This is followed by the blood–brain barrier (BBB) breakdown. BBB is a tight vascular barrier between the blood and brain that should prevent the passage of antibodies through it, but in MS patients it does not work. For unknown reasons special areas appear in the brain and spine, followed by leaks in the blood–brain barrier where immune cells infiltrate. This leads to the entrance of macrophages into the CNS, triggering the beginning of an immune-mediated attack against myelin. Gadolinium cannot cross a normal BBB and, therefore, Gadolinium-enhanced MRI is used to show BBB breakdowns.