Cell recognition and the immune system

• Defence mechanisms

  • Cell mediated responses involving T lymphocytes
  • Humoral responses involving B lymphocytes
• Non specific defence mechanisms

  Physical barrier (skin)

  Phagocytosis
• Specific defence mechanisms 


  Cell mediated response

  Humoral response
• Phagocytosis 


  Large particles can be engulfed by cells in the vesicles formed from the cell surface membrane.
• What white blood cells carry out phagocytosis

  Phagocytes
• Antigens 


  Any part of an organism or substance that is recognised as a non self by the immune system and stimulates an immune response. Usually proteins that are part of the cell surface membrane or cell walls of invading cells.
• Lymphocytes 


  Type of white blood cells, produced by stem cells in the bone marrow. Two types.
• B lymphocytes 


  Mature in the bone marrow, associated with humoral immunity.
• T lymphocytes 


  Mature in the thymus gland, associated with cell mediated immunity.
• Antigen presenting cells 


  Cells that display foreign antigens in their surface. They can present antigens of other cells on their own cell surface membrane.
• Cell mediated response


  Pathogens invade body cells or are taken in by phagocytes
• Cell mediated response


  1. Phagocyte places antigens from the pathogen on its cell surface membrane

  2. Receptors on a specific helper T cell fit exactly onto these antigens

  3. Attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells

  4. Cloned T cells develop into memory cells that enable a rapid response to future infections by the same pathogen

  5. Cloned T cells stimulate phagocytes to engulf pathogens by phagocytosis

  6. Cloned T cells stimulate B cells to divide and secrete their antibody

  7. Cloned T cells activate cytotoxic T cells
• How cytotoxic T cells kill infected cells

  The 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.
• Humoral response


  1. B cell takes up surface antigens of invading pathogen

  2. B cell processes antigens and presents them on its surface

  3. Helper T cells attach to processed antigens on B cell, activating B cell

  4. Activated B cell divides by mitosis to give clone of plasma cells

  5. Cloned plasma cells produce and secrete specific antibody that fits pathogen's surface antigen

  6. Antibody attaches to and destroys pathogen antigens

  7. Some B cells develop into memory cells that can respond to future infections by same pathogen
• Secondary immune response


  Memory cells respond to future infections by rapidly dividing and developing into plasma cells that produce antibodies
• Antibodies


  Proteins with specific binding sites synthesised by B cells
• Antibodies


  • Made of four polypeptide chains
  • One pair of chains are long and called heavy chains
  • One pair of chains are shorter and known as light chains
  • Each antibody has a specific binding site that fits very precisely onto a specific antigen
  • The binding site is the variable region because it is different on different antibodies
  • Each binding site consists of a sequence of amino acids that form a specific 3D shape
  • The rest of the antibody is known as the constant region
• Antigen-antibody complex


  Created when the antibody's binding site fits very precisely onto a specific antigen
• Antibodies assist in antigen destruction in two ways:

  • Cause agglutination of the bacteria 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.
• Monoclonal antibodies 


  An antibody produced by a single clone of cells.
• Monoclonal antibodies can be used to treat cancer in a number of ways. By far the most successful so far is direct monoclonal antibody therapy.

  • Monoclonal antibodies are produced that are specific to antigens on cancer cells.
  • These antibodies are given to a patient and attach themselves to the receptors on their cancer cells.
  • They attach to the surface of their cancer cells and block the chemical signals that stimulate their uncontrolled growth.
• Another way to treat cancer using monoclonal antibodies

  It is called indirect monoclonal antibody therapy, involves attaching a radioactive or cytotoxic drug to the monoclonal antibody. When the antibody attaches to the cancer cells, it kills them.
• Passive immunity 


  Produced by the introduction of antibodies into individuals from an outside source. No direct contact with the pathogen or its antigen is necessary to induce immunity. Immunity is acquired immediately. As the antibodies are not being produced by the individual themselves, the antibodies are not replaced when they are broken down, no memory cells are formed and so there is no lasting immunity.
• Active immunity 


  Produced by stimulating the production of antibodies by the individuals' own immune system. Direct contact with the pathogen or its antigen is necessary. Immunity takes time to develop. It is generally long term and is of two types.
• Natural active immunity


  Results from an individual becoming infected with a disease under normal circumstances. The body produced its own antibodies and may continue to do so for many years.
• Artificial active immunity 


  Form the bases of vaccination. It involves inducing an immune response in an individual, without them suffering the symptoms of the disease.
• The success of a vaccination programme depends on a number of factors:

  • A suitable vaccine must be economically available in sufficient quantities to immunise most of the vulnerable population.
  • There must be few side-effects, if any, from vaccination. Unpleasant side-effect may discourage individuals in the population from being vaccinated.
  • Means of producing, storing and transporting the vaccine must be available. This usually involves technologically advanced equipment, hygienic conditions and refrigerated transport.
• Factors for the success of a vaccination programme

  • A suitable vaccine must be economically available in sufficient quantities to immunise most of the vulnerable population
  • There must be few side-effects, if any, from vaccination. Unpleasant side-effect may discourage individuals in the population from being vaccinated
  • Means of producing, storing and transporting the vaccine must be available. This usually involves technologically advanced equipment, hygienic conditions and refrigerated transport
  • There must be the means of administering the vaccine properly at the appropriate time. This involves training staff with appropriate skills at different centres throughout the population
  • It must be possible to vaccinate the vast majority of the vulnerable population to produce herd immunity
• Herd immunity 


  Arises when a sufficiently large proportion of the population has been vaccinated to make it difficult for a pathogen to spread within that population.
• Ethics of using vaccines

  • Development of new vaccines often involves the use of animals.
  • Vaccines have side effects that may sometimes cause long term harm.
  • Vaccine programmes are expensive
• Structure of human immunodeficiency virus (HIV)

  On the outside is a lipid envelope, embedded in which are peg-like attachment proteins. Inside the envelope is a protein layer called the capsid that encloses two single strands of RNA and some enzymes. One of these enzymes is reverse transcriptase, it catalyses the production of DNA from RNA - the reverse reaction to that carried out by transcriptase. The presence of reverse transcriptase, and consequent ability to make DNA from RNA, means that HIV belongs to a group of viruses called retroviruses.
• Replication of HIV


  1. HIV enters bloodstream and circulates around body

  2. Protein on HIV binds to CD4 protein on helper T cells

  3. Protein capsid fuses with cell-surface membrane

  4. RNA and enzyme of HIV enter helper T cell

  5. HIV reverse transcriptase converts RNA into DNA

  6. DNA inserted into helper T cell's nucleus

  7. HIV DNA creates mRNA using cell's enzymes

  8. mRNA passes out of nucleus through nuclear pore

  9. Cell's protein synthesis mechanisms make HIV particles

  10. HIV particles break away from helper T cell with piece of cell-surface membrane forming lipid envelope
• ELISA test


  1. Apply the sample to a surface (a slide) to which all the antigens in the sample will attach

  2. Wash the surface several times to remove any unattached antigens

  3. Add the antibody that is specific to the antigen we are trying to detect and leave the two to bind together

  4. Wash the surface to remove excess antibody

  5. Add a second antibody that binds with the first antibody. This second antibody has an enzyme attached to it

  6. Add the colourless substrate of the enzyme. The enzyme acts on the substrate to change it into a coloured product
• The amount of the antigen present is relative to the intensity of colour that develops