3.2.4 Cell recognition and the immune system Flashcards
what is an antigen?
- a protein located on the surface of cells
- generates an immune response
what do antigens enable the immune system to identify?
- pathogens
- cells from other organisms of the same species
- abnormal body cells
- toxins
what is a pathogen?
a microorganism which causes disease
how do lymphocytes distinguish between self and non-self cells?
- each type of cell has specific molecules on its surface that identify it (antigen)
- these molecules are usually proteins
- their 3D tertiary structure enables lots of unique and identifiable shapes to be made
what is antigen variability?
- pathogens’ DNA can mutate frequently
- if a mutation occurs in the gene which codes for the antigen, then the shape of the antigen will change
what is the effect of antigen variability on disease and disease prevention?
- any previous immunity to the pathogen with variable antigens is no longer effective
- this is because the memory cells in the blood will have a memory of the old antigen shape
- e.g., the influenza virus mutates and changes its antigen very quickly which is why a new flu vaccine has to be created each year
what are the two types of white blood cells?
phagocytes and lymphocytes
what are the two types of phagocytes?
neutrophils and macrophages
what are the two types of lymphocytes?
B lymphocytes and T lymphocytes
what is the difference between neutrophils and macrophages?
both engulf and digest pathogens, but macrophages can become antigen-presenting cells
what type of response is phagocytosis?
- non-specific
- any non-self cell that is detected will trigger the same response to destroy it
describe the process of phagocytosis in detail
- phagocytes are in the blood and tissues and any chemicals/debris released by pathogens/abnormal cells attract the phagocytes
- receptor binding points on phagocytes will attach to antigens on the pathogen
- phagocyte changes shape to move around and engulf the pathogen
- once engulfed, the pathogen is contained within a vesicle called a phagosome
- a lysosome within the phagocyte will fuse with the phagosome and release its contents
- lysozyme enzyme is released into the phagosome which hydrolyses the pathogen to destroy it
- soluble products are absorbed and used by the phagocyte
describe how bacteria are destroyed by phagocytes (6 marks)
- phagocyte recognises foreign antigen
- pathogen engulfed
- enclosed in phagosome
- phagosome fuses with lysosome
- lysosome contains enzymes
- pathogen digested / molecules hydrolysed
what are antigen presenting cells (APCs)?
any cell that present a non-self antigen on their surface
what are 4 examples of antigen presenting cells?
- infected body cells will present viral antigens on their surface
- a macrophage which has engulfed and destroyed a pathogen will present the antigens on their surface
- cells of a transplanted organ will have different shaped antigens on their surface compared to our self-cell antigens
- cancer cells will have abnormal shaped self-cells
describe how phagocytosis of a pathogen leads to the presentation of its antigens
- phagosome fuses with lysosome
- pathogen destroyed by lysozymes
- antigens from pathogen are displayed on the cell membrane
what is a cell-mediated response?
involves the response of T lymphocytes to a foreign antigen
why are T cell responses described as ‘cell-mediated’?
T cells only respond to antigens which are presented on APCs, and not antigens detached from cells and within body fluids such as the blood
what are the steps of the cell mediated response?
- once a pathogen has been engulfed and destroyed by a phagocyte, the antigens are positioned on the cell surface to become an APC
- helper T cells have receptors on their surface which can attach to the antigens on the APC
- once attached, this activates the helper T cells to divide by mitosis to replicate and make large numbers of clones
- cloned helper T cells differentiate into different cells
what 3 things can cloned T helper cells do?
- some remain as T helper cells and activate B lymphocytes
- some stimulate macrophages to perform more phagocytosis
- some become memory cells for that shaped antigen
what is the purpose of cytotoxic T cells?
to destroy abnormal or infected cells
how do cytotoxic T cells work?
- release a protein (perforin) which embeds in the cell surface membrane and makes a pore so that any substances can enter or leave
- this causes cell death
what is the humoral response?
the response involving B cells and antibodies
where are lymphocytes made?
all lymphocytes are made in the bone marrow and B cells mature there too
what are the steps of the humoral response?
- antigens in the blood collide with their complementary antibody on a B cell
- the B cell takes in the antigen by endocytosis and then presents it on it’s cell surface membrane
- this B cell then collides with a helper T cell receptor, which activates the B cell to go though clonal expansion and differentiation (clonal selection)
- B cells undergo mitosis to make large numbers of cells, these differentiate into plasma cells or memory B cells
what is the purpose of plasma cells?
to make antibodies as a part of the primary immune response
what is the purpose of helper B cells?
- to divide by mitosis and make plasma cells rapidly if they collide with an antigen they have previously encountered as a part of the secondary immune response
- this results in large numbers of antibodies being produced so rapidly that the pathogen is destroyed before any symptoms can occur
what is an antibody?
- a protein specific to an antigen
- produced by B cells / secreted by plasma cells
describe the structure of an antibody
- 4 polypeptide chains - 2 heavy and 2 light
- each chain has a variable region and a constant region
- the chains are connected by disulphide bridges
- all antibodies have the same constant regions that
allow them to bind to receptors on immune system
cells - they have two variable regions which are the
antigen binding sites so they will have a unique
tertiary structure complementary to an antigen - one antibody can bind to two pathogens at once
what is agglutination?
- antibodies are flexible and can bind to multiple antigens to clump them together
- this makes it easier for phagocytes to locate and destroy the pathogens
describe and explain the role of antibodies in stimulating phagocytosis (2 marks)
- bind to antigen
- are markers
OR - cause agglutination/clumping
- attract phagocytes
what is passive immunity?
- antibodies which come from another organism are introduced into the body
- no long-term immunity as no memory cells are formed
what is natural passive immunity?
a baby gets antibodies through placenta and in breast milk
what is artificial passive immunity?
you are injected with antibodies
what is active immunity?
immunity created by your own immune system following exposure to the pathogen or its antigen
what is natural active immunity?
creation of the body’s own antibodies and memory cells following infection
what is artificial active immunity?
creation of antibodies and memory cells following the introduction of a weakened version of the pathogen or antigens (via a vaccine)
describe the differences between active and passive immunity (6 marks)
- active involves memory cells, passive does not
- active involves the production of antibodies by plasma cells / memory cells
- passive involves antibodies introduced into the body from outside
- active is long-term, because antibodies are produced in response to antigen
- passive short term, because the antibody given is broken down
- active can take time to develop / work and passive is fast acting
what are vaccines?
small amounts of weakened or dead pathogen, or antigens are administered orally / by injection
how do vaccines work?
exposure to antigens activates the humoral response
describe how B cells respond to a vaccine (4 marks)
- B cell binds to complementary antigen
- B cell divides by mitosis
- plasma cells produce antibodies
- B cells develop into memory cells
what is herd immunity?
- if enough of the population are vaccinated the pathogen cannot spread easily amongst the populations
- this provides protection for those who are not vaccinated
what does HIV stand for?
human immunodeficiency virus
what is the structure of HIV?
- core = genetic material (RNA) and reverse transcriptase enzyme
- capsid = outer protein coat
- envelope = extra outer layer, made out of membrane taken from host cell’s membrane
- protein attachments = on the exterior of the envelope, enables the virus to attach to the host’s T helper cells
how does HIV replicate in cells (long)?
- glycoprotein molecules on the virus surface bind to CD4 receptors on the surface of T helper cells
- this allows the envelope surrounding the virus to fuse with the T helper cell membrane.
- the capsid is released into the cell where it releases the RNA and reverse transcriptase
- reverse transcriptase copies the viral RNA into a DNA copy
- the DNA is inserted into the cell’s DNA which gets replicated when cells replicate
- viral DNA is used to make HIV RNA and proteins at host ribosomes
- viral particles are assembled which bud off from the cell membrane and go on to infect other cells
how does HIV replicate in cells (short)?
- attachment proteins attach to receptors on helper T cell
- RNA and reverse transcriptase enter cell
- RNA converted into DNA using reverse transcriptase
- DNA inserted into host cell’s nucleus
- DNA transcribed into HIV mRNA
- HIV mRNA translated into new HIV proteins
- virus particles assembled and released from the cell
how does HIV affect the production of antibodies when AIDS develops in a person? (3 marks)
- HIV destroys helper T cells
- few B cells are activated
- fewer antibodies are produced
how does HIV cause the symptoms of AIDS?
acquired immunodeficiency syndrome (AIDS) is when the replicating viruses in the T helper cells interfere with their normal functioning of the immune system
why are antibiotics ineffective against viruses?
- virus replicate inside of cells, so it is difficult to destroy them without harming host cells
- viruses also have different mechanisms to replicate and no cell wall, like bacteria, so cannot be destroyed by antibiotics
how can inhibiting reverse transcriptase slow the development of AIDS?
- stops new HIV particles from forming new HIV DNA
- so slows the replication of HIV
- stops the destruction of more T cells
- so the immune system continues to work
- this is how antiretroviral drugs work
what is a monoclonal antibody?
- antibodies with the same tertiary structure
OR - antibodies produced from identical/cloned B cells
what is direct monoclonal antibody therapy?
- antibodies have a binding site which is complementary in shape to the antigens on the outside of cancer cells
- antibodies are given to the patient and attach to cancer cells
- this prevents the chemicals that enable uncontrolled cell division from binding to the cancer cells
- therefore, monoclonal antibodies prevent the cancer cells growing
what is indirect monoclonal antibody therapy?
- monoclonal antibodies complementary in shape to the antigens on the outside of cancer cells have drugs attached to them
- the cancer drugs are therefore delivered directly to the cancer cells and kill them
- radioimmunotherapy is when radioactive isotopes are attached instead of drugs
how do pregnancy tests work?
- first contains a mobile antibody complementary to the antigen being tested for and has a coloured dye attached
- mobile antibody moves to first window
- first window has a second antibody which is complementary in shape to the antigen is immobilised
- if antigen binds to first antibody, it will also bind to second antibody as they are both complementary to antigen
- this means that dye stays in the first window
- in the second window, a third antibody is immobilised which is complementary in shape to the first antibody
- this means that first antibody binds to third antibody and dye stays in second window
- 2 coloured lines = positive result
- 1 coloured line in control window = negative result but test works
how does an ELISA test work?
- add the test sample from a patient to the base of the beaker
- wash to remove any unbound test sample
- add antibody complementary in shape to antigen being tested for
- wash to remove any unbound antibody
- add second antibody, which is complementary to first antibody and has an enzyme attached to it
- second antibody binds to first antibody
- wash to remove any unbound antibody
- add substrate for enzyme, which is colourless
- substrate produces coloured products in the presence of the enzyme
- intensity of colour indicates the quantity of antigens present
describe the role of antibodies in producing a positive result in an ELISA
test (4 marks)
- the first antibody binds to the antigen which is complementary in shape
- second antibody with an enzyme attached is added
- the second antibody attaches to the first antibody (indirect ELISA)
- substrate added and colour changes
what is the ethical issue with monoclonal antibodies?
monoclonal antibody production requires mice to produce the antibodies, causes the death of animals
