The Human immune system

• Adaptive Immune System

  Third Line of Defense, Specific/Acquired Immunity
• Exposure to an antigen is required for acquired immunity. This may be acquired through passive or active processes.
• Passive immunity


  May be acquired from maternal antibodies or antibody serum injection
• Active immunity


  May be acquired through natural exposure to a pathogen or through vaccination
• Adaptive Immune System


  • Involves specific antigens and pathogen recognition
  • It is highly selective, able to detect differences between pathogens
  • It retains memory to the same antigen for future infections
  • Second exposure to the same antigen induces a faster and bigger response than the first
• Lymphatic system


  A series of vessels and nodes that control fluid levels in tissues, and Transport WBCs
• Major Histocompatibility Complex (MHC)


  A group of protein markers found on the surface of cells
• MHC process


  1. Host or infected cell engulfs pathogen, breaks up its antigen

  2. The antigen then binds to the cell's MHC molecule and pushes antigen to the surface

  3. The combined MHC + antigen on the surface is recognised by a T-cell
• 1. lymphocytes (B-cells)


  WBCs that each have their own cell receptor designed to recognise one specific antigen
• 1. cell receptor


  Complementary to the antigen
• Action of B-cells


  1. Extracellular antigen binds with the specific (complementary) receptor on the surface of a B-cell

  2. This stimulates the B-cell to divide and produce two different cells: Plasma cells that produce antibodies, Memory-b cells for future (secondary) exposure to the same antigen

  3. B-cells are also activated by cytokines released by T-cells
• Memory B-cells


  Increase the response to a second exposure of the same antigen
• Primary response


  The first exposure to an antigen
• Secondary response


  The response to the same antigen produces larger quantities of antibodies, at a faster rate
• 1. lymphocytes (T-cells)


  Part of a process called "cell-mediated immunity"
• 1. cell response


  1. T-cells bind to antigens bound to MHC molecules, presented by macrophages

  2. This initiates the production of various types of T-cells
• Types of T-cells


  • Killer T-cells (cytotoxic) - identify infected cells and destroy them
  • Helper T-cells - Activate Killer T-cells, Produce Memory T-cells, Activate specific B-cells for antibody production
  • Memory T-cells - provide a 'memory bank' for future exposure
• For an individual to have immunity to a specific pathogen, (antigen) they must be exposed to that pathogen, in order to produce antibodies and memory cells
• Adaptive immunity


  Produced from exposure to a pathogen, resulting in memory B and T cells. These cells reduce the response time, and produce a larger level of protection
• Active Immunity


  B and T cells and Memory cells are produced and stored in the spleen and lymph nodes
• Methods of active immunity


  • Exposure to the live/active pathogen/antigen
  • Exposure to an inactive (harmless) version of the antigen in a vaccination
• Exposure to the antigen via vaccination ensures the individual does not suffer from effects of the disease (tissue damage or death)
• Vaccinations are usually given in two doses to initiate the secondary immune response
• Passive Immunity


  Antibodies are provided to protect against pathogens. Antibodies are effective for a short time, therefore this type of protection is temporary
• Sources of natural passive immunity


  • Across the placenta during pregnancy
  • Through breast milk in the first few months of life
• Sources of acquired passive immunity


  • Antivenom or poison from snakes/spiders etc
  • Virulent antigens such as the toxin produced by tetanus bacterium
• Polio vaccine was first used in 1957. The last reported case was in 1994.
• Polio was a debilitating disease, destroying muscle tissue.
• In 1988, when the Global Polio Eradication Initiative began, polio paralysed more than 1000 children worldwide every day. Since then, more than 2.5 billion children have been immunized against polio thanks to the cooperation of more than 200 countries and 20 million volunteers, backed by an international investment of more than US$ 11 billion
• Adaptive immune response


  • Specificity i.e. recognition of a specific antigen/pathogen
  • The second or subsequent exposure to the same antigen is quicker and larger due to the memory of the old antigen
  • There are primary and secondary organs of the lymphatic system and these are vital for the adaptive immune response: Primary e.g. bone marrow and thymus gland, Secondary e.g. lymph nodes and spleen
  • The Major Histocompatibility Complex (MHC) is a set of surface markers that are involved in self and non-self (antigen) recognition. Some cells of the immune system display sections of antigens on their surface attached to the MHC
• B and T lymphocytes


  The main cells of the adaptive immune response. They both recognise specific antigens: B cells work by releasing antibodies that bind to antigens, T cells work by direct cell action releasing chemicals that destroy infected cells
• B and T memory cells


  Specific for a particular antigen, accumulate after exposure to the antigen and are responsible for the quicker and larger response after the second exposure to the antigen
• Active immunity


  The adaptive immune response is activated either naturally or by vaccination
• Passive immunity


  Brought about by the acquisition of antibodies either naturally (e.g. breast milk) or by injection (e.g. snake venom)