15-8 The MALT and spleen are collections of immune cells that work to fight infection

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• MALT is a collection of immune cells that stands guard just beneath outer surfaces of the body to defend against invasive pathogens.
• The spleen is a highly organized collection of red blood cells, B cells and T cells. B and T cells can be activated and released by the spleen to fight infection.

Mucosal-Associated Lymphoid Tissue (MALT) is scattered throughout the connective tissues of the body, but especially beneath moist epithelial membranes such as those that line the upper respiratory tract, intestine and urinary tract. MALT is strategically distributed to help the body prevent infection by organisms that have penetrated beyond the mucosal surface. MALT consists of small masses of lymphatic tissue (up to a millimeter in diameter) containing mainly lymphocytes. These tissues are far less organized than the lymph nodes. Most MALT consists of small groups of cells, but in certain areas it is found in large clusters. For example, large aggregates of MALT occur in the wall of the lower portion (ileum) of the small intestine and are known as Peyer's patches. Tonsils and adenoids are also aggregates of MALT that protect your body from microorganisms present in the upper respiratory tract.

The spleen is a very important secondary lymphoid organ. Individuals who have had their spleens removed due to rupture caused by a car accident or atrophy from sickle cell anemia can lead nearly normal lives, but they tend to be more susceptible to infection. Some functions of the spleen are similar to those of the lymph nodes; however, it also produces lymphocytes and removes senescent (old) red blood cells from the circulation.

The spleen, shown in Figure 15-5, is highly organized and is a repository for immune cells. It plays an important role in response to pathogens. The spleen contains a circulatory system allowing access to it through the blood. Cells in the spleen organize around the blood vessels into two tissues: red pulp and white pulp. Red pulp contains mainly red blood cells, and white pulp is made up predominantly of lymphocytes. The white pulp focuses tightly around the arterioles of the spleen while the red pulp fills the rest of the interstitial space. The location of various lymphocytes in the white pulp further differentiates it. T cells are found near the arterioles and further away are areas of B cells. The spleen is a major area for B cells to congregate in the body, where they wait to be activated by antigens. During an illness, activated lymphocytes are released from the spleen to fight the infection.

This completes our tour of the anatomy of the immune system. In the next section we will more specifically examine how cells and compounds in these tissues recognize and attack invading pathogens.

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