The sulfur compounds found in garlic and onions have antiviral and antibacterial qualities and are well known to boost immunity. Garlic, onions and citrus fruits like lemon and oranges contain a natural antihistamine called quercetin, which counteracts the histamines that cause nasal allergies. Eating spices such as cayenne, ginger, onion, garlic, and horseradish help to thin mucus and relieve sinus congestion.