Adaptive Immunity

Question 1

What are interferons?

Answer 1

They are signalling proteins that are released by host cells.
They stimulate the infected cells to produce proteins that inhibit the spreading of the virus.

Question 2

What are some differences between the innate and adaptive immunity?

Answer 2

Innate
Diversity: large
Memory: No
Cells: Phagocytes

Adaptive
Diversity: limited
Memory: yes
Cells: Lymphocytes, NK cells

Question 3

What is the function of dendritic cells?

Answer 3

It is a type of phagocyte that shows antigens on its surface (APCs) to T cells.

They act as messengers.

Question 4

Which complement protein is the molecular marker for cytotoxic T cells?

Answer 4

CD8

Question 5

What is the function of cytotoxic T cells, and how do they do this?

Answer 5

They directly kill target cells which are infected (viral/bacterial), and tumour transformed cells.

They do this by releasing perforins to generate pores on the cells surface and induce apoptosis.

They can also induce Fas-mediated cytotoxicity. A Fas ligand is activated and induces apoptosis via the Fas pathway.

Question 6

Which complement protein is the molecular marker for T helper cells?

Answer 6

CD4

Question 7

What is the function of T helper cells, and how do they do this?

Answer 7

They stimulate cytotoxic T cells, and help B cells produce and secrete antibodies. This occurs due to:

1. Cytokine release
2. Binding of CD40L on the T cell to the CD40 receptor on the B cell
3. Interactions with TNF-TNF receptor ligands

Question 8

Which complement protein is the molecular biomarker for B cells?

Answer 8

CD20

Question 9

What do B cells do?

Answer 9

They differentiate into antibody producing plasma cells with correct stimulus from T helper cells.

Question 10

Which cell is mainly associated with humoral immunity?

Answer 10

B cells.

Question 11

What is the function of humoral immunity?

Answer 11

It blocks infections and eliminates extracellular microbes.

Question 12

Which cells are mainly associated with cell-mediated immunity?

Answer 12

Phagocytes and T lymphocytes (helper/cytotoxic)

Question 13

What is the function of cell-mediated immunity?

Answer 13

Activate macrophages to kill phagocytosed microbes, and to kill infected cells & eliminate reservoirs of infection.

Question 14

What do all nucleated cells have?

Answer 14

Molecular marker MHC (major histocompatibility complex)

Question 15

What is the difference between MHC I and MHC II?

Answer 15

MHC I: is a self-marker protein (expressed on all somatic cells)

MHC II: they present processed antigens on their cell surfaces for T cell recognition.

(Think of MHC II like a ‘serving plate’, where they ‘serve’ the antigen)

Question 16

What can occur when MHC molecules present antigen contents?

Answer 16

B cell activation, and T cell stimulation.

Question 17

What is HLA (Human Leukocyte Antigen)?

Answer 17

It is a complex of genes which encodes for cell-surface proteins responsible for the immune system.

Question 18

What is allorecognition?

Answer 18

It is when the host T cell receptor can distinguish between self MHC I, and non-self MHC.

This can occur from e.g. transplants

Question 19

Which cells can only bind to MHC on an APC?

Answer 19

T helper cells & B cells.

Question 20

What is the costimulator in an APC, and what does it bind to on the T helper cell?

Answer 20

Co-stimulator B7 on the APC binds to its co-stimulator ligand CD28 on the T helper cell.

Question 21

How many T cells are present in lymphoid tissue, and what do they do?

Answer 21

16 T cells are present randomly in lymphoid tissue, and generate an immune response which expels invading pathogens in <6 days.

Question 22

What are naive T cells?

Answer 22

They are matured precursors for effector/memory T cells, which have not yet encountered an antigen.

Question 23

How is the adaptive immune response initiated?

Answer 23

Lymphocytes recognise that a cell is either non-self, or contains elements of non-self presented on MHC
Adaptive immune response is then initiated.

Question 24

What is the whole process of the adaptive immune response, from start to finish?

Answer 24

1. A dendritic cell ingests the microbe and releases cytokines to induce cytokine-mediated activation (inflammation).
2. The dendritic cell migrates to the lymph nodes via lymphatic vessels
3. The dendritic cell matures into an APC and expresses MHC to display the microbial antigen & costimulators (B7)
4. Inside the lymph node, the APC presents the antigen to a naive T cell (in the T cell zone) via MHC
5. Circulating naive T cells migrate into the lymph nodes and encounter more APCs
6. A naive CD4 T helper cell recognises a peptide antigen and costimulator B7 on the APC
7. The T helper cell becomes activated and releases cytokines to stimulate its own proliferation and differentiate into effector T cells.
8. The activated T cell undergoes clonal expansion (proliferation)
9. While an APC is activating a T cell, soluble antigens begin to bind to naive B cells via surface Ig
10. MHC type 2 on the B cell binds to the antigen and it begins to migrate to the edge of the follicle.
11. Activated T & B cells move towards the edge of the follicle, where B cells present the antigen to the T cell
12. The activated B cell proliferates with T cell and cytokine stimulation
13. Proliferated B cells then differentiate into antibody secreting plasma cells, and memory cells.
14. Plasma cells secrete antibodies that enter the circulation via the efferent lymphatic vessel and target microbes for destruction
15. The antibody binds to the microbe and hinders their ability to interact with cell receptors, hence preventing host cell infection

Question 25

What can free antibodies circulating in the body do?

Answer 25

They can bind to antigens (opsonisation), and allow the microbe to be captured by a phagocyte. (Remember opsonisation 'to make food ready for-')

Question 26

What occurs when an opsonised microbe is captured by a phagocyte?

Answer 26

1. The Fc region of the antibody binds to the Fc receptors of the phagocyte 2. Signals from the Fc receptor promotes phagocytosis of the opsonised microbe. 3. Granules are released resulting in the killing of the target cell

Question 27

What is the process of complement activation?

Answer 27

1. A C1 complex binds to 2 or more antigen-bound antibodies (opsonised microbes) 2. Complement cascade is activated and a MAC is produced (membrane attack complex) 3. Pores are formed in the microbial membrane and ions/water are able to pass through, resulting in osmotic lysis of the microbe 4. C3 and C5 fragments from the C1 complex cleave to form C3a and C5a to induce acute inflammation by activating mast cells & neutrophil chemotaxis