Cell recognition and the immune system


Defence mechanisms

Immunity is where the body is prepared for the second infection by the same pathogen and can kill it before any harm is done.

There are two types of defence mechanisms, one where it is physical, immediate responses such as the skin acting a barrier or specific which is non-specific and less rapid but longer-lasting that uses a white blood cell called a lymphocyte by cell-mediated or humoral responses.

Cell-mediated is using T lymphocytes and humoral is using B lymphocytes- specific responses.

Lymphocytes are able to tell between self cells and non-self/foreign cells. 

Each cell, self or non-self, has a specific receptor on it that identifies it.

Proteins allow the immune system to identify pathogens, non-self material, toxins, and abnormal body cells.

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The two types of white blood cells involved are phagocytes and lymphocytes.

Phagocytes ingest and destroy the pathogen by phagocytosis. 

Lymphocytes are involved in immune responses.

Phagocytes have several receptors on their cell surface membrane and this is what attaches to the surface of the pathogen. They engulf the pathogen, forming a vesicle called a phagosome. Lysosomes move towards the vesicles and fuses with it. Lysozymes are digestive enzymes that break the pathogen down by hydrolysis on the cell wall. The soluble products are absorbed into the cytoplasm of the phagocyte and the debris is removed by exocytosis.

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T lymphocytes and cell mediated immunity

An antigen is recognized as a non-self cell that stimulates the immune response. They are proteins on the cell surface membrane and this presence triggers the production of antibodies.

Lymphocytes offer a specific response found in the bone marrow. There are two types:

  • B lymphocytes that are associated with humoral immunity- immunity involving antibodies in body fluids 
  • T lymphocytes that are associated with cell mediated immunity- involving body cells.

T cells can distinguish between self and non-self cells as the phagocytes that engulfed the pathogen display some of the pathogens antigens on its CSM and body cells invaded by a virus displays some of the antigens on its CSM. Different cells have different antigens on the CSM so the t cells can tell whether it is self or non-self.

T cells only respond to antigens on body cells. Phagocytes places the pathogens antigens on CSM and receptors on specific t helper cell fit onto the antigens. The attachment activates t cells to divide rapidly by mitosis and forms clones that either develop into memory cells, stimulates phagocytes to engulf, stimulates b cella to divide and secrete antibodies or activate cytotoxic t cells. These cells produce a protein called perforin that makes holes in the cell membrane.

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B lymphocytes and humoral immunity

Humoral immunity involved antibodies which are soluble in the blood and tissue fluid. B cells produce a specific antibody that responds to a specific antigen. 

The surface antigens on a pathogen is taken in by a B cell and they process and present them on their CSM. Helper T cells attach to these antigens which activate the B cell. It then divides by mitosis to give clones of plasma cells. These plasma cells then produce antibodies that fit the antigen on the pathogen's surface and it destroys them. Some of the b cells develop into memory cells so it could respond to future infection of the same pathogen. This is the secondary immune response.

Plasma cells are responsible for the primary immune response. They are for the immediate response defence of the body against infection.

Memory cells are responsible for the secondary immune response.

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Antibodies are proteins with specific binding sites. It is made of 4 polypeptide chains with heavy and light chains. When non-self material enters the body, B cells produce specific antibodies that fit onto its antigens to form an antigen-antibody complex.

The binding sites on antibodies are different on different antibodies and this is called the variable region. Each binding sites has a sequence of amino acids that form a specific 3D shape. The rest of the antibody is called the constant region

Antibodies cause destruction by:                                                                                                         

  • causing agglutination which clumps bacterial cells together so easier for phagocytes to engulf.
  • serves as markers that stimulate phagocytes to engulf the cells.                                                                                
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Monoclonal antibodies

Monoclonal antibodies are antibodies produced by a single clone of cells and consist of identical antibody molecules. 

Monoclonal antibodies can be used to target specific substances and cells. One target could be cancer cells. Monoclonal antibodies can be used to treat cancer:

- Produced that is specific to cancer antigens. The antibodies are given to a patient and it attaches to the receptors on the cancer cells. By attaching to the surface it it blocks the chemical signals that stimulate their uncontrolled growth.

As monoclonal antibodies are highly specific and not toxic, they lead to fewer side effects than other forms of therapy.

Another method is called an indirect monoclonal antibody therapy where it is attached to radioactive drugs and this then attaches to the cancer cells and kills them.  They are used in small doses as they are specific. They are cheaper and has less side effects.

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Diagnosis with monoclonal antibodies

Monoclonal antibodies are important in diagnosing certain cancers. Prostate cancer often produces more of the protein PSA (prostate-specific antigen). By using M antibodies, you can measure the levels of it in the blood. High amounts don't indicate you have the disease but shows an early warning for it.

Pregnancy tests use monoclonal antibodies. The placenta produces the hormone called hCG and is found in the mother's urine. M antibodies are on the ***** of the test and if it is present it binds onto the antibodies. This hCG antibody colour complex moves along the stick until trapped by a different antibody, created a coloured line.

The ethical use of M antibodies would be that it involves mice as they are used to produce antibodies and tumour cells. Also, a clinical trial before resulted in organ failure of 6 volunteers.

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Immunity is the organisms ability to resist infection and it comes in two forms:

- Passive immunity is produced by the introduction of antibodies from an outside source such as anti-venom. This means it will not resist it in the future as no memory cells are formed. The immunity is immediate.

- Active immunity is produced by stimulating the production of antibodies by own immune system. The immunity takes time and is long lasting and comes in two types: natural active immunity which is where the body produces own antibodies and artifical active immunity is taken in the form of vaccination. 

The intention of a vaccine is to stimulate an immune response and a vaccine contains a dead or weakened version of the pathogen or even just the pathogens antigens. This produces memory cells so when the body becomes infected the body will produce a fast response before it infects the body more.

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Herd immunity

Herd immunity is where the majority of the population has been vaccinated so it is hard for the pathogen to spread. It is important as it is impossible to vaccinate everyone. 

Features of a successful vaccine would be that it produces herd immunity. Also it should be economically available to immunise most the population. Must have few side effects and able to be produced, stored and transported.

Vaccinations may not eliminate disease as some people have defective immune systems and the pathogen may mutate and so the vaccines are no longer effective as the antigens on this pathogen won't fit the antibodies. Some pathogens can hide from the bodies immune system by living in cells.

Ethics of using vaccines would be that development of vaccines often uses animals. They also have side effects and should vaccines be compulsory? But not everyone is for vaccines.

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Structure of HIV:

  • On the outside it has a lipid envelope.
  • There are attachment proteins where the lipid envelope is embedded on.
  • Inside the envelope is a protein layer called a capsid that encloses the two single strands of RNA and enzymes.
  • One enzyme is reverse transcriptase and it catalyses the production of DNA from RNA. Transcriptase carries out the reverse of this. 

HIV belongs to retroviruses as is can make DNA from RNA.

HIV infects the body by entering the bloodstream. A protein on it binds to CD4 protein which is on many cells. HIV typically binds to T helper cells. The protein capsid fuses with the CSM and RNA and enzymes of HIV enter the t helper cell. The HIV reverse transcriptase converts the viruses RNA into DNA. Newly made DNA moves to helper t cells nucleus and is inserted into the cells DNA. HIV DNA in the nucleus creates mRNA and contains instructions to make new viral proteins and RNA goes into HIV. mRNA passes out the nucleus by a nuclear pore and uses cells protein synthesis mechanism to make more HIV. 

When you are infected you would be HIV positive.

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HIV leading to AIDS

HIV attacks the t helper cells and HIV causes AIDS by killing normal functioning helper t cells. Someone suffering from AIDS can have as low as 200mm3 of t helper cells. Without sufficient amount of t helper cells, can't stimulate B cells to produce antibodies and so the body can't produce an adequate immune response. This means HIV sufferers catch illnesses fast and have worse symptoms. 

The ELISA test stands for enzyme-linked immunosorbant assay. Uses antibodies to detect presence of a protein in a sample and the amount of it. 

Apply sample to a surface and wash it several times to remove any unattached antigens. Add the antibodies that is specific to the antigen we are trying to detect and allow it to bind. Wash it to remove any excess antibodies. Add a second antibody which has an enzyme on, that binds to the first one. Add colourless substrate of the enzyme and the enzyme acts on the substrate which causes a colour change. Amount of antigens present correlates to the intensity of the colour.

Antibiotics are ineffective on viruses as they are in host cells and so there are no metabolic mechanisms or cell structures to disrupt like with bacteria. They have a protein coat rather than murein and so has no sites on it.

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