Secondary Lymphoid Organs

The secondary lymphoid organs include the lymph nodes, spleen, tonsils, and areas called Peyer's patches under the mucosa of the intestine (see chapter 13, fig. 13.36). These organs are strategically located across epithelial membranes in areas where antigens could gain entry to the blood or lymph. The spleen filters blood, whereas the other secondary lymphoid organs filter lymph received from lymphatic vessels.

Lymphocytes migrate from the primary lymphoid organs— the bone marrow and thymus—to the secondary lymphoid organs. Indeed, lymphocytes move constantly through the blood and lymph, going from lymphoid organ to lymphoid organ. This ceaseless travel increases the likelihood that a given lympho cyte, specific for a particular antigen, will be able to encounter that antigen. This process is aided, particularly in the case of T lymphocytes, by other cells that are known as antigen-presenting cells (see fig. 15.15). Secretion of chemokines (chemical attractants) by these cells increases the chances that the appropriate lymphocyte will encounter its specific antigen.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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