IMMUNE SYSTEM

AND

LYMPHOID ORGANS

IMMUNE SYSTEM

Has the ability to distinguish “self” (body’s own molecules) from “non-self”

(foreign substance)

This system has the ability to neutralize or inactivate foreign molecules

(virus, bacteria & parasites) and to destroy microorganisms or other cells

(e.g. virus infested cells, cancer cell, cells of transplanted organs)

Continued

The cells of the immune system are:

a) distributed throughout the body in blood, lymph, epithelial &

connective tissue

b) are arranged in small spherical nodules called ‘lymphoid

nodules’ which are isolated cells of immune system found in the

mucosa of GIT, respiratory system, reproductive & urinary system

c) are organized in larger lymphoid organs- lymph nodes, spleen,

thymus, bone marrow

ANTIGENS

A molecule recognized by the body as ‘foreign’ and is capable of eliciting a

response is called an ANTIGEN.

Antigen may consist of proteins, polysaccharides & nucleoproteins

It may be bacteria, protozoa, tumor cells or virus infested cells

ANTIBODIES

Are “glycoproteins” that interacts specifically with an antigen to destroy or

inactivate it.

B lymphocytes produce plasma cells which then produce antibodies

The antibodies:

a) may circulate in the plasma and reach tissues

b) present in secretions of glands i.e. salivary and mammary gland

c) integral membrane proteins of the surface of lymphocytes

CLASSES OF ANTIBODIES

Ig G 75-80%. Is only immunoglobulin to cross the placental barrier.

Ig A is found in secretions i.e. nasal, bronchial, intestinal, prostatic &

colostrum, saliva, vaginal fluid

Ig M 10%. Found on surface of B lymphocytes functions as its specific

receptor for antigen.

Ig E. Present on surface of mast cells & basophils

Ig D. Its functions not completely understood. Found on cell membrane

of lymphocytes

CELLS OF IMMUNE SYSTEM

Lymphocytes

Plasma cells

Mast cells

Neutrophils

Eosinophils

Cell of mononuclear phagocyte system

LYMPHOCYTES

T lymphocytes

B lymphocytes

NATURAL KILLER lymphocytes

PRECURSORS OF ALL LYMPHOCYTES TYPES ORIGINATE IN THE BONE MARROW

B LYMPHOCYTES

mature & become functional in the in the bone marrow

leave bone marrow & enter circulation

colonize C.T. epithelia, lymphoid nodule, lymphoid organ

After encountering an antigen, they redifferentiate into PLASMA CELLS

Activation of B lymphocytes require assistance of T helper cells

Some lymphocyte differentiate into B memory cells

T LYMPHOCYTES

leave bone marrow & enter circulation

reach thymus & undergo intense proliferation & differentiation / die

after maturation leave thymus

distributed in C.T. & lymphoid organs

constitute 65-70% of blood lymphocytes

SUBPOPULATIONS OF T LYMPHOCYTES:

- Helper T lymphocytes

- Cytotoxic T lymphocytes

- Regulatory T lymphocytes

PRIMARY /CENTRAL LYPHOID ORGANS:

- bone marrow

- thymus

SECONDARY / PERIPHERAL LYPHOID ORGANS:

- spleen

- lymph nodes

- solitary lymphoid nodules

- tonsils

- appendix

- peyer’s pathces of ileum

TYPES OF IMMUNE RESPONSES

INNATE RESPONSE

ADAPTIVE RESPONSE

INNATE RESPONSE

Include the action of complement system & cells such as neutrophils,

macrophages, mast cells & natural killer cells

Is fast

Is non- specific

Does not produce memory cells

ADAPTIVE RESPONSE

Depends on initial recognition of antigen by B & T lymphocytes

Is more complex

Is slower

Is specific

Produces memory cells

LEADS TO ELIMINATION OF ANTIGENS

HUMORAL RESPONSE

CELLLULAR RESPONSE

HUMORAL IMMUNITY:

- accomplished by antibodies produced by plasma cells

CELLULAR IMMUNITY:

- mediated by T lymphocytes

LYMPHOID TISSUE

is C.T. charecterized by rich supply of lymphocytes

lymphocytes are lying within rich network of reticular fibres (type III

collagen produced by reticular cells)

in thymus reticulum is formed by epithelial reticular cells

NODULAR LYMPHOID TISSUE

Groups of lymphocytes are arranged as spherical masses called

LYMPHOID NODULES

Lymphoid nodules contain B LYMPHOCYTES

Antigen arrives →B lymphocytes recognizes antigen → nodule becomes

activated →B lymphocytes proliferate in centre of nodule→ it stains lighter

& is called GERMINAL CENTRE

Lymphoid nodules are found in many C.T. and within lymph nodes, spleen

& tonsils. NOT IN THYMUS

THYMUS

bilobed organ

is central or primary lymphoid organ

dual embryological origin

surrounded by connective tissue capsule that sends septae into the

parenchyma and divides it into lobules

capsule consists of collagen &

elastic fibres, blood vessels

intra-lobular trabeculae

reticular frame work

each lobule has a cortex &

medulla

cortex richer in small

lymphocytes, stains darkly

medulla lighter staining

thymic cortex contains t

lymphocytes,

macrophages, epithelial

reticular cells

epithelial reticular cells are

stellate or squamous with long

processes

have large nucle

long procecesses form

cytoreticulum

thymic medulla also

contains cytoreticulum of

epthelial reticular cells

less densly packed T

lymphocytes

has structures called thymic

(Hassall’s) corpuscles

Hassall’s corpuscles

consists of epithelial

reticular cells arranged

concentrically

they are a charecteristic

feature of medulla

T LYMPHOCYTES PROLIFERATE & MATURE IN CORTEX. ONLY 2-3% MIGRATE TO MEDULLA WHERE THE ENTER THROUGH THE WALL OF VENULES INTO THE CIRCULATION. REST DIE

BLOOD THYMIC BARRIER

arterioles & capillaries in thymic cortex are sheathed by epithelial reticular

cells with tight junctions

capillary endothelium is continuous

basal lamina is thick

these features create a blood thymic barrier

it prevents most of circulating antigens from leaving the blood vessels and

entering the cortex

NO such barrier exists in the MEDULLA

THYMUS HAS NO AFFERENT LYMPHATIC VESSELS

THYMUS OF ELDERLY

TONSILS

are organs composed of aggrates of lymphoid tissue which is incompletely

encapsulated. they lie beneath & in contact with the epithelium of the initial

part of digestive tract

Types:

palatine

pharyngeal

lingual

tubal

PALATINE TONSILS

location

epithelium is stratified

squamous

epithelium rests on basal

lamina

thin fibrous connective tissue

layer deep to epithelium

epithelium in crypts intensly

infiltrated with lymphocytes

10-20 tonsillar crypts

lining of crypt same as

surface

secondary crypts

diffuse mass of lymphoid

tissue surround crypts

lymph nodules are embedded

in them

nodules may contain germinal

centres

CAPSULE

incomplete

Made of

dense C.T.

LINGUAL TONSIL:

covered by stratified squamous epithlium

- has single crypt

PHARYNGEAL TONSIL:

- lined by pseudostratified columnar ciliated epithelium

- no crypts

TUBAL TONSIL:

- ciliated columnar epithelium

LYMPH NODE

Are encapsulated kidney shaped

organs

Lye along the course of blood

vessels

Are inline filters

Have a convex side where

afferent vessels enter

and concave side (hilus) where

efferent vessels leave

Capsule sends in trabeculae

Framework is made of reticular

fibers and macrophages

Parenchyma divided into:

Outer cortex

Inner cortex

Medulla

Outer cortex

Subcapsular sinus formed by

macrophages, reticular cells &

fibres

Communicates with medullary sinus

through intermediate sinus

Outer cortex made of reticular fibres

Populated with B lymphocytes

Lymph nodules with germinal

center present

CORTEX

Inner cortex:

Contains few if any nodules

Many T lymphocytes

MEDULLA OF LYMPH NODE

Medullary cords

Branched extensions of inner

cortex

Contain B lymphocytes

Medullary sinuses

Are dilated, irregular spaces

containing lymph

Lined by reticular cells &

macrophages that form network

Afferent lymph vessels pour

lymph into subcapsular sinus

Lymph pass through intermediate

sinus into medulla

Subcapsular sinus slows flow of

lymph through node

Lymph collected by efferent

lymphatic vessels

As lymph flows through sinuses

99% or more antigens are

removed

SPLEEN

FUNCTIONS:

Filtration of blood

Destruction of aged RBC

Produces antibodies & activated lymphocytes

SPLEEN

Location

Surrounded by capsule

Trabeculae

Incomplete division of parenchyma or splenic pulp

Hilum

SPLEEN

Splenic pulp divided into:

White pulp

Red pulp

WHITE PULP

Lymphoid nodules

Periarteriolar lymphatic

sheath

RED PULP

Sinusoids

Splenic cords

WHITE PULP

Splenic artery divide to

trabecular artery

Enter parenchyma

Enveloped by sheath of T

lymphocytes (PALS)

Called ‘central arterioles’

PALS receive B lymphocytes &

form nodules

Penicillar arterioles

RED PULP

Splenic cords composed of

reticular fibres, T & B

lymphocytes, macrophages etc

They are seperated by irregular

shaped sinusoids

Sinusoid lined by elongated

endothelium called ‘stave cells’

Incomplete basal lamina

BLOOD FLOW IN RED PULP

CLOSED CIRCULATION:

Pencilar arterioles connect

directly to sinusoids

OPEN CIRCULATION:

Pencillar arterioles dump

blood into stroma

ESOPHAGUS

Is muscular tube

Extent

Wall has four layers:

Mucosa

Epithelium

Basement membrane

Lamina propria

Muscularis interna or muscularis mucosae

Submucosa

Muscularis externa

serosa

UPPER 1/3 OF ESOPHAGUS

MUCOSA Epumithelium: Stratified

squamous non-keratinized Lamina propria: scattered

lymphocytes, few lymphatic nodules, mucous glands, blood vessels

Muscularis mucosae: thick, striated muscles

SUB-MUCOSA Collagen & elastic fibres, mucous

glands, blood vessels, nerves

MUSCULARIS EXTERNA Skeletal /striated muscles

ADVENTITIA Collagen & elastic fibers, blood

vessels

MIDDLE 1/3 OF ESOPHAGUS

MUCOSA Epithelium: stratified squamous non-

keratinized Lamina prppria: : scattered lymphocytes,

few lymphatic nodules, mucous glands, blood vessels

Muscularis mucosae: thick, striated muscles

SUB-MUCOSA Collagen & elastic fibres, mucous glands,

blood vessels, nerves

MUSCULARIS EXTERNA Skeletal striated muscles, smooth muscles

ADVENTITIA Collagen & elastic fibers, blood & lymphatic

vessels

LOWER 1/3 OF ESOPHAGUS

MUCOSA Epithelium: Simple columnar

Lamina prppria: : scattered lymphocytes, few lymphatic nodules, mucous glands, blood vessels

Muscularis mucosae: thick, striated muscles

SUB-MUCOSA Collagen & elastic fibres, mucous glands,

blood vessels, nerves

MUSCULARIS EXTERNA smooth muscles

ADVENTITIA/ SEROSA Collagen & elastic fibers, blood & lymphatic

vessels

TRACHEA

Rigid tube

Extent

Patency maintained by C shaped cartilagenous rings

Between rings is fibroconnective tissue

MUCOSA

Epithelium: pseudo-stratified columnar ciliated with goblet cells

OTHER CELLS: Columnar non-ciliated cells Immature cells Basal cells Neuroendocrine (APUD)

Lamina propria: Collagen fibres, elastic fibres, lymphatic

nodules, mucous, serous glands

Muscularis mucosae: Smooth muscles

SUB-MUCOSA

Loose connective tissue Serous & musous glands Blood & lymph capillaries

CARTILAGE

Hyline cartilage covered with perichondrium

Trachealis (smooth) muscle

ADVENTITIA: Loose connective tissue Blood & lymph vessels