Mucous membranes are found throughout the body, lining various cavities and covering surfaces that come into contact with the external environment. One of their primary functions is to act as a barrier against harmful substances, such as bacteria and viruses. However, mucous membranes can also become irritated or inflamed in response to allergens. When this happens, mucous membranes produce excess mucus in an attempt to flush out the allergen. This can lead to symptoms such as runny nose, watery eyes, and sneezing. In severe cases, mucous membrane inflammation can cause difficulty breathing.