The WBC then begin to squeeze between the contracted enothelial cells and migrate in an ameba-like fashion into the extravascular space (a process called diapedesis). Once in tissue, the WBC's are attracted by activated complement and and begin to migrate towards the site of injury or infection. WBC's (specifically PMN's) apparently have surface receptors for chemotactic agents. (C5a, TNF, IL-8, LtB4, IL-1, IFN-y) which causes them to move in the direction of increasing concentrations of the chemotactic substance (process is called chemotaxis). Intially, the first group to arrive at the site of injury are neutrophils (PMNs). Later, macrophages become more numerous. In certain parasitic infections, eosinophils predominate. In viral infections, lymphocytes rather than neutrophils usually predominate.