Histology of the immune system
1. Identify the major components of the lymphatic system and outline their
function.
Reference: Marieb, E.N. (2004). Human anatomy and physiology. San Francisco: Person Education.
Roles of the lymphatic system:
1. Lymph return of excess fluid
2. Defence – foreign material is carried in the lymph to the lymph nodes for processing and
destruction.
The lymphatic system consists of two parts:
i)
Lymphatic vessels – network of meandering vessels that transport fluids that have escaped
the vascular system back to the blood
ii) Lymphoid tissues and organs - which are scattered throughout the body and contain
phagocytic cells and lymphocytes which are essential in the body’s defence mechanisms
and resistance to disease. They consist of lymphoid cells, lymphatic tissue, lymph nodes,
spleen, thymus, tonsils, Peyers patches.
Hydrostatic and osmotic pressure at capillary beds forces fluid out of the blood at the arterial ends
of the beds and cause most of it to be reabsorbed at the venous ends. The fluid that remains behind
in the tissue spaces, plus plasma proteins that escape from the bloodstream need to be transported
back to the blood to ensure the cardiovascular system has enough blood volume to operate. The
lymphatic vessels transport this excess fluid and plasma proteins back into the blood. When fluid
enters a lymphatic vessel, it is called lymph (clear water).
Lymph only flows in one direction – to the heart.
Lymphatic vessels

Begins as lymphatic capillaries which are situated within the capillary bed. They begin as
blind ended vessels which have flaplike minivalves.

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
Weave between tissue cells and blood capillaries in the loose connective tissue of the body.

Absent from bones, teeth, bone marrow and entire CNS (excess fluid drains into the CSF)

Lymphatic capillaries are highly permeable due to:

o
Endothelial cells are not tightly joined but the edges of the cells overlap each other
loosely forming flaplike minivalves
o
Collagen filaments anchor the endothelial cells to surrounding structures so that an
increase in interstitial fluid volume opens up the minivalves, preventing collapse.
Function:
o
The minivalves open when pressure in the interstitial fluid is greater than the
lymphatic capillaries. (When pressure in the interstitial fluid is less than in the
lymphatic capillaries, the valves remain shut, preventing leakage of lymph back out).
o
Proteins are able to enter into the lymphatic capillaries
o
During inflammation, lymphatic capillaries develop openings that allow larger
particles such as cell debris, pathogens and cancer cells to enter. Although this
allows for the pathogens and cancer cells to travel through the body, the lymphatic
overcomes this by taking “detours” through lymph nodes where debris is cleaned up
and examined by the cells of the immune system.
o
In the intestinal mucosa, there are specialised lymphatic capillaries called lacteals.
The lymph here is milky white due to absorption of digested fats from the intestine.
The lymph in the lacteals is called chyle.
o
Lymph flows from the lymphatic capillaries to thicker walled vessels –collecting
vessels
trunks
ducts.
o
These vessels have the same tunics as blood vessels however they contain more
internal valves and anastomose more.

Lymphatic vessels in the skin travel with superficial veins and the deep lymphatic veels of
the trunk and digestive viscera travel with the deep arteries.

Lymphatic trunks – union of large collecting vessels and drain large areas of the body.

Eventually lymph is drained into one of two large ducts in the thoracic region.

Right lymphatic duct drains lymph from the R upper arm, R side of head and thorax

Thoracic duct drains lymph from the rest of the body and arises from an enlarged sac called
the cistern chyli that collects lymph from the two large lumbar trunks that drains the
digestive organs. It also receives drainage from the L side of the thorax, L upper limb and the
head region.

Each duct empties its lymph into the venous circulation at the junction of the internal jugular
vein and the subclavian vein.

Lymphatic tissues and organs

Function
o
Perform lymphoid functions
o
Consist of lymph nodes (glands), thymus, spleen, mucosa associated lymphoid tissue
(MALT). MALT is composed of cellular aggregates in the lamina propria of the
digestive tract lining and the respiratory tract.
o
Primary lymphoid organs – foetal liver and adult bone marrow, thymus gland
o
Secondary lymphoid organs – spleen, lymph nodes, MALT
o
Lymphoid organs are the sites for lymphopoiesis (where B and T cells mature and
differentiate from stem cells)
2. Identify lymphocytes, polymorphonuclear leukocytes, monocytes and
describe where they are found in the body.
Leukocytes are made up of:

Granulocytes
o

Neutrophils (polymorphonuclear leukocytes)

Neutrophilic band granulocytes (band neutrophil)

Neutrophilic segmented granulocyte (segment neutrophil)
o
Eosinophils
o
basophils
Agranulocytes (mononuclear leukocytes)

o
Lymphocytes
o
Monocytes
Leukocytes are found circulating in blood plasma and areas of infection and inflammation.
Lymphocytes – derived from bone marrow however mature in different areas. Make up between
20%-50% of leukocytes.

T cells – mature in thymus. Provides local defense against antigens.

B cells – mature in bone marrow. Responsible for humoral side of defense against viruses,
bacteria and allergens.

Natural killer cells
Polymorphonuclear leukocytes

60%-70% of circulating leukocytes

12-15μm with a nucleus consisting of 2-5 lobes.

Found in blood and connective tissue

Lifespan of 1-4 days in connective tissue and half life of 6-7hrs in blood

Phagocytes of bacteria and other small particles
Monocytes

Derived from bone marrow

Granulocyte

12-20μin diameter

Nucleus is horseshoe, kidney or oval shaped and eccentrically placed

Differentiate into macrophages
3. Describe the histological structure and function of the lymphoid tissues
and the lymphoid organs.
Lymphoid tissues and organs

Lymphoid cells
Function:
o
Lymphocytes – B cells and T cells, arise in the bone marrow, important in the
immune system
o
Lymphoid macrophages – phagocytose foreign material and help to activate T cells,
as do dendritic cells
o
Reticular cells – produce the reticular fibre stroma, a network that supports other
cell typs in the lymphoid organs
Structure

Lymphatic tissue
Function:
o
Houses and provides proliferation site for lymphocytes
o
Surveillance point for lymphocytes and macrophages to attack pathogens
Structure
o

Composed of loose connective tissue called reticular connective tissue
Lymph nodes
o
Large clusters occur near the body surface in the inguinal, axillary and cervical
regions where lymphatic collecting vessels converge to form trunks
o
Have two functions

Act as lymph filters. Allows macrophages to remove and destroy
microorganisms therefore preventing them from entering the blood

Help to activate the immune system
Structure

Lymphoid organs – all composed of reticular connective tissue
o
Spleen
Function:
o

largest lymphoid organ

Provides a site for lymphocyte proliferation, immune surveillance and
response

Also has blood cleansing functions – filters blood

Stores some of the breakdown products of RBC for later reuse e.g.
haemoglobin, releases others to the blood for processing by the liver

Site of erythrocyte production in the foetus

Stores blood platelets
Thymus
Function:

Important functions during the early years of life and in adolescence stops
its growth. By old age it has atrophied and is almost entirely fibrous tissue.

Site for differentiation of T cells between self and non-self antigens.

Acts on T cells to enable them to become active against
(immunocompetent) specific pathogens.
Structure

Made up of 2 lobes. Each lobe is subdivided into smaller lobes by septa.
Each small lobe (trabeculae) is made up of an outer layer (cortex) and an
inner layer (medulla)

The cortex (outer layer) consists mainly of densely packed lymphocytes

Medulla contains smaller population of lymphatic cells and differentiated
from the cortex due to the presence of Hassal cells (thought to be remnants
of epithelial cells) which are tightly packed.

o
Tonsils

Palatine tonsils – largest and most often infected

Lingual tonsils – paired lymphoid follicles of the tongue

Pharyngeal tonsil (adenoids if enlarged)

Tubal tonsils – surround openings of the auditory tubes into the pharynx
Structure:

o
The tissue contains follicles that house lymphocytes
Peyer’s patches
Structure:

Large isolated clusters of lymphoid follicles, structurally similar to the tonsils

Located in the wall of the distal portion of the small intestine

Also in the appendix

Provides a place to destroy bacteria

Generates memory lymphocytes for long term immunity

There are also lymphoid follicles in the walls of the bronchi
Function:
4. Outline the process of haemopoiesis, the sites of blood cell formation
and how these change from foetal development to skeletal maturity.
Haemopoiesis

Production of red blood cells begins in embryonic development in the yolk sac

Later the production site moves to the liver and lymphatic organs. In the newborn, most of
the red marrow is devoted to haemopoietic function. As the newborn grows, the
haemopoietic marrow sites gradually recede and by 4yo only the ribs, sternum, pelvic
girdle, flat bones of the skull and spinal column and proximal portions of the humerus and
femur.

Finally haemopoiesis occurs in the red bone marrow.

Blood cells have a limited lifespan and require rapid turnover throughout our human
lifespan.

New blood cells originate from pluripotent stem cells which are in the bone marrow. As the
cells develop they differentiate and become specialised.

These stem cells differentiate into 3 different blood cell categories

o
Red blood cells
o
White blood cells
o
Platelets
White blood cells can further be divided into
o
o
Granulocytes

Neutrophils (Polymorphonuclear neutrophils) found in blood and tissue.

Eosinophils

Basophils
Agranulocytes

Lymphocytes

Monocytes