Everything we consume is broken down by enzymes in our bodies. While some foods are broken down in the intestines, others are digested in the stomach. Alcohol does not need to pass through the digestive tract in order to be digested; rather, it is absorbed directly into the blood stream. Alcohol is a toxic substance. The body produces a special enzyme in the liver, known as Aldehyde Dehydrogenase (ALDH) enzymes, to break down and oxidize the alcohol in a process that turns the alcoholic substances into acetic acid (vinegar), from which some energy can be obtained.
Some people have malfunctioning ALDH genes. As a result, their bodies produce inactive ALDH enzymes that are unable to properly break down the alcohol into acetic vinegar. This incomplete processing of the alcohol can cause the body to react by producing symptoms that reflect an allergy to the alcohol. People with this gene deficiency suffer two-fold from allergic symptoms to alcohol. First, the body produces histamines in response to the presence of the alcohol that the body is unable to digest. Second, because ALDH is also the enzyme that is supposed to metabolise histamine, people with alcohol allergies suffer from excessive amounts of undigested histamine in their systems, leading to allergy-like symptoms such as nasal congestion and mild flushing immediately upon consuming even minute amounts of alcohol.
Although allergies to alcohol are rare, one of the most common symptoms of an allergy to alcohol is nasal congestion, also known as allergic rhinitis; caused by the inflammatory effect of the histamine produced by the body in response to the presence of alcohol. The histamine causes the blood cells in the nasal region to dilate, resulting in mucus, nasal congestion, sneezing, and a runny nose. However, the severity of these symptoms may vary from person to person.