Lymphatic System Histology

mucosa associated lymphatic tissue (MALT)

in the mucosa of different organs, aggregates of lymphatic follicles/nodules

Roles of lymph system

- protects body
- monitoring
- antigens detect foreign bodies
-

Cells of lymph system

- lymphocytes
- antigen presenting cells (APC)

diffuse lymphatic tissue

under the epithelium covering different organs

non-specific mechanisms

physical barriers and inflammation

_______→ in the layer of antibacterial substances on the surface of mucosae

lysosome

inflammation of non-specific mechanisms

→destroying dead tissue and exogenous materials and → of its replacement with a new tissue or a scar

phagocytosis and degradation of microorganisms can also trigger an

immune/specific defense mechanism

specific mechanism has targeted defense mechanism against foreign materials called

antigens

hypersensitivity reactions

intense secondary immune response in individuals immunologically sensitized by exposure to antigens

B and T lymphocytes will be activated to

destroy foreign agents

types of specific mechanisms

humoral and cell-mediated

humoral immune response

through production of antibodies, mark the antigens in order to be destroyed by other immune cells

Igs secreted by

activated B Ly + plasma cells

cell-mediated immune response

when antigens are directly destroyed by mature T lymphocytes

inflammation

the initial response to an antigen, nonspecific defense process

How is specific response induced?

antigen released, digested by enzymes, fragmented, further presented to immunocompetent lymphocytes

primary immune response

at the initial/first contact with a specific antigen or foreign agent, immunological window until detected in blood, initiated by B lymphocytes (some stay as memory cells)

secondary immune response

when the body encounters for the second time the same antigen, more rapid + intense, higher levels of secreted antibodies

Lymphocyte

immunocompetent cells, distinguish foreign cells, destroy or inactivate foreign organisms, ensures immunity

IHC markers of Ly

different lymphocytes can be identified

T Cell Receptors

on the surface of T lymphocytes

Immunoglobulins Igs

proteins secreted by B Ly and plasma cells, in blood + cell membrane of B Ly

Major Histocompatibility Complex - MHC

major antigens that are specific for each organism, Class I on the surface of every cell of the body excepting erythrocytes, Class II on the surface of APCs, B lymphocytes - that present the antigen to T Ly

Cluster of Differentiation - CD system

IHC marker, surface proteins with various functions

Lymphocytes will enter the blood or lymph flow and are transported throughout the body, towards

the secondary (peripheral) lymphatic organs and the connective tissue

examples of primary vs secondary lymph organs

primary - bone marrow, GALT, and thymus
secondary - lymphatic nodules, lymph nodes, tonsils, and spleen

T and B lymphocytes undergo antigen-dependent activation into

effector lymphocytes and memory cells

NK (Natural Killer) lymphocytes

spontaneous cytotoxic cells, able to recognize and destroy the antigen without the APCs intervention, against cancer cells, done by perforins and fragmentins (granzymes)

NK lymphocytes have on their surface

IFN receptors, IFN secreted by TH Ly will activate them

T Lymphocytes

long lifespan cells

T Lymphocytes arise from

MSC (multipotent stem cell) of the bone marrow

T Lymphocytes differentiate into + migrate to

T cell progenitor cells (= immunocompetent cells), to thymus

Maturation of T Lymphocytes

acquisition of specific antigens on their surface, self tolerance will develop

T cell specific areas

paracortex - lymph nodes o PALS (periarterial lymphatic sheath) - spleen o diffuse lymphoid tissue - MALT, between the lymphatic nodules

3 types of T lymphocytes

Cytotoxic T cells
Helper T cells
Suppressor T cells

Helper T cell

CD4 and TCR - on their surface, recognize antigens presented by APC + activate and secrete interleukins

Activation of Th Lymphocyte

CD4 binds to class II MHC molecules on the surface of the APC, signals TH Ly and determine its activation

TCR also binds to the particles presented by the APC, CD3, which is closely bound to TCR, initiates a supplementary mechanism for TH Ly activation

Th Lymphocytes activating Tc and Treg/s lymphocytes - cannot recognize antigens by themselves, only if presented by

APCs

Th Lymphocytes activating B lymphocytes - which will start secreting

Igs

Th Lymphocytes activating macrophages → which become avid of

phagocytosis

Th1 Ly vs Th2 Ly - function

Th1 - controlling intracellular pathogens
Th2 - initiating antibody-mediated immune responses against extracellular pathogens

Tc Lymphocytes - function

destroy virus infected cells, tumor cells or cells no longer recognized as self

TC lymphocytes - activation

activated by Th lymphocytes via ILs, by directly connecting to the class I MHC on APCs, secrete perforins and fragmentins

Treg Ly function

monitor the immune response by stopping it when the stimulus/antigen is extinct/removed or adapting it to the stimulus/antigen or suppressing the immune response towards autoantigens

B Lymphocytes arise from the _________ and _ ________ cells

lymph MSC, b progenitor

B cell specific areas

MALT (lymph node) + Malpighian corpuscles of spleen

maturation of B lymphocytes

to immunocompetent cells, insertion of Ig into the membrane so Ig becomes BCR

2 signals needed for B lymphocyte activation

1 - interaction between the B cell receptors and antigen, antigen engulfed + fragments display on surface + bind to MHC class II molecules
2 - o binding TH lymphocyte with TCRs to B lymphocyte, T cell can secrete cytokines

B lymphocytes - function

humoral (antibodymediated) immunity through Igs secreted by activated B cells

Antigen Presenting Cells - role

processes antigen and present them on their surface bound to a molecule of class II MHC

most APCs → belong to the

mononuclear phagocytotic system (MPS)

interdigitate (dendritic) cells iDC (MHCII)

an APC, surface facilitates the contact of the antigens with the T lymphocytes

follicular dendritic cells FDC

antigen-bearing cells, present directly of B lymphocytes

the site where undifferentiated stem lymphocytes develop/migrate and undergo proliferation and differentiation into immunocompetent cells

primary lymphatic organs

________________: organs in which lymphocytes, that have become "mature" cells in the primary lymphatic organs → become "effector" cells of the immune response

secondary (peripheral) lymphoid organs

Bone marrow function

- makes blood cells
- removes bad erythrocytes
- site of B lymphocyte differentiation

red marrow - active bone marrow: producing a great number of _____________
yellow marrow: accumulation of large quantities of _____

erythrocytes, fat

BM is isolated from the bone by the

endosteum

stroma of bone marrow contains

reticular connective tissue

interstice of bone marrow

hematopoietic cells • adipocytes • macrophages

sinusoid structure + function

continuous endothelium surrounded by reticular fibers, provide the route of exit of mature blood cells into the circulation - aperture

spleen function

filters the blood of aged blood cells and foreign particles and is site of immune reactions

stroma of spleen

consists of reticular connective tissue

capsule and trabeculae of spleen

dense connective tissue that contains also myofibroblasts

White pulp of spleen

lymph tissue, central artery (branch of the splenic artery), PALS and splenic nodule

White pulp - PALS - periarterial lymphatic sheath

lymphocytes that aggregate around the central artery, thymus-dependent zone, has T lymphocytes

White pulp - splenic nodule (Malpighian corpuscle)

territory of B lymphocytes, eccentric central artery,

Red pulp of spleen - composed of

splenic sinuses, separated by splenic cords of Billroth, vessels lined by endothelial cells (resting on discontinuous bm)

central artery sends branches to

- white pulp
- sinuses
- red pulp
- penicillar arteries
- arterial capillaries (sheathed)

Immune system functions of spleen

- antigen presentation by APCs
- initiation of immune response
- activation and proliferation of B and T lymphocytes
- production of antibodies

Hematopoietic functions of spleen

- removal and destruction of bad erythrocytes + platelets
- retrieval of iron
- formation of erythrocytes
- storage of blood

Mucosa- Associated Lymphoid Tissue (MALT)

oan accumulation of lymphatic tissue that is not enclosed by a capsule and is found in the mucosa of different organs

MALT is made of

aggregations of lymphatic nodules, disseminated lymphatic tissue, lymphocytes in epithelia

epithelia that cover the lymphatic tissue are specialized in

taking up and transporting antigens to lymphatic cells

cells of MALT sample the antigens + initiate

a cellular or humoral mediated immune response

How is IgA secreted?

by plasma cells, transported into the lumen where it acts against antigens

How are IgG and IgM secreted?

by plasma cells, secreted into the chorion/lamina propria to neutralize the antigens that have escaped the surface defense mechanisms

IgE function

regulates the release of histamine from the mast cells of the chorion

tonsils

aggregations of lymphatic nodules that guard the entrance into the oropharynx

which tonsils are covered by stratified squamous epithelium

lingual and palatine

pharyngeal tonsils (adenoids) covered by

pseudostratified ciliated epithelium with goblet cells

underlying connective tissue of tonsils contains

o lymphatic nodules (B lymphocytes region) and diffuse lymphatic tissue (T lymphocytes region)

Antigens of tonsils

on the surface of the epithelium or in crypts, transported and phagocytized to the chorion (initiates immune response)

homing

process of lymphocytes returning to the same mucosa

stimulated lymphocytes of tonsils

move along the lymphatic pathways in the cervical lymph nodes where they proliferate and differentiate into effector lymphocytes

Peyer's patches

in ileum connective tissue, consists of numerous aggregations of lymphatic nodules and diffuse lymphatic tissue o containing T and B lymphocytes

M cells (intestinal epithelial cells)

epithelial cell that takes up the microorganisms and macromolecules from the lumen in endocytotic vesicle, has microfolds, antigen-transporting-cell

after puberty, a progressive involution occurs:

o T cell differentiation and proliferation are reduced and o most of the lymphatic tissue (and the cortex in particular) is replaced by adipose tissue
- thru steroid hormones

Thymus function

- site of development and maturation of naïve TLy
- secretes hormones (which regulate TLy)

epithelioreticular cells - stromal cells of thymus

provide support for other cells of the thymus, form the Hassal bodies/corpuscles, secrete hormones, part of the blood-thymus barrier

___________ being the only lymphatic organ without reticular fibers in its stroma

thymus

Type I epithelioreticular cells

oboundary of the cortex and the connective tissue capsule as well as
obetween the cortical parenchyma and the trabeculae oaround the adventitia of the cortical blood vessels
- blood-thymus barrier

Type II epithelioreticular cells

within the cortex, divide the cortex into isolated areas for the developing T cells, express MHC I and MHC II molecules which are involved in thymic cell education

Type III epithelioreticular cells

- at the boundary of the cortex and medulla
- functional barrier
- possess MHC I and MHC II molecules

Macrophages of thymus

within the thymic cortex, for phagocytosis of T cells that do not fulfill thymic education requirements

Thymus - stroma- medulla (contains)

large number of epithelioreticular cells and loosely packed T cells • type IV, V and VI epithelioreticular cells

Type IV epithelioreticular cells

between the cortex and the medulla, close to type III-cells, create the barrier at the corticomedullary junction

Type V epithelioreticular cells:

throughout the medulla, role in the maturation of the T lymphocytes and display on their surface, class I MHC and II MHC molecules

Type VI epithelioreticular cells

forms thymic Hassall's corpuscle