3.2.4 Cell recognition and the immune system

• An antigen is a foreign molecule/ protein that stimulates an immune response leading to the production of antibodies
• Cells are identified by the immune system as each type has a specific molecule on its cell surface membrane/ cell wall that identify it. Often these proteins has a specific tertiary structure
• The cells and molecules that the immune system can identify are

  • Pathogens (disease-causing microorganisms) e.g, viruses, fungi and bacteria
  • Cells from other organisms of the same species such as organ transplant
  • Abnormal body cells such as tumour cells or virus-infected cells
  • Toxins such as poisons which are released by some bacteria
• Phagocytois - non specific immune response

  • Phagocyte attracted by chemicals and it recognises the foreign antigens on the pathogen
  • Phagocyte then engulfs the pathogen by surrounding it with its cell membrane
  • Pathogen is contained in vesicle or phagosome in cytoplasm of phagocyte
  • Lysosome fuses with phagosome and releases lysozymes
  • Lysozymes then hydrolyse and digest the pathogen
• T lymphocytes recognise antigen presenting cells e.g, infected cells, phagocytes presenting antigens, transplanted cells, tumour cells and etc
• Specific helper T cells with complementary receptors on the cell surface bind to antigen on antigen presenting cell which activates and divide by mitosis to form clones which then stimulate:

  • Cytotoxic T cells which kill infected cells and tumour cells by producing perforin
  • Specific B cells
  • Phagocytes which engulf pathogens by phagocytosis
• B lymphocytes can recognise free antigens such as blood or tissues, not just antigen presenting cells
• Response of B Lymphocytes to a foreign antigen - humoural response:

  1. Clonal selection:

  • Specific B lymphocyte with complementary receptor binds to antigen
  • This is stimulated by helper T cells which releases cytokines
  • So divides rapidly by mitosis to form clones

  2. Some differentiate into B plasma cells -> secrete large amounts of monoclonal antibodies

  Some differentiate into B memory cells -> remain in blood for secondary immune response
• Antibodies have a quaternary structure with 4 polypeptide chains which are secreted by B lymphocytes and bind specifically to antigens forming antigen-antibody complex.
• Antibodies lead to the destruction of pathogens as:

  • Antibodies bind to antigens on pathogens forming an antigen-antibody complex.
  • The specific tertiary structure so binding site or variable region binds to complementary antigens.
  • Each antibody binds to 2 pathogens at a time causing agglutination which is th eclumping of pathogens.
  • Antibodies attract phagocytes and these bind to the antibodies and phagocytose many pathogens at once.
• Antibodies have a

  • A light polypeptide chain and a heavy polypeptide chain
  • Disulfide bridges
  • Hinge Region
  • Variable Region
  • Antigen binding site
• Differences between primary and secondary immune response:

  • Primary - first exposure to antigen
  • Antibodies are produced slowly at low conc.
  • Takes time for specific B plasma cells to be stimulated to produce specific antibodies
  • Memory cells produced

  
  

  • Secondary - second exposure to antigen
  • Antibodies produced faster and at higher conc.
  • B memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies
• A vaccine is an injection of antigens from attenuated - dead/ weakened pathogens. This stimulates the formation of memory cells
• Vaccines provide protection to individuals against disease by:

  1. Specific B lymphocyte with complementary receptor binds to antigen
  2. Specific T helper cells bind to antigen-presenting cells and stimulates B cell
  3. B Lymphocyte dividies by mitosis to form clones
  4. Some differentiate into B plasma cells which release antibodies
  5. Some differentiate into B memory cells
  6. On secondary exposure to antigen, B memory cells rapidly divide by mitosis to produce B plasma cells
  7. These release antibodies faster and at a higher concentration