TOPIC 6

what is a pathogen

microorganism that causes disease

• bacteria

• virus

6.7

what is an antigen

Molecule on the surface if a pathogen / infected cell. Used for immune system to recognise the molecule as foreign/non-self

• Protein/ glycoprotein

phagocyte meaning

white blood cell - engulf + digest pathogen

• macrophage

• neutrophage

lymphocyte meaning

WBC - involved in specigic immunity

• B cells

• T cells

specific immunity meaning

immune response that targets a specific antigen

what is lysosome

organelle containing digestive enzyme

where are macrophages found

In tissues

list macrophage function

1. phagocytosis

• eat / digest pathogens

2. to present antigen cells

phagocytosis process

1. recognise foreign antigens

2. engulf pathogen: enocytosis/phagocytosis

3. Pathogen encased in phagosome; vesicle

4. lysosome fuse w phagosome

5. digestive enzyme break pathogen down

6. macrophage presents pathogen antigen on surface

becomes APC

• antigen presentng cell

APC purpose

displays foreign antige on surgace

• T helper cell recognise them

• T cell needs to be shown antigen before action; not random

• T helper release cytokines to stimulate other immune cells

neutrophil meaning

wbc

kills pathogens by phagocytosis

phagocytes; like macrophages

Neutrophil location

Mainly blood

First phagocytes to arrive to an infection

neutrophil function

non-specific immune response

attack pathogens generally - not w specific antigen

neutrophil process

1. pathogen enters body: cut

2. neutrophil moves towards chemicals released by damaged cells + pathogen

3. Neutrophil recognise pathogen as foreign

• detects foreign molecules/antigen on pathogen

4. Neutrophil engulf pathogen

• membrane surrounds pathogen + traps in vesicle (phagosome)

5. lysosome + phagosome fuse

• lysosome contains digestive enzyme

6. Enzyme digest pathogen

7. neutrophile dies

• dead neutrophil + pathogen + tissue fluid = pus

macrophage vs neutrophills

Macrophages	Neutrophils
Found in tissues	Found mainly in blood
Long-lived	Short-lived
Present antigens	Mainly kill quickly
Activate lymphocytes	First response

lymphocytes meaning

wbc

specific immune response: target specigic antigen

antibody meaning

protein - binds to specific antigen

Globular - specific tertiary structure

• specific binding site

List how antibodies help destroy pathogens

1. Agglutination

2. Neutralisation

3. Opsonisation/ marking pathogens

explain agglutination

Antibodies that can bind to multiple pathogens at once = clumping together

• easier for phagocytes to engulf the pathogen

Explain neutralisation

Some pathogens can release toxins

Antibodies bind to toxins to stop them from working

Opsonisation / making pathogens meaning

Antibodies coat proteins

B cell function

Produce antibodies

Humoral immune response

• body fluids (blood plasma + lympth)

B cell process

Contain specific receptors on surface

• complementary to an antigen

upon meeting correct antigen:

1. B cell selected

2. divides by mitosis = identical clones form

3. some become plasma cells = secrete antibodies

4. some become memory cells

T helper cell function

coordinate immune response

• stimulate other lymphocytes to carry out their function

Activated when binding to antigen on APC

T helper cell process

1. activate when binding to antigen on APC

2. Release cytokines: chemical messenger proteins

3. Cytokines stimlate:

• B cells to divide into plasma cells

• T killer cells to fivide

• more phagocytes to act

• basically clonal expansion + differentiation

T killer cell function

cell mediated immune response

Destroy infected bosy cells

• ex: infected by viruses

why t killer especially targets viral infected cells

virus hide in body cell

• antibody cannot easily reach them

T killer process

1. recognise viral antigen on infected body

2. bind to infected cell

3. release chemical to kill infected cell

4. stops virus from replicating

how APC is made

• A macrophage recognises a pathogen as foreign.

• It engulfs the pathogen by phagocytosis.

• The pathogen is trapped inside a phagosome.

• Lysosomes fuse with the phagosome.

• Digestive enzymes break the pathogen down.

• The macrophage keeps some pathogen antigens.

• These antigens are placed on the macrophage’s cell surface.

• The macrophage is now an APC.

how are T helper cells activated

• A macrophage engulfs a pathogen.

• The macrophage presents the pathogen’s antigen on its surface.

• The macrophage is now an APC.

• A T helper cell with a complementary receptor binds to the antigen.

• The T helper cell becomes activated.

• It releases cytokines.

role of cytokines

Cytokines are chemical messenger proteins released by immune cells.

They stimulate other immune cells.

Cytokines activate:

Cytokines stimulate...	Effect
B cells	divide by mitosis and form plasma cellsbbioioi
Plasma cells	produce antibodies
T killer cells	divide and destroy infected body cells
Phagocytes	become more active

Clonal expansion meaning

Selected B cell divides by mitosis to make many identical B cells.

differentiation meaning

The cloned B cells then differentiate.

Differentiate = become specialised.

They form:

Cell formed	Function
Plasma cells	Produce antibodies
Memory B cells	Stay in body long-term

plasma cells

Plasma cells secrete lots of antibodies.

Antibody = a protein that binds specifically to an antigen.

Antibodies help by:

• causing pathogens to clump together: agglutination

• neutralising toxins

• preventing viruses entering cells

• marking pathogens for phagocytosis

memory cells

A long-lived lymphocyte that remains in the body after an infection.

Function: recognise same antigen quickly if pathogen enters again

Process:

• A pathogen enters the body.

• The correct B cell is activated.

• The B cell divides by mitosis (clonal expansion).

• Some cells become plasma cells.

• Some become memory cells.

Cytokines released by T helper cells stimulate the B cell with a complementary receptor to the antigen. This B cell undergoes clonal selection and divides by mitosis in clonal expansion. The cloned B cells differentiate into plasma cells, which secrete specific antibodies, and memory B cells, which provide a faster secondary response.

what is humaoral immune response

Targets pathogens outside body cells

• tissue fluid

It involves:

• APCs

• T helper cells

• cytokines

• B cells

• clonal selection

• plasma cells

• antibodies

list humoral response steps

1. pathogen enters body

2. macrophage - phagocytosis

3. macrophage becomes APC

4. T helper binds to complementary antigen

5. T helper release cytokines

6. cytokines activate B cell = clonal selection

• clonal expansion and differentiation

7. plasma cells secrete antibodies = help destroy pathogen

8. memory cells remain in body for faster secondary immune response

Explain humoral immune response process

1. Pathogen enters body

• contains foregin antigens on surface

2. Macrophage does phagocytosis

• Macrophage recognise pathogen as foreign

• phagosome

• lysosome + phagosome

3. Macrophage becomes APC

• keeps some pathogen antigens and displays them on its surface.

4. T helper binds to antigen w complementary antigen

• t helper = activated

5. T helper release cytokines

• stimulate other immune cells

• B cell

• T killer

• more phagocytes

6. B cell activation

• via cytokines

• clonal expansion: divides by mitosis = identical bcells

• differe ntiation: plasma / memory cells

A) plasma = produces antibodies to destroy pathogen

• binds specifically to pathogen

• aggulination, neutralising toxins, marking pathogens

B) Memory cells: faster response if same pathogens come

• more antibodies produced

• disease symptoms may not occur

• Secondary immune response

what is the cell mediated response

T cell response

• cell mediated = cells directly act on infected body cells rather than thru antibodies in body fluids

T lymphocytes used to destroy infected body cells

• important for pathogens inside body cells; viruses

• antibodies cant easily reach pathos inside bodyt cells

• so whole cell is destroyed instead

cell mediated process

1. Body cell = infected

• infected cell contains viral antigens on surgace = APC

2. T helper binds to complementaryt antigen

3. T helper cells release cytokines = stimulate other immune cells

4. T killer = acttivated

• t killer binds to complementary antigen on infected cell

5. T killer destroy infected cell

• release perforins / enzymes

• protein that makes pores in infected cell membrane

humoral vs cell mediated

Humoral immune response	Cell-mediated immune response
B cells involved	T cells involved
Plasma cells produce antibodies	T killer cells destroy infected cells
Targets pathogens outside cells	Targets infected host cells
Antibodies are main product	Perforins/enzymes from T killer cells are main product
Useful against bacteria in body fluids	Useful against viruses/cancer cells

Secondary immune system meaning

The secondary immune response is the immune response that happens when the body meets the same antigen again.

It is:

• faster

• stronger

• produces more antibodies

• often prevents symptoms

Explain primary immune response

Primary immune response = the first response to a specific antigen.

Sequence:

1. Pathogen enters the body.

2. Antigen-presenting cells show the antigen.

3. T helper cells are activated.

4. Cytokines activate B cells and T killer cells.

5. B cells and T cells divide by mitosis.

6. Some become active cells.

7. Some become memory cells.

So after the first infection, you do not just make plasma cells and T killer cells.

You also keep some lymphocytes as memory cells.

B memory cells role : secondary response

When the same pathogen enters again:

1. B memory cells recognise the same antigen.

2. They divide rapidly by mitosis.

3. They differentiate into plasma cells.

4. Plasma cells produce large amounts of specific antibodies.

5. Antibodies bind to antigens on the pathogen.

6. The pathogen is destroyed quickly.

T memory cells: secondary response

When the same pathogen enters again:

1. T memory cells recognise the antigen.

2. They divide rapidly by mitosis.

3. They differentiate into active T helper cells and T killer cells.

4. T helper cells release cytokines.

5. T killer cells destroy infected host cells.

6. The pathogen cannot replicate as easily.