Chapter 10 - Lymphoid System
The main function of the lymphoid system is to protect the body from pathogens (e.g., bacteria, virus, and parasites) and diseased cells (e.g., virus-infected or tumor cells).
The immune system is organized into organs and tissues that are functionally unified via blood and lymph vascular systems.
- Primary lymphoid organs (or central lymphoid organs) - sites where lymphocytes mature and become immunocompetent - B cells in bone marrow and T cells in the thymus.
- Secondary lymphoid organs (or peripheral lymphoid tissue) - mature lymphocytes are distributed via blood or lymph to secondary lymphoid organs (e.g., lymph nodes, spleen, and diffuse lymphoid tissues) where they await activation
Mucosa-associated lymphoid tissue (MALT) is peripheral lymphoid tissue that responds to antigens that enter the body through mucosae. This includes tonsils, lymphoid aggregates (nodules), and dispersed immune cells distributed in the connective tissue underlying the mucosae of the digestive and respiratory systems.
Lymph nodes are small, bean-shaped, encapsulated organs located throughout the body along lymphatic vessels. They serve as filters of lymph to remove pathogens or other foreign substances.
Tonsils are mucosa-associated lymphoid tissue (MALT) found in the underlying connective tissue surrounding the upper part of the pharynx. These diffuse, non-encapsulated nodules are named according to their location.
The spleen is the largest secondary lymphoid organ in the body. It contains two morphologically and functionally distinct compartments:
- Red pulp - filters the blood of foreign material and old or damaged red blood cells
- White pulp - site of immune reactions to blood-borne antigens
The spleen is also a storage site of red blood cells, platelets, and iron.
The thymus is a primary lymphoid organ in which T lymphocytes proliferate and mature before distribution to peripheral lymphoid tissues. It generates a diverse population of T lymphocytes that respond to foreign antigens, but not to self-antigens.
The thymus continues to enlarge up to puberty but then undergoes a slow involution becoming replaced by adipose tissue.