Respiratory system dysfunction appears in predictable patterns, including airway obstruction, reactivity, inflammation, and susceptibility to infection. When pollutants and infection damage the lungs, the airway or the alveoli can become weak, as in obstructive lung disease, collapsing when air is trying to leave the lungs. This can lead to an accumulation of carbon dioxide in the blood, along with diminished oxygen, and lead to organ dysfunction throughout the body. In people predisposed to allergy, airborne allergens can trigger skewed immune responses, typified by a high ratio of TH2 to TH1 helper T cells, and the activation of mast cells that release histamine, leading to airway contraction, mucus production, and compromised lung function. Dysfunction of the innate immune system, and front-line NK cells and phagocytes (macrophages and dendritic cells), can lead to recurrent airway infections. Our responses to allergens and pathogens can both lead to airway inflammation, which in turn increases the likelihood of further allergy, infection, inflammation, and obstruction, in a positive feedback cycle.