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Neutrophils and monocytes migrate to sites of infection and tissue injury: inflammation

Lymph High endothelial node venule (HEV)

FIGURE 3-1 The main functions served by leukocyte migration from blood into tissues. A, Neutrophils and monocytes that arise in the bone marrow are recruited into tissue sites of infection or injury, where they eliminate infectious pathogens, clear dead tissues, and repair the damage. B, Naive lymphocytes that arise in bone marrow or thymus home to secondary lymphoid organs, such as lymph nodes (or spleen, not shown), where they become activated by antigens and differentiate into effector lymphocytes. C, Effector lymphocytes arising in secondary lymphoid organs migrate into tissue sites of infection, where they participate in microbial defense.

Infected or injured tissue

Effector and memory T cells migrate into sites of infection and tissue injury: cell-mediated immunity

FIGURE 3-1 The main functions served by leukocyte migration from blood into tissues. A, Neutrophils and monocytes that arise in the bone marrow are recruited into tissue sites of infection or injury, where they eliminate infectious pathogens, clear dead tissues, and repair the damage. B, Naive lymphocytes that arise in bone marrow or thymus home to secondary lymphoid organs, such as lymph nodes (or spleen, not shown), where they become activated by antigens and differentiate into effector lymphocytes. C, Effector lymphocytes arising in secondary lymphoid organs migrate into tissue sites of infection, where they participate in microbial defense.

ADHESiON molecules ON LEUKOCYTES AND ENDoTHEUAL cells iNvoLvED iN LEUKOCYTE RECRuiTMENT 39

Leukocyte recruitment from the blood into tissues depends first on adhesion of the leukocytes to the endothelial lining of postcapillary venules and then movement through the endothelium and underlying basement membrane into the extravascular tissue. This is a multistep process in which each step is orchestrated by different types of molecules, including chemokines and adhesion molecules. The same basic process occurs for different types of leukocytes (neutrophils, monocytes, and naive and effector lymphocytes) homing to different types of tissues (secondary lymphoid organs, infected tissues), although the specific chemokines and adhesion molecules vary in ways that result in different migration properties for each cell type. Before describing the process, we discuss the properties and functions of the adhesion molecules and chemokines that are involved in leukocyte recruitment.

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