Immunology 2

Question 1

Macrophage

- Which 2 cells can they present as and how do they switch?

Answer 1

• In form of monocyte precursor (found circulating in blood) or Mature effector cells (found in most tissues in submucosal layers.)
• Switch from monocyte to mature macrophage occurs when they move and stay in tissues.

Question 2

Macrophages:

- What is their function and some features?

Answer 2

• Early immune responders
• Relatively long life span
• Phagocytic = Engulf and kill pathogens and infected/dead/damaged host cells.
• Role in activation of other immune components:
• — Present foreign material to T cells
• — Instigate inflammatory response via cytokine secretion which also attracts other immune cells.

Question 3

What initiates inflammation?

Answer 3

Macrophages and complement initiate inflammation.
Cytokines, chemokines and complement components attract additional phagocytes and cause local vascular changes = increases diameter = slower blood flow

Question 4

Macrophage role in initiating inflammation:

Answer 4

• Following recognition of pathogens, activated macrophages release chemokines and cytokines such as interleukin 1 (IL-1) or tumour necrosis factor (TNF).
• Cytokines = signals that change the behaviour of cells and tissues. E.g. change blood vessel cells = more leaky so products and cells in the blood can pass through the epithelium and get to infection site.
• Chemokines = chemoattractant proteins e.g. interleukin 8 (IL-8) along with the activated complement anaphylatoxins C3a and C5a attract further monocytes and neutrophils to the site of infection.

Question 5

Granulocytes:

3 types?

Answer 5

Neutrophils
Eosinophils
Basophils

Question 6

What are neutrophils also known as?

Answer 6

Polymorphonuclear neutrophilc leukocytes or PMN.

Question 7

Features of neutrophils:

Answer 7

• Phagocytic- killing bacteria
• Bactericidal
• Numerous - many are recruited to the site of infection
• Short lifespan - pus = mainly dead neutrophils.

Question 8

Dendritic Cells:
Lineage?
Location?
Function?

Answer 8

Lineage: can arise from both myeloid and lymphoid lineages.

Located: immature = under surface epithelium and in solid organs. Cells migrate to the lymph nodes via lymphatic system (following uptake of antigens) where they mature.

Function:

• Most important function is to initiate the adaptive response by travelling to the lymphoid organs. They present the foreign material from the engulfed pathogens to activate adaptive cells.
• Can phagocytose (ingest and degrade)

Question 9

Natural Killer Cells

• Location
• Function

Answer 9

• 10% of lymphocytes (others = B and T cells).

Location
- In the blood, can migrate to tissues or tumours

Function
- Innate immunity
- Look for signs of infected cells and kills them
Anti viral
Anti tumour
- Extracellular release of cytotoxic granules the store in the cytoplasm - in the direction of identified targets.

Question 10

How do our phagocytic innate immune cells recognise the material that they are going to engulf?
(macrophages, neutrophils and dendritic cells)

Answer 10

• They identify items to phagocytose through binding to them with receptors the express on their cell surfaces.
• Different receptor types recognise different structures
• Some receptors recognise host antibodies or proteins that are attached to the pathogen surface (opsonisation).
• Phagocytes also recognise common pathogen structures called PAMPS (pathogen associated molecular patterns). The receptors that recognise PAMPS are Pattern Recognition Receptors on macrophages.

Question 11

PRR and PAMPS

Answer 11

Phagocytic receptors - Pattern recognition receptors PRR recognise PAMPS
PAMPS (pathogen associated molecular patterns)
- Lectin like. Dectin 1
- Scavenger receptors.
- Toll like receptors

Question 12

What receptors do Phagocytes have to recognise foreign material?

Answer 12

• Complement receptors
• Fc receptors (these bind directly to antibodies that coat pathogens)
• Pattern Recognition receptors PRRs which recognise PAMPS

Question 13

Process of phagocytosis:

Answer 13

E.g. when recognising a bacterium via the complement receptor on the phagocyte binding to activated complement.
The C3b bound to the bacterial cell surface is acting as an opsonin, like a highlighting label, enabling phagocytes that express receptors for C3b to ingest the complement-coated bacteria more easily.
- CR1 receptor on phagocyte binds to the C3b on the bacterium and initiated intracellular signalling.
- Endocytosis of the bacterium by the macrophage.
- Phagosome containing the bacterium forms in the macrophage- this fuses with lysosomes (contain degradative enzymes) which form the phagolysosome.

Question 14

Toll Like Receptors

Answer 14

• TLRs
• 9 functional TLRs in humans, each recognises a distinct set of PAMP not found in vertebrates/healthy cells.
• Eg. 1 TLR recognises LPS of gram negative bacterial outer membrane, and one that recognises lipoteichoic acids of gram positive cells walls, ds RNA, glucans such as zymosan (fungi).
• Not just expressed on innate immune cells.
• Trigger expression of antimicrobial peptides (saliva) and inflammatory cytokines.
• TLRs can trigger phagocytosis and also the secretion of antimicrobial peptides and inflammatory cytokines.

Question 15

Antimicrobial mechanisms of phagocytes:

Answer 15

• Acidification - pH 3.5 - 4
• Toxic oxygen- derived products ROS
• Toxic Nitrogen oxides.
• Antimicrobial peptides - e.g. cathelicidin (macrophages and neutrophils) and alpha/beta defensins (neutrophils store in their cytoplasm granules).
• Enzymes - Lysozyme and acid hydrolases
• Competitors (neutrophils) - lactoferrin- sequester iron in the extracellular environment, in this way they limit the nutrients available for bacterial growth and inhibit proliferation.

Question 16

PRRs intracellular action:

Answer 16

• Some PRR recognize PAMPS or DAMPS (Damage associated molecular patterns) within cells.
• Some types of TLR are expressed in the endosomes.
• TLRs can recognize the nucleic acids from ingested microbes found in endosomes.
• Bacterial products and microbial genomic material might also be found in the cytosol of infected cells and here it can also be detected by PRRs such as the NOD like receptor, RIG-like receptor and cytosolic DNA sensor.

Question 17

Anti-viral interferon response.

Answer 17

Detection of viral infection by PRRs triggers the release of type 1 IFNs.
Interferons (IFNs) are a class of cytokine.
Type I IFNs:
- IFN-α IFN-β
- Secreted from virally infected cells and Dendritic Cells (that engulfed viral pariticles).
- IFNs are then detected by other cells and induce an anti-viral state (upregulation of of genes associated with resistance to infection).
- Inhibition of the production of new virus particles (protein synthesisis) as well as inhibition of viral gene expression and virion assembly
- Increased degradation of viral mRNA
- Activate NK cells.

Question 18

Adaptive immunity overview:

Answer 18

Vast range of recognition specificity- allows us to cope with mutational capacity of pathogens.
Random generation of specificity
Each immature cell has a single and unique specificity for antigen

Question 19

Development of adaptive lymphocytes and receptor specificity:

Answer 19

• As each new immature adaptive lymphocyte develops it gains a receptor for antigen, each of these cells expresses a single type of antigen receptor and this is unique for this cell.
• The cell expresses lots of copies of this antigen receptor but on any one cell they are all identical, however unique and different to those found on other lymphocytes.

Question 20

Q1 We need more than one individual cell of each specificity to combat an infection, how is there space in the haematopoietic system?
What happens and how does this process occur?

Answer 20

• Problem - Only a small number of cells will have receptors that recognise the molecules on the pathogen currently infecting us, that’s not enough to mount effective immune response
• A pathogen specific adaptive lymphocytes binds the pathogen via binding their SPECIFIC antigen receptor, signals from the receptor trigger final differentiation and maturation into a fully mature effector cell.
• CLONAL EXPANSION- the cells undergo rounds of cell division to generate a whole clone of many daughter cells all with the same specificity to fight the detected infection.

Question 21

Q2 How is there enough space in the genome to encode all these specific receptors?

Answer 21

The DNA sequences encoding the antigen receptors in mature effector cells is different to that in progenitor/ stem cells.
These experiments revealed that these genes are rearranged to generate a vast range of receptors with different specificities.

Question 22

Q3 Random receptor generation cause problems with self / non self recognition?

Answer 22

Removal or inactivation of self reactive adaptive effectors in a process called clonal deletion.

Question 23

B Cell expansion

Answer 23

• The antigen receptor on B cells, the B cell receptor (BCR), contains a membrane bound form of antibody molecule.
• Following antigen recognition, mature effector B cells called plasma cells and long lived memory cells are produced following clonal expansion.
• The plasma B cells secrete antibodies. The antibodies they secrete have the same specificity as the antigen receptor.
• Antibody is the secreted form of the Immunoglobulin protein that is found in the antigen receptor on the B cell.

Question 24

T Cell expansion and role:

Answer 24

The T cell receptor (TCR) is not an antibody molecule, though it is related to Immunoglobulin. It is always membrane bound. Clonal expansion of T cells leads to the generation of effector T cells and long lived memory T cells.

Question 25

The protection provided by T cells is sometimes also referred to as the:

Answer 25

cell mediated immunity

Question 26

T cell receptors and recognition:

Answer 26

The TCR is only ever expressed as a membrane bound protein. It also cannot recognize antigen unless it is accompanied by a protein called MHC. The antigen has to be bound to MHC and presented in this way in order for the TCR to recognize it.

Question 27

Epitope (antigenic determinant):

Answer 27

A SITE ON AN ANTIGEN that is recognised by specific lymphocyte receptors.
These are carbohydrates, proteins or both: typically one epitope would = a cluster of amino acids or part of a CHO chain.

Question 28

Continuous / Linear Epitope:

Answer 28

a single fragment of polypeptide chain

i.e. antigen and antibody sequence matches continuously without gaps

Question 29

Conformational / discontinuous epitope:

Answer 29

amino acids that are not continuous in the polypeptide sequence but are brought together in 3D structure of the protein and it is that conformation that is recognised by the receptor or antibody.

i.e. antibody recognises and binds to a couple of sites on the antigen

Question 30

B Cell epitopes:

Answer 30

The epitopes recognised by B cells are more likely to be discontinuous and found on the surface of the antigen.

Question 31

T cell Epitopes

Answer 31

Epitopes recognized by T cells are more likely to be continuous and may be derived from parts of the antigen structure that are not on the surface of the folded antigen but could even be buried within it.