1. Cells of the immune system

Question 1

What is immunity?

Answer 1

Protection from disease especially infectious disease
Cells and molecules involved in such protection constitute the immune system and the response to introduction of a foreign agent known as the immune response

Question 2

What are markers of self?

Answer 2

Every body cell carries distinctive molecules that distinguish it as self, normally the body’s defences do not attack tissues that carry a self - marker; immune cells co-exist with other body cells in a state known as self-tolerance

Question 3

What are examples of auto-immune disease?

Answer 3

sclerosis, diabetes, rheumatoid arthritis or myasthenia gravis

Question 4

What are markers of non -self?

Answer 4

Foreign molecules carry distinctive markers
Any substance capable of triggering an immune response is known as an antigen - can be bacteria, virus or portion of product (toxin) of anyone of these organisms
Tissues or cells from other individuals also act as antigens

Question 5

What are the two levels of defence in a normal human?

Answer 5

1st = present in neonatal animals and in invertebrates namely natural or innate immunity, also referred to as non-specific 
2nd = adaptive or acquired immunity and is confined to vertebrates

Question 6

What are innate host defences?

Answer 6

Physical barriers = skin + mucous
Physiological factors = pH, temperature, acid environment of the stomach, normal flora
Protein secretions = complement, interferons
Phagocytic cells = macrophages and polymorphonuclear leukocytes

Question 7

How does the innate immune system provide the “early warning system” for infection?

Answer 7

Uses pattern - recognition receptors (PRRs) to detect microbial components which are intrinsically foreign
AND
Pathogen - associated molecular pattern (PAMPs)
- mannose (bacterial carbohydrates)
- Lipopolysaccharide (lipids)
- peptidoglycans

Question 8

What is phagocytosis?

Answer 8

Ingestion and destruction by individual cells such as macrophages, neutrophils and dendritic cells
Microorganisms release substances (or during inflammation) that attract phagocytic cells and that allow them to be readily digested (opsonisation)

Question 9

Describe the process of phagocytosis?

Answer 9

Phagocyte attaches to the bacteria known as the attachment phase
The phagosome forms
Phagosome is killed and digested
Phagosome joins with lysosome and forms phagolysosome
Then residual bacteria are released in the post-digestion phase

Question 10

What is the innate immunity-complement?

Answer 10

Complement = proenzymes that can be activated into active enzymes during innate and acquired immunity, helps in the clearance of pathogens

1. Bacteria trigger macrophages to release cytokines and chemokines.
2. Vasodilation and increased vascular permeability cause redness, heat and swelling
3. inflammatory cells migrate into tissue, releasing the inflammatory mediators that cause pain

Question 11

What is adaptive immunity?

Answer 11

Immune system adapts to infection of a particular antigen
Foreign antigen is recognised by lymphocytes, acquires memory towards it
Confers a life long protection (protective immunity) to the same antigen
1st encounter with antigen is known as primary response
Re-encounter with same antigen causes a secondary response, more rapid + powerful

Question 12

What do B and T cells release as they engage with a specific antigen?

Answer 12

B cells release antibody

T cells release cytokines

Question 13

What are the three ways antibodies work?

Answer 13

Neutrilisation
Opsonisation
Complement activation

Question 14

What is neutrilisation?

Answer 14

Blocking biological activity of the target molecule e.g. a toxin binding to its receptor

Question 15

What is opsonisation?

Answer 15

Interaction with special receptors on various cells inc. macrophages, neutrophils, basophils and mast cells allowing them to ‘‘recognise” and respond to the antigen

Question 16

What is complement activation?

Answer 16

Cause direct lysis by complement-complement recruitment also enhances phagocytosis

Question 17

What is the general structure of an antibody?

Answer 17

Antibodies have two ends

• one end = interacts with the antigen (variable)
• other end = interacts with immunoglobulin receptors on a variety of cells (constant)

Question 18

What are the two ways T-cells can work?

Answer 18

Some help CD4 regulate the complex workings of the immune system
Others are cytotoxic CD8 and directly contact infected cells and destroy them

Question 19

What are cells of the innate immune system?

Answer 19

Scavengers that hunt and destroy

• monocyte
• macrophage
• eosinophil
• mast cell
• neutrophil
• basophil

Question 20

What are cells of the adaptive immune system?

Answer 20

Antibody - destroy 
- B-cell
- plasma cell 
Killers & helpers 
- immature T cell 
- mature T helper cell
- mature cytotoxic T cell

Question 21

How do immune cells work?

Answer 21

Lymphocytes - T&B cells

• cells have receptors for antigen and confer specificity on an immune response
• lymphocytes express receptors with varying affinity for the antigen in question
• cell with the highest affinity for most abundant antigen with have a growth advantage and will preferentially generate progeny of itself (clonal expansion, antigen driven)

Question 22

What is clonal expansion?

Answer 22

• each receptor is created even though the epitope (antigen) it recognises may have never been present in the body
• if an antigen with that epitope should enter the body, the few lymphocytes able to bind will do so and will expand to give rise to effector and memory cells (acquired immunity)

Question 23

How do B cells work?

Answer 23

• by secreting soluble substances i.e. antibodies, arise in adult bone marrow
• each b cell is programmed to make one specific antibody
• When a B cell encounters its triggering antigen (along with various accessory cells), it gives rise to many large plasma cells

Question 24

What is a plasma cell?

Answer 24

Factory for producing one specific antibody

Question 25

What does each antibody consist of?

Answer 25

Two identical heavy chains
Two identical light chains
- shaped to form a Y
Variable region that binds to antigen
Stem of Y links antibody to other participants in the immune defences
Constant region is identical in all antibodies of the same class e.g. IgE’s

Question 26

What are T cells?

Answer 26

Generated in thymus
Do not produce antibody molecules, have surface receptors structurally related to Ig
- recognise peptide fragments of antigen complexes within cell surface MHC glycoproteins on neighbouring cells
– cell surface glycoproteins encoded by genes in the MHC bind fragments of antigen after it has been subject to antigen processing

Question 27

What are cytokines and how do they work?

Answer 27

Diverse and potent chemical messengers secreted by cells of the immune system

• Binding to specific receptors on target cells - recruit other cells & substances to the field of action
• encourage cell growth, promote cell activation, direct cellular traffic and destroy target cells - inc. cancer cells
• also serve as a messenger between white blood cells (leukocytes), many cytokines are also known as interleukins

Question 28

What are natural killer cells?

Answer 28

Lack T&B cell receptors - innate immune system
Cytotoxic cells containing granules with potent chemicals and kill on contact
- play a role in the rejection of tumours and cells infected by viruses

Question 29

What is a mononuclear macrophage?

Answer 29

Cells that are present in many cells of the body that take up large particulate antigens, pieces of tissue, senescent cells, bacteria etc. by phagocytosis
- called microgial cells in the CNS, Langerhan’s in the skin, kupffer cells in the liver, monocytes in blood (cells of the reticuloendothelial system) (RES)

Question 30

What are important properties of mononuclear phagocytes?

Answer 30

• front line of defence - recognise bacteria and viral proteins
• act as antigen presenting cells (APC) for T cells
• activated by T cell cytokines and inflammatory mediators
• express receptors for antibody and “complement”, bind immune complexes
• act as scavengers for cell debris and senescent cells (splenic macrophages bind to “old” erythrocytes)

Question 31

What are the cellular receptors used by macrophages to recognise bacteria?

Answer 31

Toll - like receptors (TLR’s)
- TLR2 - peptidoglycan, TLR4 - lipopolysaccharide (LPS), TLR5- Flaggellin, TLR9- CpG DNA (5’-C-PHOSPHATE-G-3’)
Scavenger receptor
- Scavenger receptor A, modified LDL, LPS
- Mannose receptor
- MMR carbohydrates (dextran)
- CD205

Question 32

How do dendritic cells initiate adaptive immune responses?

Answer 32

Immature DC reside in peripheral tissue take up antigen
Migrate to lymph node as mature DC
Activate T cells

Question 33

What are granulocytes?

Answer 33

Basophils - (very granulated, stain a darker colour)
Eosinophil - (2 lobed nucleus)
Neutrophil - (many lobed nucleus)

Question 34

What is a neutrophil?

Answer 34

Polymorphonuclear leukocytes, express receptors for immunoglobulin and complement, involved in acute inflammatory response

Question 35

What are eosinophils?

Answer 35

Carry receptors for IgE, involved in the destruction of IgE coated parasites (helminths), contribute to allergy response

Question 36

What are basophils?

Answer 36

Circulating counterpart of tissue mast cells express high affinity receptors of IgE and are stimulated to secrete chemicals responsible for immediate hypersensitivity following antigen induced aggregation of these receptors

Question 37

What are the stages of infection?

Answer 37

1. Adherence to epithelium
2. Local infection, penetration of epithelium
3. Local infection of tissues
4. Adaptive immunity