Monocytes stem from myeloid progenitors in bone marrow. There are three different populations of human monocytes: classical, intermediate, and non-classical. Human peripheral blood monocytes are defined by their expression of the cell surface antigens CD14 , CD16 , CD64 and the expression of chemokine receptors CD192 (CCR2) and CX3CR1. Monocytes express the leukocyte adhesion molecule CD226 and interaction of CD antigens with CD155 regulates monocyte migrations. Monocytes play a key role in eliminating invading bacteria, virus, fungi, and protozoans, and have negative effects on the pathogenesis of inflammatory and degenerative diseases.