cell recognition & immune system

• Lymphocytes


  Cells that are able to identify non-self cells
• Antigen


  Unique specific molecule on a cell surface membrane that acts as a marker to identify the cell
• Types of non-self cells that lymphocytes can identify


  • Pathogens (bacteria, fungi, viruses like HIV)
  • Transplanted organ cells
  • Abnormal body cells (e.g. cancer cells)
  • Toxins
• Development of lymphocytes


  1. Lymphocytes complementary to antigens on self cells die or their production is suppressed during fetal development

  2. Remaining lymphocytes are complementary to antigens on non-self cells

  3. This process continues after birth in the bone marrow
• Autoimmune diseases are caused by lymphocytes attacking self cells
• Antigen variability


  Pathogens' DNA mutates frequently, changing the shape of their antigens, rendering previous immunity ineffective
• Influenza virus mutates frequently


  Requires a new flu vaccine to be created every year
• Antigens are typically proteins that generate an immune response when detected by lymphocytes
• T lymphocytes/T cells


  Involved in the specific immune response, the second line of defense after a pathogen has entered the bloodstream
• T cells


  • Mature in the thymus organ
  • Involved in the cell-mediated immune response
• Antigen Presenting Cells (APCs)


  Any cell that presents a non-self antigen on their cell surface membrane, such as infected body cells, phagocytes, transplant organ cells, or cancer cells
• Cell-mediated immune response


  1. Antigen presented on APC

  2. Helper T cells bind to antigen on APC and become activated

  3. Activated helper T cells divide by mitosis to produce more helper T cells

  4. Helper T cells stimulate B cells, macrophages, and produce memory T cells and cytotoxic T cells
• Cytotoxic T cells


  Also called killer T cells, can destroy abnormal or infected cells by releasing perforin, a protein that creates pores in the cell membrane causing cell death
• The cell-mediated immune response involves T lymphocytes responding to antigens presented on the surface of antigen presenting cells
• T lymphocytes/T cells


  Involved in the specific immune response, the second line of defense after a pathogen has entered the bloodstream
• T cells


  • Mature in the thymus organ
  • Involved in the cell-mediated immune response
• Specific immune response


  Response to an exact antigen or cell surface molecule
• Antigen presenting cells (APCs)


  Any cell that presents a non-self antigen on their cell surface membrane, e.g. infected body cells, phagocytes, transplant organ cells, cancer cells
• Cell-mediated immune response


  1. Antigen presented on APC

  2. Helper T cells bind to antigen on APC and become activated

  3. Activated helper T cells divide by mitosis to produce more helper T cells

  4. Helper T cells stimulate B cells, macrophages, and produce memory T cells and cytotoxic T cells
• Cytotoxic T cells


  Also called killer T cells, can destroy abnormal or infected cells by releasing perforin, a protein that creates pores in the cell membrane causing cell death
• The sore throat from a cold is caused by cytotoxic T cells destroying infected cells in the throat
• Passive immunity


  When you don't actually create the antibodies yourself, but have the antibodies introduced into you (e.g. through an injection of pure antibodies)
• Active immunity


  When you have created the antibodies yourself, after being exposed to the pathogen or antigen
• Examples of passive immunity


  • Antibodies passed through the placenta to a fetus from the mother
  • Antibodies passed through breast milk to a baby
  • Injecting antibodies to neutralize snake venom
• Natural active immunity


  When you have been naturally infected with the pathogen and created antibodies and memory cells
• Artificial active immunity


  When you are exposed to a weak, weakened, or dead form of the pathogen or its antigens, usually through a vaccine (injection)
• How vaccines work


  1. Exposure to weak/weakened/dead pathogen or antigens activates B-cells to undergo clonal expansion and differentiation, creating plasma cells that produce antibodies and memory B-cells

  2. Memory B-cells remain in the blood for decades

  3. Upon re-exposure to the pathogen, memory B-cells rapidly divide and produce large quantities of antibodies, preventing symptoms
• Primary response


  Initial response to vaccine exposure, creating some antibodies and memory B-cells
• Secondary response


  Rapid and large-scale antibody production upon re-exposure to the pathogen, due to memory B-cells
• Herd immunity


  When a large enough proportion of the population is vaccinated, it protects those who cannot be vaccinated (e.g. young children, immunocompromised individuals)
• Antigen variability can compromise vaccine effectiveness long-term, as pathogens can mutate and change the shape of their antigens