Monday, November 14, 2011

Immunology II: Lymphoid Organs and Cells

As mentioned in the last post, several types of cells derive from lymphoid progenitor cells.  These cells are generated in the bone marrow in general, but only B cells mature there (hence the name B cells).  In contrast, T cells migrate to the thymus where they mature.  After full maturation of both B and T cells, they circulate in the blood system and then enter the peripheral lymphoid organs.  The central lymphoid organs are the bone marrow where the lymphocytes are generated, whereas the peripheral lymphoid organs are where T cells mature and where the adaptive immune responds to various stimuli.

The peripheral lymphoid organs
First, we will consider the components of the peripheral lymphoid system.  The lymph nodes are glands right near each armpit that is where fluid collects from the lymph system.  Lymph drains into the lymph nodes via lymphatic vessels and consists of the extracellular fluid filtered from the blood.  Thus, lymph is kind of a surveillance system for the body.  The afferent lymphatic vessels carry lymph and cells from infected tissues and drain into the lymph node.

The lymph node itself has a unique structure, illustrated to the right.  The follicles are where B lymphocytes set up shop, and T cells exist in paracortical areas (T-cell zones).  Germinal centers within the lymph node are where B cells proliferate after they have been stimulated by T cells.  Several additional tissues are organized similar to the lymph node drawn to the right, and this structure facilitates interaction between B and T cells.

The spleen is another peripheral lymphoid organ that mostly works to break down dead red blood cells.  This destruction occurs in the red pulp of the spleen, but the spleen also has white pulp where lymphocytes enter and exist within the spleen.  Within the white pulp is the periarteriolar lymphoid sheath (PALS) that contains T cells and a B-cell corona.

The digestive system is a major route for infection and has several gut-associated lymphoid tissues (GALT).  Some of these tissues include the tonsils, adenoids, and the appendix.  The intestine also has its own GALT, namely the Peyer's patches, which collect antigen directly from inside the intestine using multi-fenestrated (M) cells.

Similar to the digestive tract, the respiratory tract has its own lymphoid tissue, called the bronchial-associated lymphoid tissue (BALT).

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