ABSTRACT
In the mature organism, adhesion interactions between cells are generally strengthened by the formation of cell junctions. There are four major classes of cell adhesion molecule: Cadherins are the only class to participate in homophilic binding; Integrins are adhesion heterodimers, Selectins mediate transient cell–cell interactions in the bloodstream and Ig-CAMs possess immunoglobulin-like domains. Some cell junctions mechanically attach cells (and their cytoskeletons) to their neighbors or to the ECM using dedicated proteins, notably cadherins (cell-cell joining) and integrins (principally for cell-ECM joining). Tight junctions are primarily designed to act as barriers, and are especially prevalent in the epithelial cell sheets lining the free surfaces and all cavities of the body. There are many different types of cells in adult humans, but they are organized into just a few major types of tissue. Active immune system cells originate from hematopoietic stem cells produced by the bone marrow, but are conveyed within blood to different tissues in the body.
