After graduating from medical school, he trained in the laboratory of Dr. Samuel C. Silverstein at Columbia University. While working with Dr. Silverstein, Dr. El Khoury identified microglial scavenger receptors as key microglial receptors that mediate interactions of these brain cells with β-amyloid. Following his initial training in research, Dr. El Khoury returned to clinical medicine and completed a residency in Internal Medicine at the Beth Israel Hospital, and a fellowship in Infectious Diseases at the Massachusetts General Hospital and Brigham and Women’s Hospital. As part of his fellowship training, he worked in the laboratory of Dr. Andrew Luster studying the role of chemokines in the accumulation of microglia in the Alzheimer’s brain.