Cell recognition and the immune system

Question 1

What is immunity?

Answer 1

After the body overwhelms a pathogen, the body’s defences seem to be better prepared for a second infection from the same pathogen and can kill it before it causes any harm.

Question 2

What are the forms of lymphocytes?

Answer 2

A lymphocyte is a white blood cell and takes two forms:
Cell-mediated responses involving T lymphocytes.
Humoral responses involving B lymphocytes.

Question 3

Why must own cells be recognised?

Answer 3

To defend the body from invasion by foreign material, lymphocytes must be able to distinguish the body’s own cells and molecules.
(Self from those that are foreign (non-self).
If they could not do this, the lymphocytes would destroy the organism’s own tissues.

Question 4

What is phagocytosis?

Answer 4

Large particles, such as some types of bacteria, can be engulfed by cells in the vesicles formed by the cell surface membrane.
In the blood, the phagocytes are the types of white blood cells that carry it out.
They provide an important defence against the pathogens that manage to enter the body.
Some phagocytes travel in the blood but can move out of blood vessels into other tissues.

Question 5

What is the process of phagocytosis?

Answer 5

Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen.
Phagocytes have several receptors on their cell-surface membrane that recognise, and attach to, chemicals on the pathogen’s surface.
They engulf the pathogen to form a vesicle - a phagosome.
Lysosomes move towards the vesicle and fuse with it.
Lysozymes present within the lysosome destroy ingested bacteria by hydrolysis of their cell wall.
The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.

Question 6

What is an antigen?

Answer 6

Any part of an organism or substance that is recognised as non-self by the immune system and stimulates an immune response.
Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading cells, such as microorganisms, or abnormal body cells, such as cancer.
The presence of an antigen triggers the production of an antibody as part of the body’s defence system.

Question 7

What are the types of lymphocyte?

Answer 7

Lymphocytes are produced by stem cells in the bone marrow.
B lymphocytes (B-cells), mature in the bone marrow. They are associated with humoral immunity, immunity involving antibodies that are present in body fluids, or humour such as blood plasma.
T lymphocytes (T-cells), mature in the thymus gland. They are associated with cell-mediated immunity, immunity involving body cells.

Question 8

Why can T lymphocytes distinguish invader cells from normal cells?

Answer 8

Phagocytes that have engulfed and hydrolysed a pathogen present some of a 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.
Transplanted cells from individuals of the same species have different antigens on their cell-surface membrane.
Cancer cells are different from normal body cells and present antigens on their cell-surface membranes.

Question 9

What are antigen-presenting cells?

Answer 9

Cells that display foreign antigens on their surface.
They can present antigens of other cells on their own cell-surface membrane.

Question 10

What is the cell-mediated immunity/cellular response?

Answer 10

T lymphocytes will only respond to antigens that are presented on a body cell.
The receptors on each T cell respond to a single antigen.
There is a vast number of different types of T cell, each responding to a different antigen.

Question 11

What are the stages in the response of T lymphocytes to infection by a pathogen?

Answer 11

Pathogens invade body cells or are taken in by phagocytes.
The phagocytes place antigens from the pathogen on its cell-surface membrane.
Receptors on a specific helper T cell fit exactly onto these antigens.
This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.

Question 12

What do the cloned T cells do?

Answer 12

They develop into memory cells that enable a rapid response to future infections by the same pathogen.
They stimulate phagocytes to engulf pathogens by phagocytosis.
Stimulate B cells to divide and secrete their antibody.
Activate cytotoxic T cells.

Question 13

How do cytotoxic T cells kill infected cells?

Answer 13

They kill abnormal cells and body cells that are infected by pathogens, by producing a protein called perforin that makes holes in the cell-surface membrane.
These holes mean the cell membrane becomes freely permeable to all substances and the cell dies as a result.
This illustrates the vital importance of cell-surface membranes in maintaining the integrity of cells and hence their survival.

Question 14

Why are T cells effective against viruses?

Answer 14

The action of T cells is most effective against viruses because viruses replicate inside cells.
As viruses use living cells in which to replicate, this sacrifice of body cells prevents viruses multiplying and infecting more cells.

Question 15

Why are proteins the most important cell-surface molecule?

Answer 15

Each type of cell, self or non-self, has specific molecules on its surface that identify it.
While these molecules can be a variety of types, the proteins are the most important.
This is because proteins have enormous cell variety and a highly specific tertiary structure.
It is this variety of specific 3-D structure that distinguishes one cell from another.

Question 16

What do protein molecules allow the immune system to identify?

Answer 16

Pathogens, for example HIV.
Non-self materials such as cells from other organisms of the same species.
Toxins including those produced by certain pathogens like the bacterium that causes cholera.
Abnormal body cells such as cancer cells.

Question 17

What are the implications of the cell recognition response?

Answer 17

All of the above are potentially harmful and their identification is the first stage in removing the threat they pose.
Although this response is clearly advantageous to the organism, it has implications for humans who have had tissue or organ transplants.
The immune system recognises these as non-self even though they have come from individuals of the same species.
It therefore attempts to destroy the transplant.

Question 18

How is organ rejection reduced?

Answer 18

Donor tissues for transplant are normally matched as closely as possible to those of the recipient.
The best matches often come from relatives that are genetically close.
In addition, immunosuppressant drugs are often administered to reduce the level of the immune response that occurs.

Question 19

What are the types of defence mechanism?

Answer 19

Non-specific: response is immediate and the same for all pathogens.
Physical barrier e.g. skin; phagocytosis.
Specific: response is slower and specific to each pathogen.
Cell-mediated response - T lymphocytes; humoral response - B lymphocytes.

Question 20

How do lymphocytes respond to an infection?

Answer 20

When an infection occurs, the one type already present that has the complementary proteins to those of the pathogen is stimulated to divide to build up its numbers to be effective in destroying it - clonal selection.
This is why there is a time lag between exposure to the pathogen and body’s defence bringing it under control.

Question 21

How are lymphocytes stored?

Answer 21

Specific lymphocytes are not produced in response to an infection, they already exist - all 10 million different types.
There are so many different types of lymphocytes, so there is a high probability that when a pathogen enters the body, one of these lymphocytes will have a complementary protein on its surface to one of the proteins of the pathogen.
The lymphocyte will recognise the pathogen.

Question 22

How do lymphocytes recognise cells belonging to the body in adults?

Answer 22

In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens.
Any lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes.
No clones of these anti-self lymphocytes will appear in the blood, leaving only those that might respond to non-self antigens.

Question 23

How do lymphocytes recognise cells belonging to the body in foetuses?

Answer 23

In the foetus, the lymphocytes are constantly colliding with other cells.
Infection in the foetus is rare because it is protected by the mother and the placenta.
Lymphocytes will therefore collide almost exclusively with the body’s self-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 might fit foreign material, and therefore only respond to foreign material.

Question 24

What is the first and second line of defence in the body?

Answer 24

The body’s first defence is to form a physical or chemical barrier to entry.
Should this fail, the next line of defence is the white blood cells - two types: phagocytes and lymphocytes.
Phagocytes ingest and destroy the pathogen by phagocytosis before it can cause harm.

Question 25

What is humoral immunity?

Answer 25

It involves antibodies that are soluble in the blood and tissue fluid of the body (humour).
There are many different types of B cell, as many as 10 million, and each B cell starts to produce a specific antibody that responds to one specific antigen.

Question 26

What is clonal selection?

Answer 26

The antibody therefore attaches to this complementary antigen.
The antigen enters by endocytosis and gets presented on its surface (processed).
Helper T cells bind to these processed antigens and stimulate this B cell to divide by mitosis to form a clone of identical B cells, all of which produce the antibody that is specific to the foreign antigen.
It accounts for the body’s ability to respond rapidly to any of a vast number of antigens.

Question 27

What are plasma cells?

Answer 27

They secrete antibodies usually into blood plasma.
These cells survive only a few days, but each can make around 2000 antibodies every second during its brief lifespan.
These antibodies lead to the destruction of the antigen so are therefore responsible for the immediate defence of the body against infection.
The production of antibodies and memory cells is the primary immune response.

Question 28

What are memory cells?

Answer 28

Responsible for the secondary immune response.
They live considerably longer than plasma cells, often decades.
They do not produce antibodies directly, but circulate in the blood and tissue fluid.
When they encounter the same antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells.
The new memory cells circulate in readiness for any future infection.

Question 29

What is the role of memory cells?

Answer 29

Memory cells provide long-term immunity against the original infection.
An increased quantity of antibodies is secreted at a faster rate than in the primary immune response.
It ensures that a new infection is destroyed before it can cause any harm, and individuals are often totally unaware that they have been infected.

Question 30

How are B cells used in humoral immunity?

Answer 30

The surface antigens of an invading pathogen are taken up by a B cell.
The B cell processes the antigens and presents them on its surface.
Helper T cells attach to the processed antigens on the B cell, therefore activating the B cell.
The B cell is now activated to divide by mitosis to give a clone of plasma cells.
The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen’s surface.
The antibody attaches to antigens on the pathogen and destroy them.
Some B cells develop into memory cells.

Question 31

What are antibodies?

Answer 31

Proteins with specific binding sites synthesised by B cells.
When the body is infected by non-self material, a B cell produces a specific antibody, which reacts with an antigen on the surface of the non-self material by binding to them.

Question 32

What is the structure of antibodies?

Answer 32

Each antibody has two identical binding sites, complementary to a specific antigen.
The massive variety of antibodies is possible because they are made of proteins - molecules that occur in an almost infinite number of forms.

Question 33

What are the chains of the antibody?

Answer 33

Antibodies are made up of four polypeptide chains.
The chains of one pair are long and are called heavy chains.
The chains of the other pair are shorter and are called light chains.

Question 34

What is the antigen-antibody complex?

Answer 34

Each antibody has a specific binding site that fits precisely onto a specific antigen to form this.

Question 35

What is the variable and constant region?

Answer 35

The binding site is different on different antibodies so is the variable region.
Each binding site consists of a sequence of amino acids that form a specific 3-D shape that binds directly to a specific antigen.
The rest of the antibody is the constant region.
This binds to receptors on cells such as B cells.

Question 36

How do antibodies prepare the antigen for destruction in a bacterial cell?

Answer 36

They cause agglutination of the bacterial cells. In this way clumps of bacterial cells are formed, making it easier for the phagocytes to locate them as they are less spread out within the body.
They then serve as markers that stimulate phagocytes to engulf the bacterial cells to which they are attached.