Effector and Memory Lymphocytes Adopt Different Trafficking Patterns

The trafficking patterns of effector and memory lymphocytes differ from those of naive lymphocytes. Effector cells tend to home to regions of infection by recognizing inflamed vascular endothelium and chemoattractant molecules that are generated during the inflammatory response. Memory lymphocytes, on the other hand, home selectively to the type of tissue in which they first encountered antigen. Presumably this ensures that a particular memory cell will return to the tissue where it is most likely to reencounter a subsequent threat by the antigen it recognizes.

Effector and memory cells express increased levels of certain cell-adhesion molecules, such as LFA-1, that interact with ligands present on tertiary extralymphoid tissue (such as skin and mucosal epithelia) and at sites of inflammation, allowing effector and memory cells to enter these sites. Naive cells lack corresponding cell-adhesion molecules and do not home to these sites. Inflamed endothelium expresses a number of adhesion molecules, including E- and P-selectin and the Ig-superfamily molecules VCAM-1 and ICAM-1, that bind to the receptors expressed at high levels on memory and effector cells.

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Days following antigen exposure

Days following antigen exposure

FIGURE 15-6

T-cell activation in the paracortical region of a lymph node results in the brief loss of lymphocyte recirculation. During this shut-down phase, antigen-specific T cells cannot be detected leaving the node in the efferent lymph.

Unlike naive lymphocytes, subsets of the memory and effector populations exhibit tissue-selective homing behavior. Such tissue specificity is imparted not by a single adhesion receptor but by different combinations of adhesion molecules. For example, a mucosal homing subset of memory/effector cells has high levels of the integrins LPAM-1 (pa4p7) and LFA-1 (aLb2), which bind to MAdCAM and various ICAMs on intestinal lamina propria venules (see Figure 15-5b). However, these cells avoid direction to secondary lymphoid tissues because they have low levels of the L-selectin that would facilitate their entry into secondary lymphoid tissue. A second subset of memory/effector cells displays preferential homing to the skin. This subset also expresses low levels of L-selectin but displays high levels of cutaneous lymphocyte antigen (CLA) and LFA-1, which bind to E-selectin and ICAMs on dermal venules of the skin (see Figure 15-5c). Although effector and memory cells that express reduced levels of L-selectin do not tend to home through HEVs into peripheral lymph nodes, they can enter peripheral lymph nodes through the afferent lymphatic vessels.

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