Lymphoreticular system Flashcards
Learning outcomes
- List the cells involved in the immune system.
- Describe how these cells are organised to form lymphoid tissue.
- List the organs involved in the immune system, noting which are involved in the immune response.
- Describe the main structural features of these organs, noting the proportions of lymphoid tissue and the type of supporting network involved.
- Relate their histological structure and anatomical position to the role that each plays in monitoring foreign material.
Overview of LR system
- Specialised form of CT
- Groups of cells, tissues and organs that monitor body surfaces and internal fluid compartments, react to the presence of potentially harmful substances by initiating an immune response
- Lymphatic vessels connect parts of the system to the blood vascular system
- These vessels remove substances and fluid from the extracellular spaces of the CT, thus producing lymph
Overview of LR system- 2
Lymphatic tissues serve as sites where lymphocytes proliferate, differentiate and mature
-In addition, in the thymus, bone marrow and MALT, lymphocytes are “educated” to recognise and destroy specific antigens (Antigen–any substance that can induce a specific immune response)
-These are now immunocompetent cells that can distinguish “self” and “non-self”
-The body is constantly exposed to pathogenic organisms and hazardous substances from the external environment
In addition, changes can occur in cells, e.g. transformation of normal cells into cancerous cells, that give them characteristics of foreign cell
Primary and secondary tissues/organs of LR system
Primary-
•Where lymphocytes are produced (Bone marrow) mature
•Bone marrow (red bm, contains stem cells (Bcells))
•Thymus (T-Lymphocytes) T cells complete maturation/differentiation in T, originate in BM
Secondary
Peripheral structures that maintain mature but naïve lymphocytes and initiate immune response - final maturation process (antigen-dependent activation)
•Lymph nodes
•Spleen
-Tonsils
•Mucosa-associated lymphoid tissue (MALT) e.g. gut-associated lymphoid tissue (GALT)
Lymphocytes
B-lymphocytes
•Involved in humoral immunity
•Produce antibodies (proteins that recogniseforeign substances (antigen) and attach themselves to it
•Mark invaders for destruction by other immune cells
•In most mature form is termed a plasma cell
T-lymphocytes
•Involved in cell-mediated immunity (activation of phagocytes, antigen-specific cytotoxic T lymphocytes and the release of cytokines in response to an antigen; targets transformed and virus-infected cells for destruction by specific “killer” cells)•Do not produce antibodies
Supporting cells of the LR system
-Interact with lymphocytes and play important roles in the presentation of antigen to lymphocytes and the regulation of immune responses
Reticular cells- star shaped cells
•Secrete type III collagen (reticular fibres) and ground substance
•Form intricate network in which the immune cells reside
Epithelioreticular cells
•Specific to the thymus
•Assist with T lymphocyte education
Dendritic Cells
•Antigen-presenting cells (APCs)
•Monitor local environment for foreign substances which they process and present to lymphocytes
Macrophages
•Work both as APCs and phagocytes
Lymphoid follicle
- Spherical accumulation of lymphocytes
- Pale staining germinal centre (activated B immunoblasts)
- Dark stained outer corona/mantle–represents a ring of small transient B lymphocytes (response to antigen)
- Occur in all lymphoid organs except the THYMUS
See histological examples
Mucosa- associated lymphoid tissue (MALT)
- Lymphoid follicles occur throughout the gastrointestinal (GALT), respiratory (BALT) and urinary systems
- Found in lamina propria deep to the epithelium
- Strategically located to monitor antigen that crosses the epithelium and initiate an immune response
Tonsils
4 types in human-
Pharyngeal: lined with PSEUDOSTRATIFIED COLUMNAR CILIATED EPITHELIUM (part of resp tract)
Palatine: embedded in pharynx wall- strat. squamous non-keratinising epithelium that dips into underlying tissue- TONSILLAR CRYPT
Lingual: stratified squamous non keratinising epithelium
Tubal: ciliated pseudostratified epithelium.
Part of MALT
Lymph nodes
Spherical or kidney-shaped encapsulated organs, ranging in size from 1mm to 1-2 cm
- Deals with lymph-borne antigen -all tissue fluid-derived lymph is filtered by at least one node before returning to the circulation
- Concentrated in areas such as the axilla, inguinal region, mesenteries and cervical region
- Dense CT capsule with trabeculae
- Outer curved boundary –afferent lymphatic vessels enter
- Hilum (H) –efferent lymphatics leave (also vascular & neural connections)
Lymph nodes- contd
- Spherical or kidney-shaped encapsulated organs, ranging in size from 1mm to 1-2 cm
- Deals with lymph-borne antigen -all tissue fluid-derived lymph is filtered by at least one node before returning to the circulation
- Concentrated in areas such as the axilla, inguinal region, mesenteries and cervical region
- Dense CT capsule with trabeculae
- Outer curved boundary –afferent lymphatic vessels enter, fluid perculates through tissue into midregion and leaves cleansed efferent lympathics
- Inner Hilum (H) –efferent lymphatics leave (also vascular & neural connections)
Regions of lymph node
Outer Cortex
•Subcapsular sinus & trabecular sinuses
•Meshwork of reticular fibres, dendritic cells & macrophages
•B lymphocytes organised into follicles
Inner Cortex (Paracortex)
•Few or no follicles
•Mostly T-Lymphocytes
Medulla- paler staining
•Medullary cords –branched extensions of lymphoid tissue from inner cortex
•Separated by medullary sinuses (through which lymph flows)
•Dendritic cells & macrophages occur
Lymph node circulation
Afferent lymphatic vessels (valves- unidirectional flow into node) ↓ Subcapsular sinus ↓ Trabecular sinuses ↓ Medullary sinuses ↓ Efferent lymphatic vessels ↓ Hilum
Spleen
Immune function –detects and responds to blood-borne antigen
- Filters blood and destroys worn out red blood cells
- Dense CT capsule and trabeculae divides splenic pulp into incomplete compartments
- Hilum –site for passage of splenic artery, vein and lymphatic vessels
-Splenic pulp contains: oReticular tissue oB and T Lymphocytes oMacrophages oDendritic cells
- Spleen is organised into red pulp and white pulp
- Not a vital organ but its specific structure allows it to be an important organ in bacterial defence
White pulp
-Composed of:
o Periarterial Lymphoid Sheath (PALS) –T lymphocytes arranged around a central artery
o Lymphoid follicles –B lymphocytes
- APC can activate T lymphocytes, which in turn, activate B lymphocytes in the follicles –they convert to plasma cells and produce antibodies
- Pathogens and bacteria can also be detected by B lymphocytes and presented to the T lymphocytes
