Leukocyte integrins.

Lymphocytes, monocytes and granulocytes, which are collectively known as 'leukocytes', circulate primarily within the vascular system and lymphoid tissue but are found in essentially all tissues of the body. This mobile lifestyle necessitates the constant making and breaking of adhesive contacts with targets in their immediate environment. The adhesion receptors termed integrins, which are expressed in abundance by leukocytes, are well suited to carry out the transient interactions in which these cells engage. Major advances in the leukocyte integrin field this year have been the realization of the extensive roles for alpha 4 integrins in leukocyte function, the solution of the crystal structure of an I domain and its identification as a major ligand-binding site, and the initial understanding of how divalent cations may function in an active integrin.