Naive Lymphocytes Recirculate to Secondary Lymphoid Tissue

A naive lymphocyte is not able to mount an immune response until it has been activated to become an effector cell. Activation of a naive cell occurs in specialized microenvironments 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 activation. 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 recirculating 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 enlarge into lymphoblasts. Activation takes about 48 h, and during this time the blast cells are retained in the paracortical

Naive T cell

Mucosal-homing effector T cell

Skin-homing effector T cell

Mucosal Homing Effector Cell

GlyCAM-1

GlyCAM-1

Mucosal-homing effector T cell

Skin-homing effector T cell

Cells Skin Homing

Intestinal lamina propria endothelium

Skin dermal venule endothelium

Intestinal lamina propria endothelium

Skin dermal venule endothelium

FIGURE 15-5

Examples of homing receptors and vascular addres-sins involved in selective trafficking of naive and effector T cells. (a) Naive T cells tend to home to secondary lymphoid tissues through their HEV regions. The initial interaction involves the homing receptor L-selectin and mucin-like cell-adhesion molecules such as CD34 or GlyCAM-1 ex-

- Tertiary extralymphoid tissue -

pressed on HEV cells. (b, c) Various subsets of effector T cells express high levels of particular homing receptors that allow them to home to endothelium in particular tertiary extralymphoid tissues. The initial interactions in homing of effector T cells to mucosal and skin sites are illustrated.

region of the secondary lymphoid tissue. During this phase, called the shut-down phase, the antigen-specific lymphocytes cannot be detected in the circulation (Figure 15-6). Rapid proliferation and differentiation of naive cells occurs during the shut-down phase. The effector and memory cells that are generated by this process then leave the lymphoid tissue and begin to recirculate.

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Responses

  • thelma
    Are mucosal lymphoid tissue secondary or tertiary?
    8 years ago
  • Hanna-Mari
    Where are naive lymphocytes?
    8 years ago
  • kevin
    Where are naiive lymphocytes activated in?
    7 years ago

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