Part of the immune system that consists of 2 independent parts:

1. meandering network of lymphatic vessels2. various lymphoid tissues and organs  

Lymphatic system is located within loose connective tissue systems of the body

 

The Lymphatic The Lymphatic SystemSystem

   Form a 1-way system in which lymph fluid flows only toward the heart

   Vessels are similar to capillaries, but very permeable.

   They have the same 3 tunics, but thinner.

   They occur everywhere blood capillaries occur.

Lymphatic Lymphatic vesselsvessels

arteriolearteriole

venulevenule

lymphatic capillarieslymphatic capillaries

blood capillaries

blood capillaries

lymphatic vessel

lymphatic vessel

Fig. 22.03

Edema- swelling of lymph vessels- lymphedema

Lymph Lymph TransportTransport

• lacks pump for circulation• relies on activity of skeletal muscles and pulsation of nearby arteries for movement of fluid• 3L of lymph enters blood stream every 24 hrs• proteins easily enter lymphatic system• uptake of large particles such as cell debris, pathogens, and cancer cellslymph nodes where it is cleansed of debris and examined by cells of the immune system (WBC)

Fig. 22.04Systemic Circulation Pulmonary Circulation

FormatioFormation of n of LymphLymph

interstitial fluid

blood capillary

lymphatic capillary

tissue cell

composed of:• lymphoid cells • lymphoid tissue

Lymphoid OrgansLymphoid Organs

Lymphoid Cells

Lymphocytes the main warriors of the immune system

formed in red marrow of spongy bone (leukopoeisis)

Two main varieties• T cells (T lymphocytes)• B cells (B lymphocytes)

Lymphocytes

T cells and B cells protect against antigens• Anything the body perceives as

foreign–Bacteria and their toxins; viruses–Mismatched RBCs or cancer cells

LymphocytesLymphocytes

T cellsT cells • Manage the immune responseManage the immune response• Attack and destroy foreign cellsAttack and destroy foreign cells• 1500-3000 cells/cm3 blood

B cellsB cells • Produce plasma cells, which secrete Produce plasma cells, which secrete

antibodiesantibodies

Other Lymphoid Cells

Macrophages phagocytize foreign substances and help activate T cells

Dendritic cells capture antigens and deliver them to lymph nodes

Reticular cells produce stroma that supports other cells in lymphoid organs

Figure 20.3

Macrophage

Medullary sinus

Reticular fiber

Lymphocytes

Reticular cells onreticular fibers

Lymphoid Tissue

Houses and provides a proliferation site for lymphocytes

Furnishes a surveillance vantage point

Two main types• Diffuse lymphatic tissue• Lymphatic follicles

Reticular connective tissue found in all lymphoid organs except for thymus

Lymphoid TissueLymphoid Tissue

• Lymph nodes• Spleen• Thymus• Peyer’s patches• Tonsils• Red Bone Marrow• Appendix

Lymph Organs:Lymph Organs:

• Hundreds of small organs found imbedded in connective tissue

• Large clusters occur in inguinal, axillary, and cervical regions.

Lymph Node

Figure 20.2a

Cervical nodesEntrance of rightlymphatic duct into vein

Internal jugular vein

Entrance of thoracicduct into vein

Thoracic duct

Cisterna chyli

Lymphaticcollecting vessels

Axillary nodes

Aorta

Inguinal nodes

Regionallymph nodes:

(a) General distribution of lymphatic collecting vessels and regional lymph nodes.

Drained by the rightlymphatic ductDrained by thethoracic duct

Lymph NodesLymph Nodes

FunctionsFunctions1.1. Filter lymph—macrophages destroy Filter lymph—macrophages destroy

microorganisms and debrismicroorganisms and debris

2.2. Immune system—lymphocytes are Immune system—lymphocytes are activated and mount an attack activated and mount an attack against antigensagainst antigens

Structure of a Lymph Node

Bean shaped External fibrous capsule Trabeculae extend inward and divide

the node into compartments Two histologically distinct regions

• Cortex• Medulla

Figure 20.4

Afferentlymphaticvessels

Efferentlymphaticvessels

Capsule

Trabeculae

Hilum

Cortex• Lymphoid follicle

• Germinal center• Subcapsular sinus

Medulla:

• Medullary cord• Medullary sinus

Follicles

Trabecula

Subcapsularsinus

Capsule

Medullarycords

Medullarysinuses

(b) Photomicrograph of part of a lymph node (72x)(a) Longitudinal view of the internal structure

of a lymph node and associated lymphatics

lymphatic capillaries

blood capillaries

lymphatic vessels

lymphatic node

one-way valves

Spleen Largest lymphoid organ Served by splenic artery and vein, which enter and

exit at the hilus Functions

• Site of lymphocyte proliferation and immune surveillance and response

• Cleanses the blood of aged cells and platelets, foreign matter, bacteria, viruses and debris

• Stores breakdown products of RBC– e.g. iron

• Erythrocyte production in fetus (normally ceases after birth)

Figure 20.6c

(c) Photograph of the spleen in its normal position in the abdominal cavity, anterior view.

Diaphragm

Spleen

Adrenalgland

Splenicartery

Pancreas

Leftkidney

Structure of the Spleen

Two distinct areas• White pulp around central arteries• Mostly lymphocytes on reticular fibers and

involved in immune functions• Red pulp in venous sinuses and splenic

cords– Rich in macrophages for disposal of worn-out

RBCs and bloodborne pathogens

Figure 20.6a,b

Arterioles andcapillaries

CapsuleTrabecula

Venous sinuses

Red pulp

Splenic cords

White pulp

Central artery

Splenic arterySplenic vein

(b) Diagram of spleen histology

SplenicarterySplenicvein Hilum

(a) Diagram of the spleen, anterior view

ThymusThymusLocated: on top of heart• Secretes hormones thymosin and thymopoeitin• Causes T-lymphocytes to function against specific pathogens in the immune response (called immunocompetent) • Most active during youth; it in size during childhood, by old age, it's mostly been replaced by fibrous and fatty connective tissue

Fig. 22.05

ThymusThymus

Thymic lobes contain an outer cortex Thymic lobes contain an outer cortex and inner medullaand inner medulla

Cortex contains densely packed Cortex contains densely packed lymphocytes and scattered lymphocytes and scattered macrophagesmacrophages

Medulla contains fewer lymphocytes Medulla contains fewer lymphocytes and thymic (Hassall’s) corpuscles and thymic (Hassall’s) corpuscles involved in regulatory T cell involved in regulatory T cell developmentdevelopment

Figure 20.7

Cortex

Medulla

Thymic (Hassall’s)corpuscle

Thymus

Differs from other lymphoid organs in important ways• It functions strictly in T lymphocyte maturation• It does not directly fight antigens

The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers)

These thymocytes provide the environment in which T lymphocytes become immunocompetent

TonsilsTonsils

Prevent pathogens in respiratory tract and digestive tracts from penetrating mucous membrane lining

Tonsils

Simplest lymphoid organsForm a ring of lymphatic tissue around the

pharynx• Palatine tonsils—at posterior end of the oral cavity• Lingual tonsils—grouped at the base of the tongue• Pharyngeal tonsil—in posterior wall of the

nasopharynx• Tubal tonsils—surrounding the openings of the

auditory tubes into the pharynx

Tonsils

Contain follicles with germinal centers Are not fully encapsulated Epithelial tissue overlying tonsil masses

invaginates, forming tonsillar crypts Crypts trap and destroy bacteria and

particulate matter

Figure 20.8

Tonsil

Tonsillar crypt

Germinal centersin lymphoid follicles

Pharyngeal tonsil

Palatine tonsilLingual tonsil

Aggregates of Lymphoid Follicles

Peyer’s patches• Clusters of lymphoid follicles• In the wall of the distal portion of the small

intestine• Similar structures are also found in the

appendixPeyer’s patches and the appendix

• Destroy bacteria, preventing them from breaching the intestinal wall

• Generate “memory” lymphocytes

Figure 20.9

Smooth muscle inthe intestinal wall

Lymphoid nodules(follicles) ofPeyer’s patch

MALT

Mucosa-associated lymphatic tissue, including• Peyer’s patches, tonsils, and the appendix

(digestive tract)• Lymphoid nodules in the walls of the

bronchi (respiratory tract) Protects the digestive and respiratory

systems from foreign matter