what causes a runny nose and sneezing
This is caused by the release of histamine from allergic cells in the nasal mucous membranes, known as allergic rhinitis. Histamine acts on various nerves within the nasal mucous membranes, which causes a person to sneeze (and it also causes other typical allergy-related problems, like a runny nose). Sneezing may help prevent the inhalation of dangerous substances into the lungs, including allergens.
When these membranes become irritated, it causes you to sneeze. Sneezing can be triggered by allergens, viruses like the common cold or flu, nasal irritants, inhalation of corticosteroids through a nasal spray, or drug withdrawal.