Cell Recognition and the Immune System

Question 1

immunity - how the body deals with disease

Question 2

what is a communicable diseases

Answer 2

Communicable diseases are diseases which can be spread between organisms either of the same species of sometimes between different species

Question 3

What are communicable diseases spread by

Answer 3

Communicable diseases are spread by pathogens

Question 4

What is a pathogen

Answer 4

A pathogen is a microorganism which can cause a disease

Question 5

What does ‘host’ mean

Answer 5

The organism which the pathogen infects is called the host

Question 6

state the four main categories of pathogens

Answer 6

Four main categories of pathogens are bacteria, viruses, fungi and Protoctista - also called Protista

Question 7

describe how bacteria can act as pathogens

Answer 7

bacteria are prokaryotic cells (bacterial chromosomes)

Once they enter the body, pathogenic bacteria can release toxins
Toxin are chemicals which damage host cells and tissues leading to the symptoms of the disease

Some bacteria can enter host cells and prevent them from functioning normally e.g. the bacteria which cause tuberculosis

Question 8

describe how bacteria can act as pathogens

Answer 8

Viruses prevent a host cell from functioning normally. In many cases a virus can lead to the death of the host cell

Viruses cannot reproduce outside of a host cell
To reproduce, a virus attaches to the host cell and then passes through the cell membrane
The virus then copies itself using the enzymes of the host cell
The virus particles now leave the host cell and can go on to infect new host cells and continue reproducing

Viruses consist of genetic material which can be DNA or RNA
The genetic material is wrapped in a protein structure called a capsid

Viruses also have attachment proteins which allow the virus to attach to host cells

Some viruses also contain a lipid envelope

Question 9

describe how fungi can act as pathogens

Answer 9

Fungi are eukaryotic organisms - can be unicellular or multicellular

Fungi obtain nutrients by releasing enzymes and digesting the material around them
The products of digestion are then absorbed back into the fungal cells

This process of digestion can cause damage to host cells and tissues

When they reproduce fungi release a large number of spores so fungal diseases can spread very widely

many species of fungi are found on the remains of dead organsims where they take part in the decay process

However pathogenic fungi are found on living organisms where they cause disease e.g. in humans thrush and athletes foot

fungi in plants - damages leaves, reduces rate of photosynthesis - severely reduce the yield of plant crops

Question 10

how can protoctista (or protista) act as pathogens

Answer 10

Protoctista (or protista) are eukaryotic organisms

Protoctista can act as pathogenic parasites in humans

Question 11

Describe the body’s non-specific defences against pathogens - SKIN

Answer 11

Skin - provides a protective layer for the majority of the body surfaces

The skin has many layers of cells, including a surface layer of dead cells. This makes the skin difficult for pathogens to penetrate

The surface of the skin is covered with oil sebum produced by sebaceous glands
This forms a slightly acidic layer which can prevent the growth of harmful bacteria.

Healthy skin is also covered with harmless microorganisms which reduce the growth of pathogens by competing for resources

Question 12

features of non-specific defences

Answer 12

the non-specific defences are the same for all pathogens (unlike specific immune system)

non-specific defences are rapid and prevent pathogens from gaining access to the body

Question 13

Describe the body’s non-specific defences against pathogens - Lining of trachea and stomach

mucous membranes

Answer 13

Pathogens can enter the body via openings for example the breathing and digestive systems

These (e.g. trachea in breathing system) are lined with a protective layer called a mucous membrane.

Mucous membranes secrete mucus which traps microorganisms

These microorganisms can then be destroyed by white blood cells called phagocytes

The mucus also contains the enzyme lysozyme which destroys bacteria by digesting the bacterial cell wall

In the stomach, the mucus contains hydrochloric acid. This hydrochloric acid helps to kill any pathogens in food or water

Question 14

Describe the body’s non-specific defences against pathogens - expulsive reflexes

Answer 14

Sometimes pathogens can invade the body through opening such as the mouth or nose
In this case, the body tries to expel the pathogen. These are called expulsive reflexes. These can include sneezing, coughing, vomiting or diarrhoea

Question 15

Describe the body’s non-specific defences against pathogens - tears

Answer 15

lysozyme is also found in tears where it helps to prevent pathogens from entering via the eyes

Question 16

Describe the body’s non-specific defences against pathogens - blood clotting

Answer 16

Another way that pathogens can enter is if the skin is damages e.g. a cut

In this case, the body responds to try to seal the damaged area before a large number of pathogens can enter

This is the process of blood clotting involving platelets in the blood

Question 17

what are the two categories of white blood cells

Answer 17

The two categories of white blood cells are phagocytes and lymphocytes

Question 18

state a type of phagocyte

Answer 18

Phagocytes include macrophages and neutrophiles

Question 19

what type of white blood cells form part of the non-specific defences

Answer 19

Phagocytes form part of the non-specific defences

macrophages also play a role in the specific immune system

Question 20

State examples/types of lymphocytes

Answer 20

Lymphocytes include B lymphocytes and T lymphocytes

Question 21

what type of white blood cells form the specific immune system

Answer 21

Lymphocytes form the specific immune system

Question 22

describe the roles of neutrophils and macrophages

Answer 22

neutrophils can rapidly engulf and destroy pathogens at the site of an infection

Macrophages can also destroy pathogens by phagocytosis but macrophages also have an additional function called antigen presentation

Question 23

describe the stages of phagocytosis

Answer 23

phagocytes are attracted to molecules/chemicals produced by the pathogens

phagocytes have several receptors on their cell-surface membrane that recognise, and attach to , chemicals on the surface of the pathogen and engulf it

the pathogens are contained in a phagocytic vacuole/vesicle called a phagosome

now lysosomes move towards the phagosome and fuse with it, forming a phagolysosome

Lysosomal enzymes (lysozymes) destroy ingested bacteria by hydrolysis of their cell wall/

now break down the pathogen and destroy it

the soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte

the phagocyte then presents the pathogens antigens - it sticks the antigens on its surface to activate other immune system cells/lymphocytes - macrophages

Question 24

what does the specific immune system include

Answer 24

Antibodies
Lymphocytes (B and T)

Question 25

What does the specific immune system do

Answer 25

The specific immune system responds to each pathogen in an individual way

Once the specific immune system has defended against the pathogen, it will then produce a more effective response if it encounters the same pathogen again

Key feature - it can recognise individual pathogens

Question 26

describe what is meant by an antigen

Answer 26

Molecules (such as proteins and polysaccharides) which cover the surface of all cells
These molecules often carry out normal cellular functions - called antigens

Question 27

what leads to an immune response

Answer 27

The immune system can detect antigens on the surface of pathogens
The immune system can see these antigens as foreign or ‘non-self’

This leads to an immune response

Question 28

describe what happens during an immune response

Answer 28

During an immune response, antibodies will be produced which specifically bind to the antigen

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pathogen
‘non-self’ antigen
antibody

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antibodies are specific

Question 29

Explain why the antigens on your body cells do not trigger an immune response

Answer 29

Antigens on the surface of your body cells do not trigger an immune response

That is because your immune system identify these antigens as ‘self’ - a normal part of your body

The immune system does not produce antibodies against self antigens

Question 30

what are antibodies also referred to as

Answer 30

Antibodies are also referred to as immunoglobulins

Question 31

describe what is meant by an antibody

Answer 31

Antibodies are glycoproteins consisting of four polypeptide chains

Question 32

describe the structure of antibodies

Answer 32

Antibodies are glycoproteins consisting of four polypeptide chains

It has two long heavy chains, which are identical to each other, and it has two shorter light chains which are also identical to each other

The chains are held together by disulphide bridges

It also has two antigen binding sites
This means that one antibody molecule can bind to two identical antigen molecules

When antigens bind, it is called an antigen-antibody complex

The tertiary structure of the antigen-binding site is complementary to the structure of the antigen (the antigen fits perfectly into the antigen-binding site)

This means that antibodies are highly specific for the antigen they bind to

The hinge region (bend) is flexible allowing the distance between the two antigen binding sites to vary

Question 33

draw an antibody

Answer 33

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Question 34

What are antibodies produced by

Answer 34

Antibodies are produced by B lymphocytes

Question 35

describe the roles of antibodies in the specific immune system

Answer 35

The role of antibodies in the immune system is referred to as humoral immunity

Question 36

explain the similarities differences between antibodies from two different B lymphocytes

Answer 36

Both antibodies have a region which is the same. This is called the constant region.

The constant region has the same structure for every antibody no matter which B lymphocyte produced it

However, the ends of the antibody molecules are different. These are called the variable regions. These are about 110 amino acids long on each chain

The variable regions form the antigen-binding sites

The shape of the variable regions are different for the antibodies produced by different B lymphocytes

This means that the antibodies produced by different B lymphocytes will bind to different antigens

human body contains millions of different B lymphocytes so there are millions of different antibodies targeting a vast range of antigens

Question 37

functions of antibodies in the human body

Answer 37

Antibodies
act as markers that stimulate phagocytes to engulf the bacterial cells to which they are attached - for phagocytosis

Antibodies can stick pathogens together preventing them from spreading around the body. This makes it easier for phagocytes to locate them as they are less spread-out within the body. This is called agglutination

By sticking to pathogens such as viruses, antibodies prevent them from invading host cells

Antibodies can stick to bacterial toxins preventing the toxins from harming body cells.

These antibodies are called antitoxins