What Is Quercetin?
It’s a plant-derived flavonoid present in fruits and vegetables such as:
apples (red)
berries
cherries
asparagus
bell peppers (green and yellow)
broccoli
brussels sprouts
cabbage
citrus fruits
kale
green tea 
It’s also an antioxidant that fights against inflammation reduces itch and allergy symptoms.  
Quercitrin is a glycoside flavonoid with antioxidant properties. Quercitrin has been reported to have anti-viral and anti-inflammatory effects.
Allergies, asthma, hay fever and hives
In test tubes, quercetin prevents immune cells from releasing histamines, which are chemicals that cause allergic reactions. Allergies, asthma, hay fever and hives
As a result, researchers think that quercetin may help reduce symptoms of allergies, including runny nose, watery eyes, hives, and swelling of the face and lips. 
Heart disease
Test tube, animal, and some population-based studies suggest that the flavonoids quercetin, resveratrol, and catechins (all found in high concentrations in red wine) may help reduce the risk of atherosclerosis, plaque build up in arteries that can lead to heart attack or stroke. These nutrients appear to protect against the damage caused by LDL (bad) cholesterol and may help prevent death from heart disease. However, most human studies have looked at flavonoids in the diet, not as supplements. Animal studies have used extremely large amounts of flavonoids (more than you could get through a supplement). More studies in people are needed to see if flavonoid supplements can be effective.
High cholesterol
Test tube studies show that quercetin prevents damage from LDL cholesterol, and population studies show that people who eat diets high in flavonoids have lower cholesterol. One study found that people who took quercetin and an alcohol-free red wine extract (which contains quercetin) had less damage from LDL cholesterol. Another study found that quercetin reduced LDL concentrations in overweight subjects who were at high risk of heart disease. More studies are needed to show whether taking a quercetin supplement will have the same effect.
Hypertension
Studies show that quercetin supplementation reduces blood pressure in people who have hypertension.
Interstitial cystitis
Two small studies suggested that people with interstitial cystitis might benefit from consuming flavonoids. People with this condition have bladder pain similar to that from a bladder infection, and often experience an urgent need to urinate. In both studies, those who took a supplement containing quercetin appeared to have fewer symptoms. However, the studies included other flavonoids. So it is not known which flavonoid offers the most benefits. More and better designed studies are needed.
Prostatitis
Preliminary evidence indicates that quercetin might reduce symptoms of prostatitis, or inflammation of the prostate. One small study found that men who took quercetin experienced fewer symptoms than men who took placebo. More research is needed.
Rheumatoid arthritis (RA)
There are reports of people with RA who had fewer symptoms when they switched from a typical Western diet to a vegan diet with lots of uncooked berries, fruits, vegetables, nuts, roots, seeds, and sprouts containing quercetin and other antioxidants. But there is no evidence that the positive effects were due directly to antioxidants, and no evidence that quercetin supplements help treat RA.
Cancer
Scientists have long considered quercetin, and other flavonoids contained in fruits and vegetables, important in cancer prevention. People who eat more fruits and vegetables tend to have lower risk of certain types of cancer. Animal and test tube studies suggest that flavonoids have anti-cancer properties. Quercetin and other flavonoids have been shown in these studies to inhibit the growth of cancer cells from breast, colon, prostate, ovarian, endometrial, and lung tumors. One study even suggests that quercetin is more effective than resveratrol in terms of inhibiting tumor growth. Another found that frequent intake of quercetin-rich foods was associated with lower lung cancer risk. The association was even stronger among subjects who smoked more than 20 cigarettes daily, and a third suggests that quercetin slows tumor growth in the laboratory (in leukemia cells). More research is needed.