Cell recognition and the immune system

What two main things can monoclonal antibodies be used for?

• Targeting specific cells

• Medical diagnosis

Explain why the method used to treat cancer by administering only the monoclonal antibody is effective.

Monoclonal antibody binds to complementary antigens on the cancer cell’s surface. This prevents molecules controlling cell division from binding

Explain why the method used to treat cancer by administering the monoclonal antibody attached to a drug is effective.

Monoclonal antibody targets the cancer cell specifically, so brings the drug directly to the cancer cell, where it is killed

Why can monoclonal antibodies be used for medical diagnosis?

Many conditions produce molecules with specific antigens which the monoclonal antibodies can bind to. The formation of an antigen-antibody complex enables the condition to be detected

Why are there ethical issues surrounding the use of monoclonal antibodies?

• Mice are deliberately infected in the production of monoclonal antibodies

• There are sometimes dangerous side effects in human trials

What are the two types of immunity?

Active and passive

What is passive immunity?

When antibodies are introduced into the body

What is an advantage of passive immunity?

It is fast-acting

What is a disadvantage of passive immunity?

No memory cells or new antibodies are produced, and therefore it does not provide long-term immunity

What is active immunity?

When the body produces its own antibodies in response to exposure to an antigen

What is a disadvantage of active immunity?

It is slow-acting

What is an advantage of active immunity?

Memory cells are produced, so it provides long-term immunity

What is herd immunity?

When a sufficiently large proportion of the population has been vaccinated against a specific disease

Why might a disease be difficult to eradicate using a vaccine?

The vaccine may become less effective over time if the pathogen mutates

Why might a vaccine become less effective if the pathogen mutates?

The pathogen will have different antigens to the ones in the vaccine, and therefore the body will produce antibodies that are not complementary to the new antigen

What are the ethical issues surrounding vaccines?

• They use animal testing

• They often have side effects

What does HIV stand for?

Human immunodeficieny virus

What can HIV lead to?

AIDS

What does AIDS stand for?

Acquired immunodeficiency syndrome

What are the structures found in a HIV virus?

Attachment proteins, lipid envelope, matrix, capsid, reverse transcriptase, RNA

What do viruses use in order to copy themselves?

The host cell’s replication machinery

What is the first stage of HIV replication?

Attachment protein on surface of HIV binds to CD4 receptor on helper T cell

What is the second stage of HIV replication, after HIV has bound to the helper T cell?

RNA and reverse transcriptase from the HIV enter the helper T cell

What is the third stage of HIV replication, after RNA and reverse transcriptase have entered the helper T cell?

Reverse transcriptase converts viral RNA to viral DNA

What is the fourth stage of HIV replication, after viral RNA has been converted to viral DNA?

The viral DNA enters the helper T cell’s nucleus, where it is integrated into the helper T cell’s DNA

What is the fifth stage of HIV replication, after viral DNA has been integrated into helper T cell’s DNA?

The viral DNA is transcribed to produce viral mRNA

What is the sixth stage of HIV replication, after viral DNA has been transcribed to produce viral mRNA?

The viral mRNA is translated to viral proteins

What is the seventh stage of HIV replication, after viral mRNA has been translated to viral proteins?

The viral proteins are assembled to form new HIV, which causes the helper T cell to burst. The HIV then spreads to other parts of the body

What can a critically low number of helper T cells lead to, and why?

AIDS- without helper T cells, B cells are not stimulated to divide and cytotoxic T cells are not stimulated to kill pathogens

What does ELISA stand for?

Enzyme-linked immunosorbent assay

What is ELISA used for?

To detect specific antigens

What is the first stage in ELISA testing?

Antibodies complementary to the specific antigen are bound to the bottom of a reaction vessel

What is the second stage in ELISA testing, after the antibodies have been bound?

Sample is introduced and any specific antigens present bind to the antibodies. SAMPLE IS WASHED

What is the third stage in ELISA testing, after antigens have bound and sample has been washed?

A second complementary antibody is introduced, this time with an enzyme attached. SAMPLE IS WASHED AGAIN

What is the fourth stage in ELISA testing, after antibody with enzyme has been introduced?

A substrate is added which reacts with the enzyme. If antigen is present, a colour change takes place

How do antibiotics treat bacteria?

By disrupting cell wall synthesis and interfering with their metabolism

Why do antibiotics not treat viruses?

Viruses do not have a cell wall or their own metabolism pathways

Where are the RNA and reverse transcriptase stored in HIV?

The capsid

What is the capsid?

A protein shell

What is the lipid envelope?

A protective phospholipid layer

Why might someone infected with HIV not contract AIDS for years?

Helper T cells may be dormant for decades

What does it mean if helper T cells are “dormant”?

Their replication machinery stops working for a short time, meaning they are only able to replicate HIV years later

Where are all lymphocytes produced?

In the bone marrow

Where do B cells develop?

In the bone marrow

Where do T cells mature?

In the thymus

What are the two main types of lymphocyte?

B cells and T cells

What is the first stage of the primary immune response, after a B cell first encounters a pathogen?

Its receptors bind to a complementary antigen on the surface of the pathogen

What is the second stage of the primary immune response, after a B cell’s receptors have bound to the antigen?

B cell internalises the pathogen and processes its antigens

What is the third stage of the primary immune response, after the B cell has processed the anitgens?

B cell presents processed antigens on its surface

What is the fourth stage of the primary immune response, after B cell has presented antigens on its surface?

Helper T cell’s receptors bind to complementary anitgens on B cell’s surface

What is the fifth stage of the primary immune response, after T cell has bound to antigens on B cell’s surface?

Helper T cell releases chemicals to stimulate the B cell to clone itself

What is the sixth stage of the primary immune response, after B cell has cloned itself?

Clones differentiate into plasma cells or memory B cells

What type of molecule are antibodies?

Proteins

How many polypeptide chains does an antibody have?

4

What region are the 4 polypeptide chains known as in an antibody?

The constant region

How many heavy and light chains are there in an antibody?

2 heavy chains and 2 light chains

What are the polypeptide chains in an antibody held together by?

Disulphide bridges

What region are the antigen-binding sites in an antibody known as?

Variable regions

How many antigen-binding sites does an antibody have?

2

How many antigens can an antibody bind to at a time?

2

Which cells secrete antibodies?

Plasma cells

How long typically does a plasma cell survive for in the body?

A few days

Give an example of how an antibody can destroy a pathogen.

Agglutination

What is agglutination?

The process of clumping pathogens together using antibodies

Why is agglutination useful?

• Becomes harder for pathogen to spread around the body

• Makes it easier for phagocytes to locate and destroy them

How long can a memory B cell survive for in the body?

Years

What do memory B cells do after the pathogen is gone, following the primary immune response?

Remain in the body

Which immune response do B memory cells carry out when the same pathogen re-invades the body?

The secondary immune response (same as primary immune response, with a few differences)

What are the two key differences between the primary and secondary immune response?

• Secondary immune response is quicker

• Secondary immune response produces more plasma cells

What is a potential consequence of the secondary immune response being quicker?

You may not experience symptoms, as the non-self cell is wiped out before it can make a visible impact

What is a consequence of the secondary immune response producing more plasma cells?

A greater concentration of antibodies in the body

What is antigenic variability?

The ability for antigens to mutate and vary in shape

What are the primary and secondary immune responses collectively known as?

The humoral response

What can T cells bind to?

Can only bind to antigens on antigen-presenting cells

Which three cell types can act as antigen-presenting cells?

Phagocytes, B cells, virus-infected body cells

How does a cell become an antigen-presenting cell?

First processes the antigens, then presents them on its surface

What 2 types of T cell can T cells in the thymus develop into?

Cytotoxic T cells and helper T cells

What does naive mean in terms of B cells and T cells?

It hasn’t encountered an antigen before

What is the helper T cell stimulated to do when it binds to an antigen on an APC?

Clone itself

What 3 things can cloned helper T cells stimulate?

Phagocytosis, B cells to clone, cytotoxic T cells

How do cloned helper T cells stimulate phagocytosis?

They release chemicals that attract phagocytes to a pathogen

What do cytotoxic T cells kill?

Virus-infected body cells

What does a cytotoxic T cell do once it has bound to an antigen on a virus-infected body cell?

Releases perforin

What does perforin do?

Causes holes to form in the virus-infected body cell’s cell membrane. This disrupts the cell’s balance, as it allows substances to uncontrollably enter and leave.

What is the response involving T cells known as?

The cell-mediated response

What are two examples of white blood cells?

Lymphocytes and phagocytes

What are lymphocytes and phagocytes both able to do?

Tell the difference between self cells and non-self cells

What happens when lymphocytes recognise non-self cells?

An immune response is triggered

Which four main threats can lymphocytes identify?

pathogens, toxins, cells from other humans, abnormal body cells

What are pathogens?

Microorganisms capable of causing disease

Antigens are often which type of molecule?

Proteins

What do lymphocytes use to identify non-self cells?

Antigens

What is an antigen?

A protein on the surface of a cell that triggers an immune response

Which type of immune response do lymphocytes trigger?

Specific

Why do lymphocytes trigger a specific immune response?

Each lymphocyte has receptors that are complementary to one specific antigen

Why are lymphocytes able to recognise non-self cells?

They have receptors that are complementary to antigens on non-self cells

What happens when the complementary receptor on the lymphocyte binds to the antigen on the non-self cell?

An immune response is triggered

Which pathogens do phagocytes recognise?

All pathogens

Which type of immune response do phagocytes trigger?

Non-specific

What do we call a vesicle containing a pathogen?

A phagosome