Mast Cell Activation Syndrome: The Hidden Culprit Behind Mysterious Multi System Symptoms Dr. Jill Carnahan, MD Mast cell activation syndrome, neck pain and the vagus nerve Mast Cell Activation Syndrome and Its Impact on the Lymphatics Mast cells are hypersensitive immune cells that release histamine and other inflammatory mediatorsoften in response to oxidative stress, environmental toxins, and immune imbalance. Many patients with MCAS have low levels of glutathione.