Immunology 3- complement system and connection to acquired responses

Question 1

Are there normally low or high levels of inactive complement system proteins in the extracellular fluid?

Answer 1

Low.

Question 2

How can the complement system become activated?

Answer 2

With hydrostatic pressure some of these proteins can leak out from the capillary beds into extracellular fluids. If a pathogen is present , the complement system can be activated which causes a cascade of chemical reactions.

Question 3

What do the chemical reactions caused by the activated complement system prompt?

Answer 3

• Opsonization of pathogens
• Direct pathogen killing
• Acute inflammation
• Leukocyte recruitment

Question 4

What is C3, what happens to it, and what does it do?

Answer 4

It is a protein of the complement system.

C3 (inactive form) ——> C3b + C3b (active) by cleaving.

Causes down stream pathways.

Question 5

What are the three ways in which C3 can be cleaved?

Answer 5

• Classical pathway
• Mannose-binding lectin pathway
• Alternative pathway

Question 6

What do C5b and C3b do?

Answer 6

Are a good opsonins which means they enhance phagocytosis

Question 7

What do C5a and C3a do?

Answer 7

Are involved in acute inflammation create a positive feedback loop, changing the vasculature etc. Also have an effect on mast cells.

Question 8

What are dendritic cells?

Answer 8

Phagocytic cells which create a link between the innate and the acquired immune systems. They are activated by the acute inflammatory signals and play an important role in activating T cells. Present antigens on their cell surfaces (professional antigen presenting cell)

Question 9

Describe the alternative pathway.

Answer 9

C3b is unstable. If there is a pathogen present, it can bind to the molecules on the pathogen’s surface and become stable. It can feed back into the alternative pathway with chemical signals from C3b) which generates an enzyme complex called C3 convertase which can then make more C3.

Question 10

Why in a non infectious environment is the complementary system inactive?

Answer 10

Cells have inhibitory molecules on them that make sure C3b doesn’t activate and cause down stream pathways without the presence of a pathogen.

Question 11

Describe the Mannose-binding lectin pathway.

Answer 11

Mannose binding lectin (MBL) Binds to certain sugars including mannose found specifically on types of bacteria. Mannose is not shown on human cells. Binding of Mannose-binding lectin to mannose it will activate C3 convertase and cause a cascade of down stream events which will lead to the formation of an enzyme which will cleave C3 to C3b and C3a.

Question 12

Describe the downstream complement pathway.

Answer 12

C3b binds to pathogen and interacts with other inactive molecules which will generate an enzyme complex to form C5a and C5b. C5b activates MAC (membrane attack complex) which forms pores and can directly kill pathogens.

Question 13

What are the two types of lymphocytes?

Answer 13

B cells and T cells.

Question 14

What do B cells do (general)?

Answer 14

They are responsible for humoral (through fluids) immune responses. They play a key role in attacking extraceullar pathogens and they produce antibodies that attack pathogens circulating in the blood and lymph.

Question 15

What do T cells do (general)?

Answer 15

They are responsible for cellular immune responses. They play a key role in defence against intracellular pathogens.

Question 16

What are the two types of T cells?

Answer 16

CD4+ T cells and CD8+ T cells

Question 17

Antigen receptors can only bind to one specific antigenic epitope. True or false?

Answer 17

True.

Question 18

What is the B cell receptor (BCR)?

Answer 18

A membrane bound antibody.

Question 19

Describe the structure of an antibody.

Answer 19

• It has a unique variable region (antigen binding site)
• it has 2 identical light polypeptide chains and 2 identical heavy polypeptide chains.
• a protein

Question 20

What is the only type of antigen that a T cell can recognise?

Answer 20

peptide antigens (presented by MHC molecules)

Question 21

Describe a T cell receptor (TCR).

Answer 21

• unique to one peptide antigen

- tips of the alpha and beta TCR chains form the hyper-variable binding site.

Question 22

What are MHC molecules?

Answer 22

• Major histocompatibility complex

- display peptide antigens to T cells

Question 23

What are the two classes of MHC molecules?

Answer 23

MHC class I and MHC class II

Question 24

Which type of T cell recognises MHC I molecules?

Answer 24

CD8+ T cells

Question 25

Which type of T cell recognises MHC II molecules?

Answer 25

CD4 + T cells

Question 26

Describe MHC I.

Answer 26

They are expressed on ALL nucleated cells

Question 27

Describe MHC II.

Answer 27

They are expressed ONLY on professional antigen presenting cells (APCs) e.g. dendritic cells and macrophages etc.

Question 28

Where are B cells made?

Answer 28

Bone marrow.

Question 29

Where are T cells made?

Answer 29

Thymus.

Question 30

What are primary lymphoid tissues?

Answer 30

Bone marrow and Thymus.

Question 31

What are secondary lymphoid tissues?

Answer 31

Spleen and lymph nodes.

Question 32

Where are T and B cells activated by antigens?

Answer 32

Secondary lymphoid tissues.

Question 33

What vessels do T and B cells enter and exit the lymph nodes from?

Answer 33

Afferent lymphatics (entry) and Efferent lymphatics (exit)

Question 34

How do B cells encounter antigens?

Answer 34

After a pathogen has been phagocytosed and the debris (antigens) has been released, the opsonised antigens travel to the B cell zones in lymph nodes where a B cell can detect the antigen.

Question 35

How do T cells encounter pathogens?

Answer 35

Dendritic cells phagocytose pathogen-derived particles and antigens. TNFa then stimulates immature tissue-resident Dendritic cells to increase expression of co-stimulatory molecules. Dendritic Cells digest ingested proteins and display small peptides derived from these on their cell surface in complex with MHC proteins. Dendritic cells with the digested proteins then goes into a lymph node.