Revise the structure and function of primary and secondary lymphoid organs and describe the major recirculation pathways of lymphocytes around the body

- An organ or tissue where lymphocytes are produced.

What defines a primary lymphoid organ?

- Liver and spleen.

What are the primary lymphoid organs in a foetus?

- Thymus and bone marrow.

- By puberty most is produced in the bone marrow of flat bones (particularly the vertebrae).

What are the primary lymphoid organs after birth?

- T and B cells.

- Natural killer cells.

Lymphoid progenitor cells can differentiate into what cells?

- Are both produced and mature in the bone marrow.

Where are B cells produced and where do they mature?

- Produced in the bone marrow but mature in the thymus.

Where are T cells produced and where do they mature?

- Thymosin.

Important hormone secreted by the thymus that helps with maturation of T cells

- Maturation of T cells.

- Also selects T cells that are able to recognise MHC complexes (called positive selection), and destroys T cells that recognise self-antigens.

What is the role of the thymus?

- Starts at the subcapsular zone and moves through the cortex and finally onto the medulla.

What regions of the thymus does the T cell move through as it matures?

- The earliest progenitor cells.

Subcapsular zone of the thymus contains what?

- Densely packed with T cells undergoing selection.

Cortex zone of the thymus contains what?

- Fewer but more mature T cells than the cortex.

- These are about to be released.

Medulla zone of the thymus contains what?

- Around 95% die by apoptosis.

How many T cell progenitors actually go on to become mature T cells?

- Organs and tissues where lymphocytes come in contact with antigens and differentiate and mature into effector cells.

What defines a secondary lymphoid organ?

- Spleen and lymph nodes.

- Also, mucosa-associated lymphoid tissue (MALT) lines most of the bodily systems.

What are the main secondary lymphoid organs?

- Where numerous blood and lymph vessels converge.

Lymph nodes are found at sites where what occurs?

- Meeting place for antigen with T and B cells.

- Also filters lymph and removes bacteria and other pathogens.

Function of the lymph node

- In the subcutaneous tissue beneath skin.

Where are the superficial lymph nodes found?

- Anything below the level of the subcutaneous tissue.

Where are the deep lymph nodes found?

- Cortex, paracortex and medulla.

Three regions of the lymph node from outer to inner

- Predominantly the site of B cells.

What does the cortex of the lymph node contain?

- Predominantly the site of CD4+ T cells.

What does the paracortex of the lymph node contain?

- Mixture of T and B cells.

- Also APC's.

- Importantly macrophages.

What does the medulla of the lymph node contain?

- Upper left quadrant of the abdomen.

- Left hypochondriac region.

Where is the spleen located?

- Red pulp.

Is the spleen made up of more white or red pulp?

- Dense lymphoid tissue surrounding arteries.

- Also arterioles extending into lymphatic nodules.

What does the white pulp of the spleen contain?

- Mainly macrophages.

- RBC's in the process of disposal.

- Also veins of the spleen.

What does the red pulp of the spleen contain?

- Antigen and lymphocyte interactions.

- Activation of lymphocytes and proliferation.

Role of the spleen in immunity

- Because the spleen is not connected to the lymphatic system, lymphocytes and antigens arrive and leave through the blood.

What is a functional difference between the spleen and the lymph nodes?

- Response to blood-borne allergens.

- Importantly, is the source of B cell help in absence of T cells in response to bacterial cell wall polysaccharide antigens.

- Therefore is effective against encapsulated bacteria.

The spleen is the main site of what?

- In the ileum.

Where are Peyer's patches found?

- They have follicle associated epithelium (FAE) which contains M cells.

- These M cells take up these antigens through pinocytosis.

- Through a process call transcytosis, these antigens are transported into sub-epithelial tissues where they can be picked up by lymphocytes.

How are antigens taken up in the Peyer's patches?

- Epidermal layer of skin which numerous dendritic cells called Langerhans cells.

Describe the cutaneous immune system

- Most of the mature lymphocytes are in constant circulation in blood and lymph.

- Naive lymphocytes circulate among the secondary lymphoid organs until they encounter an antigen or die.

Describe the recirculation pathway of lymphocytes

- T effector cells move to sites of inflammation and infection, and naive T cells traverse secondary lymphoid organs.

- Plasma cells remain in lymphoid organs as they secrete antibodies which can travel in the blood.

Difference between recirculation of T and B cells