Lymphocyte Homing Is Directed by Receptor Profiles and Signals

The general process of lymphocyte extravasation is similar to neutrophil extravasation. An important feature distinguishing the two processes is that different subsets of lymphocytes migrate differentially into different tissues. This process is called trafficking, or homing. The different trafficking patterns of lymphocyte subsets are mediated by unique combi-

High Endothelial Venules

passing across the wall

Basement membrane

High endothelium

passing across the wall

Basement membrane

High endothelium

Basement Membrane Lymphocyte

FIGURE 15-4

Diagram Lymph Node

FIGURE 15-4

(a) Schematic cross-sectional diagram of a lymph-node postcapillary venule with high endothelium. Lymphocytes are shown in various stages of attachment to the HEV and in migration across the wall into the cortex of the node. (b) Scanning electron micrograph showing numerous lymphocytes bound to the surface of a high-endothelial venule. (c) Micrograph of frozen sections of lymphoid tissue. Some 85% of the lymphocytes (darkly stained) are bound to HEVs (in cross section), which comprise only 1%-2% of the total area of the tissue section. [Part (a) adapted from A. O. Anderson and N. D. Anderson, 1981, in Cellular Functions in Immunity and Inflammation, J. J. Oppenheim et al. (eds.), Elsevier, North-Holland; part (b) from S. D. Rosen and L. M. Stoolman, 1987, Vertebrate Lectins, Van Nostrand Reinhold; part (c) from S. D. Rosen, 1989, Curr. Opin. Cell Biol. 1:913.]

nations of adhesion molecules and chemokines; receptors that direct the circulation of various populations of lymphocytes to particular lymphoid and inflammatory tissues are called homing receptors. Researchers have identified a number of lymphocyte and endothelial cell-adhesion molecules that participate in the interaction of lymphocytes with HEVs and with endothelium at tertiary sites or sites of inflammation (see Table 15-1). As is described later, in the section on chemokines, these molecules play a major role in determining the heterogeneity of lymphocyte circulation patterns.

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