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The human body has a range of defences to protect itself from pathogens. What are the two main types?

Non-specific mechanisms that do not distinguish between one type of pathogen and another, but respond to all of them in the same way. These act immediately and take two forms:
Barrier to the entry of pathogens

Specific mechanisms that do distinguish between different pathogens. The responses are less rapid but provide long-lasting immunity. The responses involve lymphocytes and take two forms:
Cell-mediated responses involving T lymphocytes
Humoral responses involving B lymphocytes


Are specific lymphocytes produced in response to an infection or do they already exist in the blood stream?

They already exist - all 10 million different types


Why is there a time lag between exposure to the pathogen and body's defences bringing an infection under control?

When an infection occurs, the one type already present that has complementary proteins to those of the pathogen is stimulated to build up its numbers to a level where it can be effective in destroying it.


How do lymphocytes recognise their own cells?

-in the foetus lymphocytes constantly collide with each other
-infection in the foetus is rare because it is protected by the mother and placenta.
- lymphocytes will therefore only collide the body's own material
- some of the lymphocytes will have receptors that exactly fit those of the body's own cells
- these lymphocytes either die or are suppressed
- the only remaining lymphocytes are those that fit foreign material (non-self) and therefore only respond to foreign material.


What are the barriers to pathogens trying to enter a human?

A protective covering - skin is difficult to penetrate
Epithelial cells covered in mucus - difficult to penetrate
Hydrochloric acid in the stomach - in a low pH the enzymes of most pathogens are denatured.


Why do bacteria have to be engulfed by phagocytes instead of diffusing or being actively transported across a cell membrane?

They are too large


Where are phagocytes located?

Some phagocytes travel in the blood but can move out of blood vessels into other tissues.


Describe phagocytosis

- chemical products of the pathogen are chemoattractants, causing phagocytes to move towards the pathogen
- phagocytes attach themselves to the surface of the pathogen
- they engulf the pathogen to form a vesicle (aka a phagosome)
- lysosomes move towards the vesicle and fuse with it.
- enzymes within the lysosomes break down the pathogen by hydrolysis from large insoluble molecules to small soluble ones
- the soluble products are absorbed into the cytoplasm of the phagocyte.


What does phagocytosis cause at the site of infection?

Inflammation - the swollen area contains dead pathogens and phagocytes which are known as pus. Inflammation is the result of histamine which causes dilation of blood vessels. This speeds up the delivery of phagocytes to the site of infection.


Define immunity

Immunity is the ability of organisms to resist infection by protecting against disease causing Microorganisms that invade their bodies.


Define antigen

An antigen is any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response.


What are the two types of lymphocyte?

B lymphocytes (B cells)
T lymphocytes (T cells)


Which type of immunity are B cells associated with?

Humoral immunity (involving antibodies that are present in body fluids)


Which type of immunity are T cells associated with?

Cell-mediated immunity (immunity involving body cells)


Where are T cells and B cells formed?

They Aretha formed from stem cells found in the bone marrow


Where do B cells develop and mature?

In the bone marrow


Where do T cells develop and mature?

In the Thymus gland


T cells respond to...

An organism's own cells that have been invaded by non-self material e.g virus, cancer, transplanted material


T lymphocytes can distinguish between normal cells and invader cells because...

-phagocytes that have engulfed and broken down a pathogen present some of the pathogen's antigens on their own cell-surface membrane
- body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane
-cancer cells likewise present antigens on their cell surface membranes


What are cells called that can present antigens of other cells on their own cell-surface membrane?

Antigen-presenting cells


T lymphocytes will only respond to antigens that are attached to...

A cell body (rather than to antigens within the body fluids) (cell-mediated immunity)


Describe the response of T lymphocytes to infection

-Pathogens invade body cells or are taken in by phagocytosis
-The phagocyte or body cell displays antigens from the pathogen on its cell-surface membrane
-Receptors on certain T cells fit exactly onto these antigens
-This activates other T cells to divide rapidly by mitosis and form a clone
- The cloned T cells
- develop into memory cells that enable a rapid response to future infections
- stimulate phagocytes to engulf pathogens by phagocytosis
- stimulate B cells to divide
- kill infected cells


How do T cells kill infected cells?

T cells kill body cells infected by pathogens. They do not kill by phagocytosis but by producing a protein which makes holes in the cell surface membrane - the cell becomes fully permeable and dies as a result. T cells are most effective in viruses as they need a host cell.


Humoral immunity involves .................. which are soluble in blood and tissue fluid



How many different types of B cells are there?

10 million


B cells and T cells divide by....



What are differences between T cell and B cell response?

B cells - humoral
T cells - cell-mediated

T-cells react to antigens presented on a cell/phagocyte that has the pathogen in it
B-cells take up antigens from the blood/tissue fluid

T-cells kill the body cells containing the pathogen
Antibodies from plasma B cells attach to pathogen and destroy


Toxin molecules can also act as...



Describe a B cell response

- The surface antigens of the invading pathogen are taken up by B cells.
- the B cells process the antigens and present them on their surfaces
- T helper cells attach to the processed antigens on the B cells thereby activating them
- The B cells are now activated to divide by mitosis to give a clone of plasma cells
- The cloned plasma cells produce antibodies that exactly fit the antigens on the pathogen's surface.
- The antibodies attach to antigens on the pathogen and destroy them
- Some B cells develop into memory cells. They can respond to future infections by the same pathogens by dividing and developing into plasma cells that produce antibodies. This is the secondary immune response.


How many different strains does influenza have?

Over 100 different strains


When antigens on viruses constantly change what is this called?

Antigenic variability


If a pathogen which has gone through antigenic variability what happens if a person is infected by the pathogen when they have been infected previously?

Primary response will occur - there are no appropriate memory cells to stimulate antibody production.


Describe a plasma cell

They secrete antibodies directly and survive for only a few days but release 2000 antibodies every second. The antibodies destroy the pathogen and it's toxins.

They are therefore responsible for primary immune response.


Describe a memory cell

Memory cells can live for decades
They circulate in blood and tissue fluid
If they encounter the same pathogen later on they will multiply and develop into plasma and memory cells
This is secondary immune response
It has more intense antibody production


Antibodies are...

Very specific , each antigen having it's own individual antibody


How is it possible to have such variation in antibodies? (They are specific)

Because they are made of proteins - which occur in almost infinite number


How many polypeptide chains make up an antibody?



On an antibody how do the polypeptide chains differ in length?

One pair is long - heavy chains
One pair is short - light chains


Can antigens change shape?

Antibodies can change shape to help it fit around the antigens


When the antibody binding site fits precisely onto the antigen, what is formed?

An antigen-antibody complex


On every antibody, what is different?

The binding site - it is called the variable region. (The rest of the antibody is the same in all antibodies and is known as the constant region. It binds to receptors on cells such as B cells).


What does each binding site of an antibody have?

A sequence of amino acids that form a specific 3D shape


Each antigen (there are different ones) on a pathogen will induce a different B cell to multiply and produce different antibodies. What are they collectively known as?

Polyclonal antibodies


What are monoclonal antibodies?

When a single type of antibody is isolated and cloned


Monoclonal antibodies have a number of useful functions in science and medicine:

Separation of a chemical from a mixture
Immunoassay - method of calculating the amount of substance in a mixture
Cancer treatment - monoclonal antibodies can be made which attach themselves only to cancer cells. They can then be used to activate a cytotoxic drug.
Transplant surgery - transplant organs often suffer some rejection due to T cell action. Monoclonal anti body's can be used to 'knock out' these T cells.


Immunity takes two forms:

Passive immunity - antibodies externally being introduced to to individuals from an outside source - not produced by the person

Active immunity - produced by stimulating the production of antibodies by the individuals own immune system.


What does the success of a vaccination programme depend on?

-must be economic to immunise all the vulnerable population
-few side effects
- means of producing, storing and transport the vaccine
- means of administrating the vaccine
- must be possible to vaccinate the vast majority of the vulnerable population. So there are no individuals in the population with the disease - known as herd immunity.


It is difficult to irradiate disease because...

-Vaccination fails to induce immunity in certain individuals
-some people might get the disease immediately after vaccination, but their immunity levels are not high enough to prevent it - they harbour the pathogen and reinfect others.
- the pathogen might mutate frequently
- many varieties of a particular pathogen e.g common cold virus
- certain pathogens hide or are out of reach from the immune system
- objections to vaccines for religious reasons


Control of cholera by means of vaccination is difficult because:

- Cholera is an intestinal disease and so it is not easy for the immune system to reach it. Any oral treatment doesn't really have time to be effective as it is flushed from the intestines by the diarrhoea.
- the antigens of the cholera pathogen change rapidly
- mobile populations resulting from war, tourism, trade make it difficult to ensure all individuals are populated.


Control of TB by vaccination is difficult because:

- more people with reduced immune systems make bt more likely to be contracted
- poverty and wars have created refugees who move around frequently and are often housed in overcrowded accommodation
-mobile populations make it difficult to make sure all individuals are vaccinated.
- the proportion of elderly people is increasing - these people have less effective immune systems and so the vaccine is less effective at stimulating immunity.