Cell-Surface Antigens

• Four possible sources of antigens:

  • pathogens (micro-organisms that cause disease)
  • cells from other organisms of the same species (e.g. in transplanted organs)
  • abnormal body cells (e.g. cancerous cells)
  • toxins (poisonous molecules often released by bacteria)
• Anything that is non-self is destroyed as it is seen as a threat to the body. These foreign molecules are called antigens (a contraction of the term "antibody generating").
• An antigen is a foreign protein molecule that triggers an immune response by lymphocytes.
• What is a pathogen?

  A bacterium/microorganism that causes disease.
• Which types of cells stimulate an immune response?

  • toxins
  • pathogens
  • abnormal body cells
  • foreign/transplanted cells
• Proteins and glycoproteins in the cell-surface membrane of all body cells identify them as being 'self'.
• Any cell or object carrying other, non-self antigens, is attacked and destroyed by the immune system. This includes pathogens, cells from other organisms of the same species, cancer cells and toxins.
• Cell-surface antigens are cell-specific proteins and glycoproteins attached to the cell-surface membrane of cells.
• The destruction of a foreign cell by the immune system is called the immune response.
• Cell-surface antigens identify the cell as 'self'.
• An antigen is a substance that causes an immune response.
• A pathogen is an organism that causes disease.
• Pathogens are usually bacteria or viruses but can be fungi and protoctists too.
• Pathogenic bacteria carries antigens in their cell-surface membranes and cell walls, whereas viruses have antigens in their protein coats.
• Pathogens can enter the body through many ways, including:

  • in the air you breathe
  • in your food or drink
  • transfer of body fluids
  • directly through the skin
• If a person suffers a failure of a particular organ or tissue, they may have a healthy organ or tissue transplanted into their body from a donor.

  However, the transplanted cells have their own cell-specific antigens on their surfaces which the recipient's immune system will identify as foreign.

  The transplant will therefore be attacked and destroyed, a process called rejection.
• Even if an organ or tissue donor has very similar antigens to the recipient, it will never be exact (unless identical twins). The recipient must therefore take immunosuppressants - drugs which tone down the immune response - for the rest of their life.

  However, this also makes their immune system less effective at destroying invading pathogens, so there is an increased risk of suffering bacterial or viral infections.
• Some types of cells undergo a tightly controlled cell cycle in which they repeatedly grow and divide. If the DNA in these cells become damaged, then the genes controlling the cycle may be altered. The cell cycle may take place too rapidly and too frequently, producing a tumour.

  Cells in which the DNA has been altered will have different antigens. The immune system will therefore recognise the cells as 'foreign' and destroy them before they can develop into cancer.
• A toxin is a poison.
• Many pathogenic bacteria produce toxins that are released into the blood and cause damage to cells/metabolic processes, making us feel ill.
• Toxin molecules act as antigens and cause an immune response, in which the immune system destroys the toxins.
• Why will an antibody only detect a specific antigen?

  • the variable region of the antibody has a specific primary structure
  • therefore the tertiary structure of the binding site is complementary to the antigen
  • this forms an antigen-antibody complex
• What is meant by an antigen?

  A protein on the surface of a cell that initiates an immune response and causes antibody production.
• How does antigenic variability cause some people to become infected more than once with the same virus?

  • memory B/T cells do not recognise the new antigens
  • antibodies previously produced are not effective as their shape is not complementary to the new antigen
  • evades herd immunity
• What are antigens?

  Molecules that trigger an immune response by lymphocytes.
• What are the four sources of antigens?

  • pathogens (microorganisms that cause disease)
  • cells from other organisms of the same species (e.g. transplanted organs)
  • abnormal body cells (e.g. tumours)
  • toxins (poisonous molecules often released by bacteria)
• Antigens are involved in the specific immune response (SIR) e.g. the cell-mediated and humoral responses.
• Self antigens are antigens from our own body cells.
• Non-self (foreign) antigens are antigens from a different species or a different individual (e.g. pathogens).
• Once macrophages have hydrolysed a bacterium, they push an antigen from the bacteria through their cell membrane to communicate the presence of the bacterium to the rest of the immune system.