Lymphoid Organs Flashcards
Lymphoid Organs
Red bone marrow Thymus Spleen Lymph nodes Diffuse Lymphatic tissue: tonsils, adenoids, Peyers patches
Lymphoid tissue is _________ with rich ________ produced by_____________, which have little cytoplasm thus stain dark in H & E.
connective tissue; reticular fibrils; fibroblast and lymphocytes
Two main architecture types of lymphoid tissue:
(1) diffuse – uniform appearance; (2) follicular – consists of lymphoid follicles.
Two structural types of lymphoid tissue:
- Encapsulated – with connective tissue capsule including thymus, spleen, and lymph nodes
- Unencapsulated or partially encapsulated - Tonsils, Peyer’s patches, lymphoid nodules in GI, respiratory, urinary & reproductive tracts.
Consist of densely packed lymphocytes located in all peripheral lymphoid tissues. Most lymphocytes are B cells.
Two distinct areas:
Lymphoid nodules (Follicles)
- Mantle – darker stained, mainly small, resting lymphocytes
- Germinal Center
- -Defined as “reactive” or “secondary” lymphoid follicles
- -Lighter stained, larger and activated B cells
Functions of the Lymphoid System
- Draining excess interstitial fluid & plasma proteins from tissue spaces
- Transporting dietary lipids & vitamins from GI tract to the blood
- Facilitating immune responses
–recognize microbes or abnormal cells & responding by killing them directly or secreting antibodies that cause their destruction
Lymphoid Organs
- Central (primary):
- Peripheral (secondary):
- Central (primary) lymphoid organ: where lymphocytes undergo maturation
- red bone marrow gives rise to mature B cells
- thymus is site where pre-T cells from red bone marrow mature
- Peripheral (secondary) lymphoid organs:
- lymph nodes
- spleen
- Peyer’s patches
- lymphoid nodules in GI and other tracts
•Immature lymphocyte precursors (cells) develop initially in _________
bone marrow
- Some immature cells develop into ________ in bone marrow and migrate to other organs
- Some immature cells develop into _______ that must migrate to the thymus for further development
B lymphocytes; “pre-T” cells
Special Features of Lymphocytes
- Each B lymphocyte produces numerous identical antibodies that bind one specific antigen.
- Each T lymphocyte produces numerous identical membrane-bound receptors that bind one specific antigen.
Lymphatic Terminology
- Resistance
- Nonspecific resistance
- Specific resistance
- Resistance is the ability to ward off disease
- lack of resistance is termed susceptibility
- Nonspecific resistance to disease
- general defensive mechanisms effective on a wide range of pathogens (disease producing microbes)
- Specific resistance or immunity is the ability to fight a specific pathogen
- cell-mediated immunity
- antibody-mediated immunity
A primary lymphoid organ in that it supplies other lymphoid organs and tissues with T-lymphocytes.
Thymus
•The thymus is enclosed by a thin CT capsule from which numerous septae extend into the thymus subdividing 2 lobes into numerous lobules.
Organiation of Thymus
- Capsule, septa, and lobule
- Each lobule is divided into:
- Cortex:
- Darker peripheral zone with densely packed lymphocytes (no lymphatic nodules)
- Medulla:
- Lighter central zone with fewer lymphocytes but more epithelial reticular cells.
the functional tissue of an organ
parenchyma
Cell Types in the Thymus
- Pre-T cells (bone marrow origin)
- T lymphocytes (T cells)
- Epithelial reticular cells
- Thymic nurse cell
- Dendritic cells
supporting cellular framework of connective tissue which contains blood vessels and nerves
Stroma
•Parenchyma of thymic cortex:
consists mostly of ___________
- T cell receptor (TCR) genes are rearranged so that mature T cells obtain specific surface markers.
•Stroma: - sparse, delicate epithelial cells obscured by _________
- epithelial cells form support structure for developing T cells
- play important role in isolating T cells from foreign antigens during their development.
developing T lymphocytes
lymphocytes
Form the main structural support elements of the thymic parenchyma, which LACKS COLLAGEN!!
Epithelial Reticular Cells of Thymus
- Formed by epithelial reticular cells (ERCs)
- ERCs secrete thymic stromal lymphopoietin (TSL)
TSL stimulates dendritic cells to hasten the maturation of regulatory T cells that suppress auto-immune reactions against “self”
Hassall’s Corpuscles in Medulla
Functions of Epithelial Reticular Cells
- A subtype of Epithelial Reticular Cells expresses a huge variety of proteins under the command of a transcription factor called AIRE (Autoimmune REgulatory protein)
- About 3000 proteins, coded for by ~10% of all genes, are expressed!
- These proteins are a sampling of tissue-specific (“self”) proteins found throughout the body
- If a T-cell binds tightly to any of these “self” proteins, it is killed via apoptosis
- Epithelial reticular cells have a critical role in attracting lymphocytes to the thymus and guiding their development
- Lack of functioning epithelial reticular cells has a devastating effect on thymic function
- Mutations in an epithelial transcription factor called Foxn-1 damages both epithelial cells and the thymus of the “nude mouse”
“Nude mouse” has defects in keratin-containing hair follicles and in keratin- containing thymic
Thymic Nurse Cells
Specialized subtype of reticular epithelial cells
Engulf as many as 20 lymphocytes and carry them within their cytoplasm
Stimulate apoptosis in a fraction of these lymphocytes and release the others
Originate in bone marrow
Stimulate development of regulatory T cells that suppress reactions to self-proteins
Thymic Dendritic Cells
