Lecture 37 - immune response to bacteria

Question 1

Innate immunity overview

Answer 1

Rapid response (hours)
Relatively non-specific (look for basic patterns)
No memory
Particularly important in our response against bacterial pathogens

Question 2

Why is innate immunity particularly important?

Answer 2

Particularly important in our responses against bacterial pathogens. The adaptive immune system is important for the long term control of bacterial infections

There are important innate responses associated with viruses.

Question 3

Innate immunity best deals with what stages of pathogenesis

Answer 3

Innate immunity best deals with the earlier stages of pathogenesis and some effect the earlier stages of replication

Question 4

Places where their are physical and chemical barriers to bacterial attachment and invasion

Answer 4

Skin
Airways
Gut

Question 5

Skin

Answer 5

Dead cells and keratin (performs the most basic role of stopping the pathogens from getting in)
Salt - osmotic control (osmotic potential inhibits the growth of many types of bacteria)
Sebum- trapping and pH (sticky goo that comes out of hair follicles and the pH inhibits the growth of pathogenic bacteria)

Question 6

Airways

Answer 6

Mucus - trapping

Beating cilia moves trapped bacteria up the throat where they are swallowed (moves it up the respiratory tract)

Question 7

Gut

Answer 7

Constant flow of fluids (especially if gut inflammation results in diarrhoea) (thats why it is not always a good idea to take antidiarrhoeal medicines as plaques can build up which can cause toxic megacolon and can kill)
Stomach acid (stops growth)
Digestive enzymes
Bile

Question 8

Antimicrobial peptides

Answer 8

AMPs
Such as defensins
Active against gram positive and gram negative bacteria
Excreted in the skin, respiratory tract and the gut

antimicrobial peptides will preferentially interact with the bacterial cell to the mammalian cells, which enables them to kill microorganisms without being significantly toxic to mammalian cells.

Question 9

AMPs

Answer 9

Antimicrobial peptides

Question 10

Defensins

Answer 10

antimicrobial peptides, which are able to disrupt bacteria by forming pores in the cell membranes of these pathogens.

Question 11

Lysozymes

Answer 11

Excreted in the skin and the respiratory tract (not in the gut)
Especially active against gram positive bacteria (important innate immune response especially against gram positive bacteria which have a thick layer of peptidoglycan)
Lysozyme beaks the bonds between the glycopeptides which make up the peptidoglycan layer (i.e. it breaks the NAM and NAGs apart)
Lysozyme is most effective against Gram positive bacteria since the peptidoglycan layer is relatively accessible to the enzyme; lysozyme is effective against Gram negative bacteria only after the outer membrane has been compromised.

Question 12

What happens when bacteria make it past our first defences?

Answer 12

Inflammatory chemicals diffusing from the inflamed site act as chemotactic agents
Leukocytosis - neutrophils enter blood from the bone marrow
Margination - neutrophils cling to capillary wall
Diapedesis - neutrophils flatten and squeeze out of the capillaries
Chemotaxis - neutrophils follow chemical trail
Then phagocytosis

Question 13

Events of phagocytosis

Answer 13

1- phagocyte adheres to pathogens or debris
2- Phagocyte forms pseudopods that eventually engulf the particles forming a phagosome
3- Lysosome fuses with the phagocytic vesicle forming a phaglysosomme
4- Toxic compounds and lysosomal enzymes destroy pathogens
5- Sometimes exocytosis of the vesicles removes indigestible and residual material

Occurs in neutrophils, macrophages etc.

Question 14

Phagocytosis is made much more effective by

Answer 14

Increase in temperature/fever will cause phagocytosis to be much quicker
Complement system

Question 15

Complement

Answer 15

A set of proteins in the blood that float around in the inactivated form and somehow become activated by three pathways - alternative, classical and lectin pathway

The proteins after becoming activated eventually cause lysis of the microbe

Question 16

Pathways of complement activation

Answer 16

The activation of the complement system may be initiated by three distinct pathways (alternative, classical and lectin pathway), all of which lead to the production of C3b. C3b later initiates the late steps of complement activation, culminating in the production of numerous peptides and polymerised C9 (which forms the MAC which creates holes in plasma membranes)

Question 17

Describe how the steps to getting to the lysis of a microbe in terms of complement

Answer 17

Alternative, classical and lectin are the three distinct pathways that can activate the complement system. All of which leads to the production of C3b from the fragmentation of C3 to form C3b and C3a. C3a causes inflammation. C3b is responsible for opsonisation and phagocytosis by adhering the C3b molecules to the microbe. Then C5 is fragmented to form C5a which causes inflammation. Then the C9 molecules polymerise to form a MAC or membrane pore which causes the lysis of a microbe.

Question 18

Alternative pathway

Answer 18

Microbe is directly causing complement to occur. Microbe surface proteins that immune cells recognise

Question 19

Classical pathway

Answer 19

Antibodies are connected to a microbe which stimulates the firing up of the complement pathway. This pathway requires antibodies.

Question 20

Lectin pathway

Answer 20

Looks for a particular sugar that is attached to certain microbes to activate complement.

Both alternative and lectin pathways are looking for basic patterns except lectin pathway is specifically looking for mannose.

Question 21

C3

Answer 21

Fragments once activated to form C3a and C3b

Question 22

C3a

Answer 22

Causes inflammation

Fragment of C3

Question 23

C3b

Answer 23

Causes opsonisation and phagocytosis

Fragment of C3

Question 24

C5a

Answer 24

Causes inflammation

Fragment of C5

Question 25

C9

Answer 25

These complement proteins form a MAC or a membrane pore which causes the lysis of the microbe

Question 26

How does the adaptive immune response interlink with the innate immune response?

Answer 26

Adaptive immune response interlinks with the innate immune response - complement pathway

The classical pathway needs antibodies and therefore cannot be considered only a part of the innate immune response because you need antibodies

Question 27

Opsonisation (label)

Answer 27

Opsonisation labels pathogens which bind to complement receptors on phagocytes
Mediated by C3b
Coating a microbe with antibody and/or complement protein C3b

Question 28

Destroy (lysis)

Answer 28

Membrane attack complex formation (pores in the bacterial cells) which results in death
Mediated by C9
Complement that has a direct effect on destroying the bacteria
Lets fluid into the bacterial cell

Question 29

Recruit

Answer 29

Complement proteins act as peptide mediators of inflammation and recruit phagocytes
Mediated by C3a and C5a
Complement can increase the overall inflammation of an area

Question 30

Aggulation test

Answer 30

In this technique, small latex beads are coated with a bacterial antigen. When the corresponding antibody is added to a suspension of antigen coated latex bead, each antibody molecule combines with more than one antigen molecule so that masses of latex beads become linked together in a readily visible angulation reaction

The latex beads are added to make the antigen and antibody reaction visible to the naked eye (visually seen as white clumps when it occurs)

This test just gives a positive or negative result - it is a qualitative rather than quantitive method of detecting antibody

Measure of the presence or absence of antibody. Only antibody specific to that antigen will bind.

Question 31

ELISA

Answer 31

ELISA = Enzyme Linked Immuno Sorbent Assay

Quantifies how much antibody there is in the blood that is specific for the bacterial antigen

The intensity of colour produced is proportional to the amount of enzyme and therefore to the amount of antibody present

Saline blank, patient A and patient B - saline blank is used as a negative control as it will have a negative result always as it does not contain any antigen

Blue colour means that the substrate and enzyme are working together

Question 32

ELISA and structural features

Answer 32

Antigen is present which is then bonded to the antibody in a patients serum and then a detector antibody which has an enzyme on it connects to the antibody and then a substrate that is specific to the enzyme and is what gives us the colour change binds

If the antibody in the patient’s serum isn’t present then nothing else can bind hence there is no colour change

Question 33

Classical pathway is the most…

Answer 33

Effective way to produce complement products, including the opsonin C3b. This is due to the ability of antibody-antigen complexes to amplify early steps in complement activation

Question 34

Name molecules of the immune response along with the name of the tissue or cells from which they originate

Answer 34

Cytokines from APCs

Complement proteins from liver

Antibody from B cells

Question 35

Name two opsonins

Answer 35

C3b

Antibody IgG

Something that can opsonise is something that can adhere to a pathogen that makes it more obvious to phagocyte so the phagocyte is more likely to take it up and phagocytose it.

Question 36

Briefly outline the sequence of events in the innate immune response that occur immediately following skin infection

Answer 36

Bacteria breaks the skin barrier and enters tissues, start to replicate and release metabolic products. Upon recognising these products, tissue macrophages phagocytose the bacteria and release cytokines. These cytokines act to increase body temperature and increase the production and release of neutrophils. Diapedesis allows more immune cells into the area of infection which increase phagocytosis. APCs migrate to the lymph node and present antigen to the T cells which leads to the activation of the adaptive immune response

Question 37

What is complement and what does it do?

Answer 37

Series of 9 major proteins/protein complexes (C1-9) that act in sequence to clear pathogens from blood and tissues

Question 38

Outcomes of complement

Answer 38

Enhances the inflammatory response e.g. attracts phagocytes

Increases phagocytosis through opsonisation or immune adherence

Formation of MAC (membrane attack complex)

Question 39

Cut on hand…several days later pain, swelling, heat and redness…What is happening to the wound?

Answer 39

Given that it is several days since the cut was formed we can assume that the adaptive immunie response has been activated. This activation was started by the dendritic cells at the wound site phagocytosing the bacteria and travelling to the nearest lymph node to alert the T cells. The T cells can recognise specific peptides in association with MHC-II molecules. THese activated T cells will secrete cytokines to start the process of activation of B cells. When B cells are also exposed to native antigen they become fully activated and can opsonise bacteria to increase phagocytosis. A proportion of B cells will remain in circulation as memory B cells for a faster immune response should be the same bacteria be encountered again.