Most people get allergic conjunctivitis when their eyes come into contact with an allergen - a substance which makes the body’s immune system overreact. The eye becomes sore, inflamed and sometimes painful. Symptoms occur because the overreacting immune system makes the body release histamine and other active substances by mast cells - these cause dilation of blood vessels (blood vessels expand, widen), which irritates the nerve endings and causes increased secretion of tears.