Cell recognition and the immune system

cell-mediated response

a response involving antigens that are attached to body cells associated with T cells involving antigen presenting cells (APCs)

differences between active and passive immunity

active immunity is produced by stimulating the production of antibodies by the individuals own immune system, where direct contact with the pathogen or its antigen is necessary. whereas, passive immunity is instead produced by the introduction of antibodies into the individual from an outside source. active immunity takes time to develop and is generally long-lasting. whereas, passive immunity is acquired immediately and is not long-lasting, since no memory cells are formed. active immunity also has two types; natural and artificial. whereas, passive immunity doesn't have two types.

Response of T lymphocytes to a foreign antigen

receptors on each T cell respond to a different antigen 1) pathogens invade body cells or are taken in by phagocytosis 2) the phagocyte presents antigens from the pathogen on its cell-surface membrane 3) receptors on a specific helper T cell fit exactly onto these antigens 4) this attachment activates the T cell to divide rapidly by mitosis and form clones of genetically cells 5) the cloned T cells: - develop into memory cells that enable a rapid response to future infections by the same pathogen - stimulate phagocytes to engulf pathogens by phagocytosis - stimulate B cells to divide and secrete their antibody - activate cytotoxic T cells

The use of antibodies in the ELISA test

ELISA stands for enzyme linked immunosorbant assay and uses antibodies to detect the presence of a protein in a sample and also the quantity of it. the antibodies are extremely sensitive and so can detect very small amounts of a molecule can be used to detect HIV and the pathogens of tuberculosis and hepatitis the procedure: - apply sample to a surface e.g. a slide, to which all the antigens in the sample will attach - wash surface several times to remove any unattached antigens - add antibody that it specific to the antigen we are trying to detect and leave the two to bind together - wash surface to remove excess antibody - add second antibody that binds with first antibody and has an enzyme attached to it - add colourless substrate of the enzyme, which the enzyme acts on to change it into a coloured product - amount of antigen present is relative to the intensity of the colour that develops

how HIV causes the symptoms of AIDS

HIV specifically attacks helper T cells and causes AIDS by killed and interfering with the normal functioning of them. a person suffering from AIDS has a significantly lower amount of helper T cells in their blood than a healthy person. without a sufficient number of helper T cells, the immune system cannot stimulate B cells to produce antibodies or cytotoxic T cells that kill cells infected by pathogens. memory cells may also become infected or destroyed. consequently, the body becomes unable to produce an adequate immune response and becomes susceptible to infections and cancers. many AIDS sufferers develop infections in the lungs, intestines brain and eyes, as well as experiencing weight loss and diarrhoea. these secondary diseases can ultimately cause death. HIV infects the immune system and prevents it from functioning normally. as a result, HIV sufferers are unable to respond effectively to pathogens and it is these infections, rather than the HIV itself, that ultimately leads to ill health and eventual death.

describe the structure of the human immunodeficiency virus

On the outside of the human immunodeficiency virus is a lipid envelope, which has attachment proteins embedded in it. inside the lipid envelope is a protein layer called a capsid that contains two single strands of RNA and some enzymes. one of the enzymes is reverse transcriptase that catalyses the production of DNA from RNA. beneath the lipid envelope is the matrix, which contains many enzymes involved in respiration.

the replication of HIV in helper T cells

since HIV is a virus, it can only replicate itself by using its genetic material to instruct a host cell to produce other components needed to make a new HIV. how HIV does this is as follows: - following infection, HIV enters the bloodstream and circulates around the body - a protein on the HIV readily binds to a protein called CD4 on helper T cells - the protein capsid fuses with the cell-surface membrane and the RNA and enzymes of HIV enter the helper T cell - the HIV reverse transcriptase converts the virus's RNA to DNA, which is moved into the helper T cell's nucleus where it is inserted into the cell's DNA - the HIV DNA in the nucleus creates mRNA using the cell's enzymes, which contains the instructions for making new viral proteins and the RNA to go into the new HIV - the mRNA passes out of the nucleus through a nuclear pore and uses the cell's protein synthesis mechanisms to make HIV particles - the HIV particles break away from the helper T cell with some of its cell-surface membrane surrounding them, which forms their lipid envelope

describe the two main types of lymphocyte

T lymphocyte - cell-mediated response B lymphocyte - humoral response

release of monoclonal antibodies (the humoral response)

the humoral response includes antibodies being produced by plasma cells. monoclonal antibodies are produced by B cells and develop into either plasma cells or memory cells. the production of antibodies and memory cells is called the primary immune response. plasma cells secrete antibodies usually into blood plasma very rapidly, which leads to the destruction of the antigen. plasma cells are responsible for the immediate defence of the body against infection. memory cells are responsible for the secondary immune response and do not produce antibodies directly but circulate in the blood and tissue fluid. when memory cells encounter the same antigen at a later date they divide rapidly and develop into plasma cells and more memory cells. an increased quantity of antibodies is produced at a faster rate in the secondary immune response than the primary immune response.

Ethical issues associated with the use of vaccines and monoclonal antibodies

the production of vaccines and development of new ones raises the question of whether it is acceptable to test on animals for that reason. vaccines have side-effects that can sometimes cause long-term harm, which raises the question of whether the benefits of a vaccine against a virus outweigh the potential side-effects.

response of B lymphocytes to a foreign antigen

the surface antigens on an invading pathogen are taken up by a B cell, which processes the antigens and presents them on its surface. helper T cells attached to the processed antigens on the B cell and activate the B cell. the B cell can then divide by mitosis to give a clone of plasma cells that produce and secrete the specific antibody that exactly fits the antigen on the pathogen's surface. the antibody can then attach to antigens on the surface of the pathogen and destroy them.

lymphocytes

a form of white blood cell, produced in the bone marrow (from stem cells) but develop and mature in different places 2 types: B lymphocytes and T lymphocytes

Antibody

antibodies are proteins with specific binding sites synthesised by B cells

agglutination

antibodies attach to bacterial cells to cause agglutination, where clumps of the bacterial cells are formed. this makes it easier for phagocytes to locate them as they are less spread out around the body.

Antigen

any part of an organism or substance that is recognised as non-self and stimulates an immune response

explain the role of T cells in cell-mediated immunity

cell-mediated immunity does not involve antibodies, instead it involves the activation of antigen-specific helper T cells. helper T cells help activate B cells to secrete antibodies and macrophages to destroy pathogens, as well as helping to activate cytotoxic T cells to kill infected target cells.

response of B lymphocytes to clonal selection

clonal selection accounts for the body's ability to respond rapidly to any of a vast number of antigens.

how cytotoxic T cells kill infected cells

cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens by producing a protein called perforin. this makes holes in the cell-surface membrane, which makes the cell-surface membrane permeable to all substances and the cell dies. the action of cytotoxic T cells is most effective against viruses because they replicate inside body cells.

explain the role of B cells in humoral immunity

each B cell starts to produce a specific antibody that responds to one specific antigen

formation of antigen-antibody complex

each antibody has a specific binding site that fits precisely onto a specific antigen to form an antigen-antibody complex.

the role of helper T cells (TH cells) in stimulating cytotoxic T cells (TC cells), B cells and phagocytes

helper T cells attach to phagocyte which activates it to clone by mitosis the cloned T cells activate cytotoxic T cells, stimulate B cells to divide and secrete their antibody and stimulate phagocytes to engulf pathogens by phagocytosis

humoral response

immunity involving antibodies in body fluid or a humor e.g. blood plasma associated with B cells

roles of plasma and memory cells in producing primary and secondary immune response

in the primary immune response, plasma cells secrete antibodies into blood plasma rapidly, that only last for a few days. these antibodies lead to the destruction of the antigen and are responsible for the immediate defence of the body against infection. in the secondary immune response, memory cells do not produce antibodies directly but circulate in the blood for a long time. when memory cells encounter the original antigen at a later date, they divide rapidly and develop into plasma cells and more memory cells. the plasma cells then produce the antibodies to destroy the pathogen while the new memory cells circulate for future infection. memory cells provide long-term immunity against original infection and an increased quantity of antibodies can be produced at a faster rate than in the primary immune response.

role of antigen-presenting cells in the cellular response

when pathogens invade body cells or are taken in by phagocytosis, the phagocyte places antigens from the pathogen on its cell-surface membrane. receptors on a specific helper T cell fit exactly onto these antigens. this activates the T cell to divide rapidly by mitosis and form a genetically identical clone of itself. the cloned T cells are then able to develop into memory cells that enable rapid response to future infections by the same pathogen, as well as stimulate phagocytes to engulf pathogens by phagocytosis, B cells to divide and secrete their antibody and activate cytotoxic T cells.

Destruction of ingested pathogens by lysozymes

lysosomes release their lysozymes into the phagosome, where they hydrolyse the pathogen

Antibody structure

made up of 4 polypeptide chains. the chains of one pair are long and are called heavy chains, while the chains of the other pair are shorter and are light chains. each antibody has a specific binding site that fits exactly onto a specific antigen to form an antigen-antibody complex. the binding site is different on different antibodies and is called the variable region - consists of a sequence of amino acids that forms a specific 3D shape that binds directly to a specific antigen. the rest of the antibody is the constant region which binds to receptors on B cells.

4 ways that body cells end up with antigens on their surface

1) when a body cell is invaded by a virus, it presents some viral antigens on its cell surface membrane 2) phagocytes that have engulfed or hydrolised a pathogen present some of the pathogens antigens on its cell surface membrane 3) cancer cells present antigens on their cell surface membranes, making them appear as foreign cells 4) body cells from a different individual from the same species have different proteins on their cell surface membranes which are antigens

explain the role of memory cells in the secondary immune response

memory cells 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 plasma cells produce the antibodies needed to destroy the pathogen while the new memory cells circulate for future infection. memory cells provide long term immunity against the original infections and allow more antibodies to be secreted a faster rate than in the primary immune response, ensuring the infection is destroyed before it can cause any harm.

state two differences between a specific and non-specific defence mechanism

non specific response is immediate and the same for all pathogens. specific responses are slower and specific to each pathogen

phagocytosis of bacterial cells

phagocyte attracted to pathogen by chemical products of the pathogen. phagocyte has several receptors on its cell surface membrane that attach to chemicals on the surface of the pathogen. lysosomes within the phagocyte migrate towards the phagsome formed by engulfing the bacterium. the lysosomes releases their lysozymes into the phagosome where they hydrolysis the bacterium. the hydrolysis products of the bacterium are absorbed by the phagocyte.

When a pathogen enters the body it may be destroyed by phagocytosis. describe how (4 marks)

phagocytes are attracted to pathogens by chemical products of the pathogen and move towards the pathogen along a concentration gradient. receptors on the surface of the phagocyte's cell-surface membrane attach to chemicals on the surface of the pathogen. lysosomes within the phagocyte migrate towards the phagosome formed by engulfing the pathogen and the lysosomes release their lysozymes into the phagosome where they hydrolyse the pathogen. the hydrolysis products of the bacterium are absorbed by the phagocyte.

explain the roles of plasma cells and antibodies in the primary immune response

plasma cells secrete antibodies into blood plasma and these lead to the destruction of the antigen. plasma cells are responsible for the immediate defence of the body against infections. the production of antibodies and memory cells is known as the primary immune response.

B lymphocytes (B cells)

produced by stem cells in bone marrow mature in bone marrow associated with humoral immunity

T lymphocytes (T cells)

produced by stem cells in bone marrow mature in the thymus gland associated with cell mediated immunity respond to antigens presented on a body cell rather than just in body fluid

Explain why specific antibodies only damage specific cells (4 marks)

Antibody has a specific tertiary structure and so only binds with the complementary antigen. The nerve cells have antigens which fit the variable region (antigen binding site) and make antibody-antigen complexes with the specific antibody. so antibody only binds to antigen-antibody complex with these nerve cells.