Lymphoid System Flashcards
Antigens (Ag)
. Molecules recognized by cells of immune system
Cells of immune system
. Lymphocytes
. Macrophages
. Dendritic cells
. Granulocytes
Innate immunity
. Nonspecific, quickly utilized
. No previous exposure needed
. Macrophages, dendritic, granulocytes involved
. Have physical and chemical barriers of skin and mucous membranes to contribute
Adaptive immunity
. Specific response mediate by lymphocytes
. Need previous exposure of antigen
. Only small number lymphocytes will recognize particular antigen
. Slower response
. Generates immunologic memory
. Uses cellular and humoral immune responses
What occurs when lymphocyte recognizes particular antigen?
. Activates and divides
. Undergoes clonal expansion where millions of identical effector cells are made
Cellular immune response
. Mediated by T lymphocytes
Humoral immune response
. Mediated by plasma cell-produced antibodies
. Bind to antigens to mark for destruction
Antigen presenting cells
. Antigens processed before recognition
. Found in skin, lymph nodes, spleen, thymus, mucosal epithelia
. Dendritic, langerhans, macrophages, and B cells
. Recognized by T lymphocytes as MHC molecule
Antigen presenting cell mechanism
. Initiated when APC ingests antigen (usually protein)
. Ag broken down into small peptide-fragments in cell
. Fragments bind to molecule called histocompatibility complex (MHC)
. Ag-MHC complex is transported to APC surface
MHC in humans also called ____
Human leukocyte antigen (HLA)
Population percentage of lymphocytes in circulation in blood or lymph
70%
Lymphocyte classification
. T cells
. B cells
Lymphocyte general functions
. Immunologic surveillance
. Display cell surface receptors that facilitate specific recognition of antigens
T cells
. Cell-mediate immunity
. Differentiate in thymus, majority of lymphocytes
. Detect cell-bound Ags presented by MHCs
. T helper cells (Th cells)
. T cytotoxic cells (Tc cells)
. Regulatory T cells
B cells
. Humoral immunity
. Differentiate into plasma (antibody-producing)
. Recognize soluble or cell-bound Ags
Antibody
. Protein that interacts w/ antigen (Igs do this too)
. Secreted by plasma cells
Reticular cells
. In majority of lymphoid tissues/organs
. Produce reticular fibers (collagen III) that for scaffold for lymphoid cells
Epithelioreticular cells
. Stellate cells in thymus
. Produce keratins
. Joined by desmosomes
Lymph Nodules
. Densely packed, spherical clusters . Embedded in diffuse lymph tissue . Not covered by capsule . Found in CT . Not found in thymus . Primary or secondary
Primary nodule
. Homogenous
. Formed by small B cells
Secondary nodule
. Pale center, darker periphery
. Develop from primary nodules upon Ag stimulation
. Have mantle of just naive B cells
Germinal center of secondary nodule
. Dark zone: activated B cells proliferate (centroblasts) and undergo hypermutation where high affinity antibodies enervated
. Light zone: B cells lower profile ration rate and migrate (centrocytes), further differentiate into plasma cells and memory B cells or return to dark zone and become centroblasts again
Primary lymphoid organs
. Bone marrow, thymus, fetal liver
. Initial formation of lymphocytes
. Don’t contact Ags
Secondary lymphoid organs
. MALT, lymph nodes, spleen
. Cells migrate to here from primary sites
. Lymphocytes activated by encounters w/ Ag to start immune responses
. Ags transported through lymph
. MALT encounters Ags that entered from external environment via M cells
More than 50% lymphoid tissue located in ___
Mucosal system of digestive, respiratory, and GU tracts
Lymphocytes in lamina propria
. Mostly activated T cells, sometimes B cells and plasma cells
Tonsil lymph tissue
. Waldyer’s ring: diffuse and modular tissue in mucosa
. Tonsil epithelium invaginates into underlying lymph to form crypts/pits
. Lymph tissue separated by incomplete CT capsule
. Only have efferent lymph vessels
Types of tonsil
Palatine
Lingual
Pharyngeal
Palatine tonsils
. Btw palatopharyngeal and palatoglossal arches at post. Aspect of oral cavity
. Paired
. Covered by SSNK
Lingual tonsil
. On base of tongue
. Paired
. Converted by SSNK
Pharyngeal (adenoids) tonsil
. In post. Wall of nasopharynx
. Unpaired
. Psuedostratified ciliated epithelium w/ goblets (respiratory type)
. Can hypertrophy and obstruct nasal openings
Lymph nodes
. Kidney bean shaped along lymph vessels
. Has hilum
. Blood vessels enter and leave at hilum, lymph vessels leave at hilum
. Surround by dense CT capsule
. Capsule sends trabeculae into node where stroma of reticular cells and fibers attach
. Macrophages and dendritic cells in reticular fiber network
. Afferent and efferent lymph vessels
Lymph node parenchyma
. Diffuse and modular lymph tissue divided into cortex/medulla
Lymph node cortex
. Area beneath capsule
. Outer cortex: diffuse T cells and modular B cells (B-lymphocyte area)
. Inner cortex/paracortex: diffuse only, thymus-dependent area, has T cells
Lymph node medulla
. Pale staining
. Diffuse tissue forms medullary cords (strands)
. Cords contain plasma cells, lymphocytes, and macrophages
. Cords separated by medullary sinuses
Subcapsular (marginal sinus)
. Btw capsule and lymph tissue of outer cortex
Trabeculae sinuses
. Cortical/intermediate
. Btw trabeculae and lymph tissue in cortex
Medullary sinuses
. Btw medullary cords
Afferent lymphatics
. Bring lymph to node for filtering
. Come from other organs or other nodes
. Numerous vessels pierce capsule of node at periphery to drain subcapsular sinus
Efferent lymphatics
. Fewer
. Formed by coalescence of medullary sinuses
. Exit at hilum and carry lymph away
. Efferent lymphatic plus node can become afferent lymphatic of next node
T/F lymph flow multidirectional
F, unidirectional, towards hilum
Pattern of lymph flow
. Afferent lymphatics -> subcapsular sinus -> cortical sinus -> medullary sinus -> efferent lymphatic
Circulation of lymphocytes through nodes
. Some enter lymph, most enter from blood stream in deep cortex area
. Cross walls of cuboidalish endothelium of high endothelial post-capillary venules (HEV) by diapedesis
. T cells stay in deep cortex
. B cells migrate to nodules of superficial cortex
HEV cells
. high endothelial cells of lymph nodes
. Have surface receptors for lymphocytes
. Gate for circulating lymphocytes
Lymphadenopathy
. Lymph node enlargement
. Common consequence of infection
Lymphoma
. Primary tumor of lymphoid tissue
Spleen functions
. Filters blood and detects blood-borne antigens
. Removal and destruction of damaged RBCs and platelets
. Retrieval of iron from Hb
Spleen characteristics
. Largest lymphoid organ
. Splenic a. Enters hilum and splenic v. Exits spleen at hilum
. Has no direct connections w/ lymph vessels
. Lymphocytes enter spleen via blood vessels
. Surrounded by dense CT capsule that extends into it via trabeculae
. Trabeculae partially divide parenchyma
. Rich reticular network
. White pulp and red pulp
White pulp of spleen
. Lymphoid tissue
. Cuff of lymphocytes around central arterioles running through spleen
. Accumulations called periarteriolar lymphatic sheaths (PALS)
Lymphoid nodules at intervals along PALS that have mostly B cells
. Marginal zone of nodule surrounds follicle and is rich in macrophages
. Blood borne Ags filtered from blood in marginal zone
Arterial circulation in spleen
. Splenic a. Branches into trabecular aa. That follow trabeculae
. Become central arterioles after they leave trabeculae and are surrounded by PALS
. Some branches of central arterioles end as marginal sinuses that supply marginal zone
. Central arterioles lose PALS and terminate as small penicillar aa/ entering red pulp
Class I MHC
. Expressed by all nucleated cells
. HLA-A, B, C
Class II MHC
. Expressed by APC
. HLA-DR, DQ, DP
T helper cells
. CD4+
. Helps via contacts with APCs and other lymphocytes
. Help II: helps B lymphocytes to be plasma cells
. Helper I: activates macrophages
Tc cells
. CD8+
. Cytotoxic functions
. Recognition and lysis of virally infected cells
T regulatory cells
. CD4+, FoxP3
. Limit immune response, produce inhibitory cytokines
Classes of Ig
. IgA: secretory on mucosal surfaces . IgE: basophil cell importance . IgG: most prevalent in blood plasma . IgM . IgD
NK cells
Kills virally infected cells and tumor cells
Assists Tc cells
Frequency for lymphocyte to recognize specific Ag
1 in 10,000 to 100,000 chance
. Even less for both B and T cells to recognize
T/F there are reticulocytes I all lymphoid tissue
F, there aren’t in thymus
Follicular dendritic cells
. In lymphoid nodules
. Mesenchymal origin, NOT bone-marrow derived
Ig class-switch recombination (CSR)
. Centrocytes become plasma cells
3
T/F neutrophils are rare in healthy MALT
T
Red pulp of spleen
. Splenic sinusoids and cords (cords of Billroth)
. Cords supported by reticular cells/fibers, contain B and T cells, macrophages, plasma cells, dendritic cells, granulocytes, and rbcs
. Lined by stave cells
Macrophage role in red pulp
. Phagocytosis old rbcs and begin breakdown of Hb and Fe recycling
Stave cells
. Loosely connected, elongated endothelial cells
. Oriented parallel to blood flow
. Spaces btw cells allow blood to leave or enter sinus
Closed circulation through red pulp
. Capillaries branching from pencillar arterioles connect directly w/ sinusoids
. Blood alway in blood vessel
Open circulation in red pulp
. Capillaries from 0.5 penicillar arterioles are open-ended
. Blood enters splenic cords
. Cells return to circulation by squeezing spaces between stave cells of sinusoid
. Platelets, leukocytes, and young rbcs are flexible to do it
. Old rbcs are blood from reentry and phagocytosed
Thymus embryological origin
. Lymphoid cells from hematopoietic mesenchyme
. Stroma made of epithelioreticular cells from endodermal epithelium of 3rd laryngeal pouch (no reticular fiberS)
. CT capsule and trabeculae from mesoderm
Thymus structure
. In mediastinum
. 2 lobes divided into incomplete lobules
. CT capsule w/ trabeculae that have blood vessels, nerves, and efferent lymphatics
. CT in capsule has reticular fibers and fibroblasts
. Cortex w/ thymocytes(dark)
. Medulla less cellular (light)
Epithelioreticular cells
. Provide framework for T cell development
. 6 cel types
. Form Hassall’s corpuscles (characteristic of thymus)
.
Thymic education steps
. T cell precursors from bone marrow enter corticomedullary junction of thymus and migrate to outer cortex (thymocytes)
. Thymocytes interact w/ APCs and selected (positive selection) to mature further and enter medulla (10% survive, rest undergo apoptosis)
. Thymocytes that would mount immune response against itself is eliminated (neg. selection) in deeper cortex
. Cells that made it leave thymus and populate secondary lymphoid organs as naive T cells
What mediate positive selection in thymus?
. epithelioreticular cells that display both MHC I and II
.
What mediates negative selection in thymic education?
. Bone marrow-derived dendritic cells ad macrophages are main APCs
Percentage of thymocytes that survive education process?
2%
Thymus Medulla contents
. Paler
. Contains fewer cells
. Cells larger, have lighter nuclei and more cytoplasm
Blood-thymus barrier
. Thymocytes mature into T cells in absence of Ags
. Barrier prevents contact btw thymocytes and Ags
. Created by endothelium (continuous w/ tight junctions), macrophages in perivascualr tissue, and epithelioreticular cells w/ tight junctions surrounding capillary wall in cortex
