Color enhanced scanning electron micrograph (SEM) of ragweed pollen. Ragweed pollen is the number one cause of Hay Fever. An allergen, ragweed pollen causes the body's immune response system to react. The body perceives pollen to be a threat and releases histamine and other chemical mediators, leading to the symptoms of nasal allergy. Ragweed is native to North America and rapidly spreads through disturbed soil along driveways, roads, rail beds, sidewalks & cultivated fields.