The adhesion of leukocytes to endothelial cells is mediated by complementary adhesion molecules on the two cell types whose expression is enhanced by secreted proteins called cytokines. Cytokines are secreted by cells in tissues in response to microbes and other injurious agents, thus ensuring that leukocytes are recruited to the tissues where these stimuli are present. The initial rolling interactions are mediated by a family of proteins called selectins. There are three types of selectins: one expressed on leukocytes (L-selectin), one on endothelium (E-selectin), and one in platelets and on endothelium (P-selectin). Ref: Robbins 8th edition, Chapter 2.