Montelukast works by blocking cysteinyl leukotriene receptors, which can be activated by the chemical leukotriene D4. When the body is exposed to an allergen, immune cells (including mast cells and basophils) release leukotriene D4. Since montelukast blocks leukotriene D4 from activating the cysteinyl leukotriene receptors, the symptoms that are typically caused by leukotriene D4 are lessened.