2. Antigen processing & Presentation

Question 1

What are the three major classifications of cell types involved in the immune response?

Answer 1

• Polymorphonuclear leukocytes PMNs (granulocytes)
• Monocytes
• Lymphocytes

Question 2

Which cell type can be considered the first line of defence against an invading pathogen?

Answer 2

PMNs

• Neutrophils in particular
• Drawn towards infection by chemotaxis

Question 3

Describe PMNs…

Answer 3

• Can discriminate between self and nonself
• Not programmed to ingest specific antigens
• Ingestion is facilitated when the antigen is coated by antibody or by complement protein
• Don’t possess immunological memory

Question 4

What are the two types of T helper cells and what do they do?

Answer 4

Th1: Promotes cell mediated immunity

Th2: Promotes antibody production

Question 5

Describe Neutrophils…

Answer 5

• Lifespan of only a few days
• Contain primary and secondary granules in their cytoplasm
• Possess many receptors on the cell surface
• Receptors for Ig
• Capture foreign material through phagocytosis

Question 6

Describe the process of Neutrophils phagocytosing…

Answer 6

1. Chemotaxis
2. Adherence & Opsonization
3. Ingestion
4. Digestion

Question 7

What is the fate of neutrophils?

Answer 7

• They are capable of undertaking only a limited number of phagocytic events
• After phagocytosis many undergo apoptosis and are removed by macrophages

Question 8

What is Opsonization?

Answer 8

Coating of a particle with molecules that promote phagocytosis
- molecules are called opsonins

Question 9

What are the three major types of PMNs?

Answer 9

1. Neutrophils
2. Eosinophils
3. Basophils

Question 10

Describe Eosinophils…

Answer 10

• Second major PMN
• 12 day half life in tissues
• 30s half life in circulation
• Phagocytic cells that can ingest and destroy foreign material
• More effective in the extracellular digestion/ damage of infectious agents
• Contain two types of granules
• Possess surface receptors for immunoglobulin and complement

Question 11

What is the major function of Eosinophils?

Answer 11

The destruction of parasitic helminths
- Antibody coated parasites attach to the eosinophil which degranulates and releases its granule contents into the worm cuticle

Question 12

Describe Basophils…

Answer 12

• PMN
• Found primarily circulating in the blood
• Contains a complex of vasoactive molecules
• Have high affinity IgE receptors on their cell surface

Question 13

What are lymphocytes…

Answer 13

Cells that originate from bone marrow and lymphoid stem cells

Question 14

What are the primary lymphoid organs?

Answer 14

Organs that regulate the production and differentiation of lymphocytes

• Thymus
• Bursa (birds)
• Peyer’s patches
• Bone marrow

Question 15

What are the secondary lymphoid organs?

Answer 15

Tissues where antigen processing occurs and where lymphocytes respond to antigen

• Tonsils
• Spleen
• Peyer’s patches
• Bone marrow

Question 16

How can B cells and T cells be differentiated from each other?

Answer 16

By their surface components (antigens and receptors)

Question 17

Describe a B cell receptor…

Answer 17

BCR

• Consists of antibody molecules bound to the cell membrane in such a way that they are able to bind antigen to the B cell surface
• Can bind to free antigen molecules

Question 18

Describe a T cell receptor…

Answer 18

TCR

• Not an immunoglobulin molecule
• Always associated with a protein called CD3
• Found on all T cells and not on any other cell type
• Can only bind to processed antigen that is associated with MHC molecules on the surfaces of APCs

Question 19

CD4+8-

Answer 19

T helper cell

• CD4
• Involved in positive regulation of the acquired immune system
• Without them, the immune response to a particular foreign antigen would be weak/ non-existent
• Able to communicate with other cells of the immune response and activate/ regulate them by cytokine release
• TH1 and TH2 cells secrete different sets of cytokines

Question 20

CD4-8+

Answer 20

Cytotoxic T cell

• CD8
• Lytic capability
• Important for the recognition and elimination of virus infected and tumour cells
• Requires antigen to be presented to them by MHC I molecules on other cells

Question 21

Describe TH1 cytokines…

Answer 21

Key mediators of cell-mediated immunity

e.g. macrophage activation and delayed-type hypersensitivity

Question 22

Describe TH2 cytokines…

Answer 22

Stimulate B cell proliferation and immunoglobulin secretion

Question 23

What are regulatory T cells?

Answer 23

T cells that are involved in controlling the intensity of the immune response
- Secrete cytokines that turn down the immune system

Question 24

What is a Natural T reg cell?

Answer 24

T cells that originate in thymus and are present in the host before pathogen exposure

Question 25

What is an Inducible Treg cell?

Answer 25

T cell produced in the secondary lymphoid tissue and acquire regulatory functions in the context of a given infection

Question 26

What are natural killer cells?

Answer 26

A subset of lymphocytes that arise from precursor cells in the bone marrow - Present in blood, bone marrow and spleen - Defense against viral infection - May have a role in immune surveillance to protect against metastasis of tumours

Question 27

What are the three major routes an antigen can follow once it gains access to the body?

Answer 27

1. Access via the bloodstream 2. Access via epidermal, dermal or subcut tissue 3. Access via GIT or respiratory tract

Question 28

Describe monocytes...

Answer 28

- Develop from immature hematopoietic progenitor cells in the bone marrow - Progenitor cell is the monoblast which develops into a promonocyte then into a monocyte - Mature monocytes enter the circulation

Question 29

Where are MHC I found?

Answer 29

On the surface of most nucleated cells (including APCs)

Question 30

Where are MHC II found?

Answer 30

Only on the surface of lymphocytes, dendritic cells an macrophages

Question 31

Where are MHC III found?

Answer 31

Not found specifically on cell surfaces

Question 32

What is a thymus dependent antigen?

Answer 32

Molecules that are composed of repeating units that can directly trigger the antibody producing B cell to produce IgM

Question 33

When does the T helper cell recognise an antigen?

Answer 33

Only when it is together with MHC antigens on the surface of the APC - Ensures no self antigens evoke an immune response

Question 34

What happens once an antigen has been recognised?

Answer 34

the antigen is fragmented in an acidic compartment that is produced when the endosome fuses with the lysosome - Here the individual epitopes from the antigen are thought to become associated with MHC II

Question 35

What are the three major functions of dendritic cells?

Answer 35

1. Act as sentinel cells and activate immune responses 2. Process exogenous antigen for presentation to T cells 3. Direct the emphasis of the immune response to be predominantly antigen mediated or cell mediated

Question 36

What are plasmacytoid dendritic cells?

Answer 36

Found in blood, bone marrow and lymphoid tissue | - derived from lymphoid precursor

Question 37

What are myeloid dendritic cells?

Answer 37

Differentiate from blood monocytes | - Derived from myeloid precursor

Question 38

What are follicular dendritic cells?

Answer 38

Found in secondary lymphoid organs | - Derived from myeloid precursor

Question 39

What are langerhaans cells?

Answer 39

Found in epidermis and mucous membranes - Derived from myeloid precursor - They can pick up and process antigen and travel to lymph nodes where they present antigen to activate T cells

Question 40

What does a T helper cell require for it to become activated?

Answer 40

- Binding of TCR to MHC II on the APC | - Cytokines

Question 41

What happens when T cells kill?

Answer 41

They exocytose granules that release their contents (cytotoxins) into the intracellular space between the target cell and the effector - Among granule contents are proteases which degrade proteins - Apoptosis can also be triggered

Question 42

What is the sequence of events that occur to initiate the antibody production by plasma cells?

Answer 42

1. T helper cell activated following antigen presentation by APC 2. Relevant B cells are engaging antigens through their immunoglobulin receptors 3. Antigen-receptor complex is drawn into the cell by endocytosis 4. Full activation of B cell is achieved when cytokine signals are provided by a T helper cell 5. B cells also function as APCs 6. Effector T cells can then deliver the appropriate cytokines that can affect other T and B cells that are in close proximity