Leukocyte Migration and Inflammation CHAPTER 15 343 Basement across 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 mi 学 crograph showing numerous lymphocytes bound to the surface of a high-endothelial venule (c)Micrograph of frozen sections of lym- phoid tissue. Some 85% of the lymphocytes(darkly stained)are 代个 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 Immi lity and Inflammation, 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 nations of adhesion molecules and chemokines; receptors lymph nodes, Peyer's patches, tonsils, and spleen) that direct the circulation of various populations of lympho- these microenvironments, dendritic cells capture cytes to particular lymphoid and inflammatory tissues are and present it to the naive lymphocyte, resulting in its activa called homing receptors. Researchers have identified a num- tion. Naive cells do not exhibit a preference for a particular ber of lymphocyte and endothelial cell-adhesion molecules type of secondary lymphoid tissue but instead circulate hat participate in the interaction of lymphocytes with HEVs indiscriminately to secondary lymphoid tissue throughout and with endothelium at tertiary sites or sites of inflamma- the body by recognizing adhesion molecules on HEVs tion(see Table 15-1). As is described later, in the section The initial attachment of naive lymphocytes to HEVs is chemokines, these molecules play a major role in determin generally mediated by the binding of the homing receptor ing the heterogeneity of lymphocyte circulation patterns L-selectin to adhesion molecules such as GlyCAM-1 and CD34 on HEVs(Figure 15-5a). The trafficking pattern of Naive Lymphocytes Recirculate naive cells is designed to keep these cells constantly recircu to Secondary Lymphoid Tissue lating through secondary lymphoid tissue, whose primary function is to trap blood-borne or tissue-borne antigen. A naive lymphocyte is not able to mount an immune re- Once naive lymphocytes encounter antigen trapped sponse until it has been activated to become an effector cell. secondary lymphoid tissue, they become activated and en- Activation of a naive cell occurs in specialized microenviron- large into lymphoblasts. Activation takes about 48 h, and ments within secondary lymphoid tissue(e.g, peripheral during this time the blast cells are retained in the paracorticalnations of adhesion molecules and chemokines; receptors that direct the circulation of various populations of lympho￾cytes to particular lymphoid and inflammatory tissues are called homing receptors.Researchers have identified a num￾ber 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 inflamma￾tion (see Table 15-1). As is described later, in the section on chemokines, these molecules play a major role in determin￾ing the heterogeneity of lymphocyte circulation patterns. Naive Lymphocytes Recirculate to Secondary Lymphoid Tissue A naive lymphocyte is not able to mount an immune re￾sponse until it has been activated to become an effector cell. Activation of a naive cell occurs in specialized microenviron￾ments within secondary lymphoid tissue (e.g., peripheral lymph nodes, Peyer’s patches, tonsils, and spleen). Within these microenvironments, dendritic cells capture antigen and present it to the naive lymphocyte, resulting in its activa￾tion. Naive cells do not exhibit a preference for a particular type of secondary lymphoid tissue but instead circulate indiscriminately to secondary lymphoid tissue throughout the body by recognizing adhesion molecules on HEVs. The initial attachment of naive lymphocytes to HEVs is generally mediated by the binding of the homing receptor L-selectin to adhesion molecules such as GlyCAM-1 and CD34 on HEVs (Figure 15-5a). The trafficking pattern of naive cells is designed to keep these cells constantly recircu￾lating through secondary lymphoid tissue, whose primary function is to trap blood-borne or tissue-borne antigen. Once naive lymphocytes encounter antigen trapped in a secondary lymphoid tissue, they become activated and en￾large into lymphoblasts. Activation takes about 48 h, and during this time the blast cells are retained in the paracortical Leukocyte Migration and Inflammation CHAPTER 15 343 Lymphocytes passing across the wall Basement membrane High endothelium (a) (b) (c) 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 mi￾crograph showing numerous lymphocytes bound to the surface of a high-endothelial venule. (c) Micrograph of frozen sections of lym￾phoid 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 Immu￾nity 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.]