Immune responses

The previous sections have indicated that antigen-driven T- and B-cell activation leading to antibody production or the generation of effector T cells requires a complex series of interactions between cells. In most primary immune responses this will occur only within secondary lymphoid tissue. The lymph nodes, mucosal lymphoid tissues and spleen contain the bulk of secondary lymphoid tissue. The outward appearance of the various secondary lymphoid tissues is markedly different, but their fine structure has many common features. In this section, the way in which immune responses occur in the different compartments of lymphoid tissues are considered.

There are two main sites of T-cell activation and T cell-dependent B-cell activation. These are the T zones and follicles. The general structure of a lymph node (Figure 20.10) is given first to provide a context for the detailed description that follows of the sites where immune responses occur and the molecular basis of T- and B-cell activation.

Lymph nodes have an afferent lymphatic supply that is fed by lymph draining the extravascular tissue spaces. This lymph

Afferent lymphatics

Afferent lymphatics

Subcapsular sinus Marginal zone

T zones Follicles Intranodal lymphatic Medullary cords

Subcapsular sinus Marginal zone

T zones Follicles Intranodal lymphatic Medullary cords

Figure 20.10 The main compartments of a lymph node. Note that the size of the marginal zone is variable; although it is often obvious in mesenteric lymph nodes, it may not be obvious in small nodes such as the popliteal nodes, particularly if these have not been sites of recent immune responses.

provides the main source of antigen for the node. Langerhans cells activated by local disturbance take up and process antigen, and then pass through afferent lymph into lymph nodes (Figure 20.11). The afferent lymph passes into the subcapsular sinus, which forms a lake of lymph that covers the cortical surface of the node. From the subcapsular sinus, lymph passes through intranodal lymph sinuses that surround and separate the conelike segments that make up the solid tissue of the node. The intranodal lymphatics, as they pass the follicles and T zone, are difficult to see as they are crossed by fibrous cords. Attached to these and the walls of the tissue cones are macrophages and other poorly defined cells. Increased numbers of these cells in the intranodal lymph sinuses and similar cells in the subcapsular sinus are described by histopathologists as sinus histiocytosis. The intranodal lymph sinuses passing the medullary cords contain fewer fixed cells and are easier to identify. In the medulla, the intranodal lymph sinuses feed into the efferent lymphatic vessel that returns the lymph to the venous blood supply; in the case of the gut and lower half of the body, via the thoracic duct to the left subclavian vein.

The solid tissue of the node is made up of variable numbers of roughly cone-like segments (Figure 20.10). The base of each cone abuts onto the subcapsular lymph sinus in the cortex of the node and the apex is in the medulla. These cones fit together, but are separated by the intranodal lymphatic sinuses, to form the roughly kidney-shaped structure of lymph nodes. The cones have three main zones: the follicles in the cortex, the T zones and the medullary cords. The medullary cords form a convoluted apex to the cone. The contents and functions of each of these zones are described in detail in subsequent sections. The blood supply to the node enters and leaves the node through the medulla, and the specialized high endothelial venules through which recirculating B and T cells and newly produced virgin B cells enter the node are located in the T zones.

Was this article helpful?

0 0
How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Post a comment