Lymphoid Organs Flashcards
What is the innate immune system?
Immune system that retains the phagocytic system in the form of monocytes/macrophages and neutrophils
Lacks immunological memory
What is the adaptive immune system?
The innate immune system with an added form of defence that has high specifity and memory.
Based on the presence of trillions of lymphocytes and cell surface recptors
Slower to react but highly flexible and very specific
Components of innate immune system
Barriers against invasion: skin, mucosa, stomach acid
Phagocytes: macrophages and neutrophils
Complement: plasma proteins that form an enzyme system to defend against bacteria
Extracellular killers: natural killer lymphocytes and eosinophils
What do extracellular killer lymphocytes kill?
Virus infected cells
Some tumour cells
What do extracellular killer eosinophils do?
Attack larger parasites
What is the main role of the adaptive immune system?
Destruction
What skill is critical for destruction?
The ability to distinguish ‘self’ from ‘non-self’
What happens when the immune system fails to distinguish between self and non-self?
It attacks the body resulting in autoimmune disease
What tissue is attacked from rheumatoid arthritis?
Synovial joints
What tissue is attacked in addison’s disease?
Adrenal cortex
What tissue is attacked in multiple sclerosis?
CNS myelin
What tissue is attacked in Hashimoto’s disease?
Thyroid gland
What tissue is attacked in grave’s disease?
Thyroid gland
What tissue is attacked in myasthenia gravis?
Neuromuscular junction
What is a primary lymphoid organ?
An organ which is the site of lymphocyte production and maturation
In humans - bone marrow and thymus
What is a secondary lymphoid organ?
Sites to which lymphocytes migrate and where they aggregate in large numbers
In humans - spleen, lymph nodes, lymph nodules
What forms do lymphocytes come in?
B lymphocytes - produce antibodies
T lymphocytes - participate in cellular immunity, found in 3 forms
Natural killer cells - kill virus infected cells and some tumor cells
Development of lymphocytes
B and T cells produced from stem cells in the bone marrow
B cells remain in the bone marrow to continue their development
Immature T cells migrate to the thymus to mature and be ‘educated’
Both then move through the bloodstream to secondary lymphoid organs
Properties of bone marrow
Compact bone
Endosteum
Hemopoietic cells embedded in reticular fibres
Blood sinusoids
Properties of the thymus
Located in mediastinum
Increases in size from birth to puberty then regresses and gets more and more fatty
Has two lobes subdivided by septa
Lobules have outer highly cellular cortex and a less cellular inner medulla
What does the thymus cortex contain?
Large numbers of T cells
Epithelioreticular cells
Macrophages
What does the thymus medulla contain?
Mainly T cells but larger and less densely packed
Epithelioreticular cells
What happens when T cells first enter the thymus?
Begin proliferation, maturation and selection processes.
Those that fail (98%) will undergo apoptosis and be phagocytosed by macrophages
What happens to T cells that survive the cortex?
They enter the medulla and interact with the epithelioreticular cells
They’re presented with ‘self’ antigens and any cells which become activated are destroyed (clonal deletion)
What happens to T cells that survive the medulla?
Enter the blood cells
What is a Hassals corpuscle?
Whorls of epithelial cells with keratin at their core
Accumulate in the medulla
What is positive selection?
In the cortex T cells must express correct surface antigens and recognise self and foreign antigens to survive
What is negative selection?
In medulla T cells that recognise self antigens are eliminated
What are the functions of the lymphatic system?
Drainage of lymph into vascular system
Surveillance of tissue for signs of antigens from foreign invaders e.g. bacteria
Delivery of absorbed fats from small intestine into vascular system
Structure of lymph vessels
Thin walled
Lined by endothelium
Anchored to tissue by filaments
Structure of lymph nodes
Encapsulated
Highly organised structures
Interposed along larger lymph vessels
Found in clusters in areas like neck, axillae and groin
How lymphocytes enter lymph nodes?
Via incoming lymph
Via bloodstream
What are lymph follicles?
Circular aggregations containing mostly B cells found beneath the capsule
What are primary follicles?
Uniformly, densely packed follicles
Unstimulated
What are secondary follicles?
Less densely packed centres consisting of actively dividing B cells = germinal centre
Outer mantle zone containing resting B cells
Active follicles
What populates the paracortical region of a lymph node?
Mostly T cells
What does the medulla of a lymph node contain?
Medullary sinuses
Medullary cords that contain plasma cells and macrophages
What is MALT?
Aggregations of lymph tissue associated with the GI tract, respiratory tract and genitourinary tract
Mucosa-associated lymphatic tissue
e.g. palatine tonsil, payer’s patch in ileum
What lines the crypts in the palatine tonsil?
Stratified squamous epithelium
Functions of the spleen
Produce immune response against blood-borne antigens
Removal of particulate matter and aged blood cells
Production of blood cells during fetal life
What is white pulp?
Whitish nodules in spleen made of T cells or B cells
What is red pulp?
Deep red tissue in spleen containing mostly blood filled capillaries
What happens to old RBCs when leaving the spleen?
Because they cannot deform to fit through the small spaces, they will lyse and be phagocytosed by macrophages within the wall of the sinuses
