It is well established that cluster headache shows impaired functions at their neuroimmunomodulatory system level. Defect in receptor expression for 5-HT, IL-1 and IL-2 have been found in these patients. Sumatriptan, a molecule with agonistic activity for 5-HT1D receptor, truncates cluster headache attacks in 74% of patients. Flow cytometric analysis of monocytes expressing 5-HT receptor in cluster headache patients showed different trends clearly correlated with the clinical response to sumatriptan. Our findings strongly support the concept that cluster headache patients who are non responders to sumatriptan could present a block in their 5-HT receptor possibly due to specific autoantibodies for this receptor site.