In Vivo Sites for Induction of Humoral Responses

In vivo activation and differentiation of B cells occurs in defined anatomic sites whose structure places certain restrictions on the kinds of cellular interactions that can take place.

Primary follicle

Afferent lymphatic > vessels

Primary follicle

Afferent lymphatic > vessels

Peripheral Lymph Nodes Humoral Response

Secondary follicle

lymphatic vessel

Plasma-cell secretion of antibody

FIGURE 11-16

Secondary follicle

lymphatic vessel

Plasma-cell secretion of antibody

FIGURE 11-16

Schematic diagram of a peripheral lymph node showing anatomic sites at which various steps in B-cell activation, proliferation, and differentiation occur. The cortex is rich in B cells and the paracortex in T cells; both B and T cells are present in large numbers in the medulla. A secondary follicle contains the follicular mantle and a germinal center.

When an antigen is introduced into the body, it becomes concentrated in various peripheral lymphoid organs. Blood-borne antigen is filtered by the spleen, whereas antigen from tissue spaces drained by the lymphatic system is filtered by regional lymph nodes or lymph nodules. The following description focuses on the generation of the humoral response in lymph nodes.

A lymph node is an extremely efficient filter capable of trapping more than 90% of any antigen carried into it by the afferent lymphatics. Antigen or antigen-antibody complexes enter the lymph nodes either alone or associated with antigen-transporting cells (e.g., Langerhans cells or dendritic cells) and macrophages. As antigen percolates through the cellular architecture of a node, it will encounter one of three types of antigen-presenting cells: interdigitating dendritic cells in the paracortex, macrophages scattered throughout the node, or specialized follicular dendritic cells in the follicles and germinal centers. Antigenic challenge leading to a humoral immune response involves a complex series of events, which take place in distinct microenvironments within a lymph node (Figure 11-16). Slightly different pathways may operate during a primary and secondary response because much of the tissue antigen is complexed with circulating antibody in a secondary response.

Once antigen-mediated B-cell activation takes place, small foci of proliferating B cells form at the edges of the T-cell-rich zone. These B cells differentiate into plasma cells secreting IgM and IgG isotypes. Most of the antibody produced during a primary response comes from plasma cells in these foci. (A similar sequence of events takes place in the spleen, where initial B-cell activation takes place in the T-cell-rich periarterial lymphatic sheath, PALS; see Figure 2-19).

A few days after the formation of foci within lymph nodes, a few activated B cells, together with a few TH cells, are thought to migrate from the foci to primary follicles. These follicles then develop into secondary follicles, which provide a specialized microenvironment favorable for interactions between B cells, activated TH cells, and follicular dendritic cells. Note that although they share the highly branched morphology of dendritic cells derived from bone marrow, follicu-lar dendritic cells do not arise in bone marrow, do not express class II MHC molecules, and do not present antigen to CD4+ T cells. Follicular dendritic cells have long extensions, along which are arrayed Fc receptors and complement receptors. These receptors allow follicular dendritic cells to retain and present antigen-antibody complexes for long periods of time, even months, on the surface of the cell. Activated B cells (together with some activated TH cells) may migrate towards the center of the secondary follicle, forming a germinal center.

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