Cell Recognition and the Immune System

• A disease may be caused by a pathogenic (disease-causing) microorganism. When a pathogen is transferred to another individual it is called transmission.
• Defence Mechanisms can be non-specific or specific.

  • Non-specific - physical barriers e.g. skin, mucus, cilia, stomach acid OR phagocytosis
  • Specific - cell-mediated = T lymphocytes, Humoral = B lymphocytes
• Phagocytosis

  1. Detection - pathogen is detected by chemicals (chemotaxis) being given off
  2. Ingestion - phagocyte engulfs pathogen
  3. Phagosome forms - phagosome is formed
  4. Fusion with Lysosome - phagosome fuses with lysosome (containing lysosomal enzymes)
  5. Digestion - Microbes are broken down by enzyme
  6. Discharge - Indigestible material is discharged from phagocyte
• Specific Defence Mechanisms

  They are a specific response against the pathogen and involve B cells and T cells which are both a type of white blood cell and lymphocyte. Humoral involves B cells and Cellular involves T cells.
• Humoral Response: Structure and Functions of Antibodies

  The basic structure of the antibody molecule consists of four polypeptide chains - 2 heavy and 2 light chains joined by disulfide bonds. Each polypeptide chain consists of a constant and variable region.

  • constant - the amino acid sequence is the same
  • variable - the amino acid sequence is different

  The heavy and light chains form two antigen-binding sites which have a specific tertiary structure complimentary to the structure of the antigen.
• B Cells in Humoral Immunity

  1. A B cell detects a pathogen and interacts directly to the pathogen's antigen as it is complimentary in shape.
  2. The pathogen is then engulfed by the phagocyte and then presents its antigen on the cell-surface membrane to become an antigen-presenting cell.
  3. Activated T helper cells bind to the B cell, causing activation of this B cell. It has a receptor which is complimentary in shape to pathogen's antigen to help activate the B cell.
  4. The B cell then divides by mitosis to form a plasma cell and memory cell by clonal expansion.
• B Cells in Humoral Immunity

  • Plasma Cells - secrete antibodies. These antibodies attach to antigens on pathogens and destroy them
  • Memory Cells - circulate in the blood, ready to divide if body is re-infected by the same pathogen
• T Cells in Cell Mediated Response

  1. Phagocytes engulf pathogens and display their antigens on the cell-surface. They are now known as antigen-presenting cells.
  2. Helper T cells with complimentary receptors bind to these antigens.
  3. On binding, the helper T cell is activated to divide by mitosis to form genetically identical clones.
• Types of Immunity

  • Passive - the individual's immune system does not make these antibodies
  • Active - makes its own antibodies after direct contact to a pathogen's antigens
  • Natural - naturally exposed to pathogen
  • Artificial - man-made
• Steps of Vaccination

  1. The vaccine, containing antigens, is injected into the blood. 
  2. This stimulates the primary immune response to produce antibodies against the pathogen. 
  3. Memory cells, capable of recognising these antigens, are produced. 
  4. On second exposure to this pathogen, memory cells rapidly divide into plasma cells. 
  5. Plasma cells rapidly produce antibodies against the pathogen. 
  6. The pathogen is destroyed before any symptoms are experienced.
• Herd Immunity

  Vaccines provide immunity for those who receive them and those who don't so there is a lower risk of a pandemic. Those who don't include: new born babies, the elderly, people with cancer or an organ transplant and people who cannot produce memory B lymphocytes.

  A suitable Vaccine Programme consists of:

  • a suitable vaccine to immunise most of the vulnerable population
  • very little/few side effects
  • means of producing, storing and transport must be available
  • medically trained staff
• Disadvantages of Herd Immunity

  • causes antigenic variation
  • medically trained staff is needed
  • ethical issues such as the use of animals
  • how the money is spent (money could've gone towards education and other healthcare problems e.g cancer treatment, surgery)
• Uses of Monoclonal Antibodies:

  • diagnosis of diseases - monoclonal antibodies bind to specific cell types
  • treatment of disease - monoclonal antibodies bind to specific cells
  • pregnancy testing - bind to pregnancy hormone
  • detecting cancers - bind to prostate specific antigens to identify prostate cancer in men
• The ELISA (enzyme-linked immunosorbent assay) uses antibodies to detect specific antigens in a sample.
• How ELISA works

  1. Add the sample (containing the target protein) to a well plate where the target protein can attach to the well.
  2. Wash the well to remove unbound antibodies
  3. Add second antibody which will bind to the first. The second antibodies are attached to an enzyme
  4. Wash the well to remove any secondary unbound antibodies
  5. Add colourless substrate and the enzyme attached will cause a change in colour
• Viruses are all acellular, non-living particles which are smaller than bacteria. They all contain nucleic acid (DNA or RNA) but can only multiple inside a host cell.
• Replication of HIV


  1. Attachment proteins on the HIV attach to the CD4 receptor on a helper T cell
  2. The protein capsid fuses with the cell membrane ans the RNA and reverse transcriptase enter the T cell
  3. Reverse transcriptase converts this RNA into DNA and then the viral DNA is inserted into the helper T cell's genome. 
  4. The new viral DNA is used to make new viral proteins needed for HIV
  5. The proteins are used to assemble new HIV particles which then leave the cell ready to infect other cells
• An antigen is a non-self protein with a specific tertiary structure which stimulates an immune response
• HIV causes AIDS by killing/interfering with the helper T cell. People normally have 800-1200 helper T cells in mm^3 but a person with AIDS can have as low as 200 mm^3.