While a malfunctioning dryer vent cover can cause house fires and elevated humidity, it does not necessarily cause allergic reactions. Lint and moisture in the duct can cause the vent to keep from opening properly, making the whole system dangerous.
If the external vent is not opening, then the moisture from the dryer doesn't leave the system. Lint can gather in the ductwork leading from the dryer to the outside vent, trapping moisture and keeping the clothes from drying efficiently. The dryer then has to work harder with less air, often leading to higher temperatures. The lint can catch on fire, threatening the entire home.