Cells and Organs of Immune System

Question 1

1.Structural and functional complexity of immune system (innate vs adaptive immunity)

Answer 1

The Immune System (IS)

• Composed of many interacting organs, cells and proteins
• Recognises invading pathogens (non self)/ foreign substances and defends body against these by producing an immune response
• In many species, IS can be classified into sub systems: innate (nonspecific) and adaptive (specific) IS

Pathogens

• Pathogens: invading microorganisms that cause disease (virus, fungi, bacteria, protozoa, parasites)
• Foreign substances include: toxins, pollution

How does the IS recognise a pathogen?

• The IS recognises antigens (Ag) on surfaces of Invading pathogens as non self
• Ag can be: cell surface protein, glycoprotein, lipopolysaccharide, lipids. Non specific innate immune cell receptors bind Ag directly.
• Presence of Ag causes IS to be activated to destroy the invading pathogen
• Innate immune cells rapidly recognise pathogen Ags directly via invariant (non specific) receptors that trigger: killing, phagocytosis (destruction) and/ or Ag presentation.
• Adaptive immune cells recognise pathogen Ags following their presentation by APCs via specific receptors to activate cell expansion & pathogen killing

Immune Systems: Innate vs Adaptive

Complexity of Immune responses

1st line of defense: Mechanical/ Chemical

• Surface barriers protect against invading pathogens: hair, keratin (key structural material making up skin outer layer), mucous membranes, antimicrobial secretions destroy pathogens before they enter body
• Damage to surface barriers allow pathogens to pass surface barriers

2nd line of defense: Innate Immunity

3rd line of defense: Adaptive Immunity

Question 2

1.Describe structure and distribution of lymphatic network linking all lymphoid organs

Answer 2

Lymphatic System

• Lymph system similar to cardiac system: one way flow ( valves) that begins in tissue and ends → heart.
• 3 main parts: Vessels carry fluid, Lymph (fluid in vessels), Lymph Nodes (filters lymph of pathogens)
• Plasma from blood vessels seeps into tissues
• Lymphatic vessels depend on muscle contractions for lymph to flow back to heart
• Lymph and lymphocytes empty into left subclavian vein of the heart

Question 3

1.Describe basic structure & cellular population of primary, secondary, tertiary lymphoid organs

Answer 3

Specific organs needed for development and function of IS:

1. Primary Lymphoid organs: Bone marrow and thymus, site of immune cell origin and lineage commitment (differentiation of HSC into lymphoid or myeloid progenitor cells)
2. Secondary Lymphoid Organs: Lymph nodes and liver, traps Ag and provides environment for interaction with mature immune cells
3. Tertiary Lymphoid Tissues: Import immune cells and APC* during an inflammatory response

• MALT- mucosal associated lymph tissue
• GALT- gut associated lymph tissue
• BALT- bronchial associated lymph tissue

Lymphoid Organs

• Primary organs (development) : Bone marrow, thymus
• Secondary organs (maturation): lymph nodes, liver and spleen
• Tertiary tissues: MALT, GALT, BALT ( aggregates of cells in lamina propria of mucosa-, gut-, bronchus)
• All lymphoid organs are connected by the lymphatic system.

Question 4

1.The process of haematopoiesis (immune cell production)

Answer 4

Primary Organ (development)

Bone Marrow (BM):

• BM is site of : Haematopoiesis (formation of all blood cells from CD34+ HSC*)
• BM MSC (major histocompatibility complex) assist B cell maturation by: providing source of self Ag → B cell apoptosis & Cytokine (IL-7) for B cell development

Haematopoiesis

• HSC (CD34 + Haematopoietic Stem Cell): reside in bone marrow, multipotent, 1 HSC per 50,000 BM cells (~ 3x 10^8 cells in mouse BM), extremely proliferative if need arises
• HSC differentiate into common lymphoid or myeloid progenitor cells = lineage commitment
• Mesenchymal stromal cells (MSC) support HSC development in BM and in vitro

Hematopoietic Growth Factors

• Growth factors and cytokines determine cell lineage commitment:
• Myeloid Growth Factors: Multi CSF (IL-3), M-CSF (Macrophage CSF), G- CSF (Granulocyte CSF), GM- CSF ( Granulocyte Monocyte CSF)
• Erythropoietin (Epo): induces production of RBCs