experimental Allergy.  December 1993, 23(12):  982 - 985.  The abstract is available here.