Knowledge-8 TIS

• Immunity is the ability to resist infection from invading disease causing micro-organisms (pathogens).
• An immune response is where an organisms recognizes and responds to antigens , destroying substances carrying non-self antigens through the activation of lymphocytes.
• T lymphocytes: these mature in the thymus gland and are associated with cell mediated immunity.
• B lymphocytes: these mature in bone marrow and are associated with humoral immunity.
• Antigens are specific molecules on a cells surface that identify it, they are usually molecules that include proteins found on the cell-surface of the membrane or cell wall.
• The immune system can recognize antigens as belonging to the organism and cell that do not (non-self or foreign).
• The immune system can identify antigens belonging to:

  • pathogens
  • cells from other organisms of the same species
  • abnormal body cells such as diseased or infected cells
  • toxins
• non-self antigens will trigger an immune response to destroy the antigen and the cell hosting it.
• Phagocytosis is a non-specific response to an invading pathogen that has entered the body.
• Phagocytes are a type of white blood cell , they engulf and destroy pathogens.
• Phagocytosis has five stages:

  1. Chemical products from the pathogen attract a phagocyte to move towards the pathogen along the conc gradient.
  2. The phagocyte attaches to the surface of the pathogen.
  3. the phagocyte surrounds the pathogen and engulfs it in a vesicle called a phagosome.
  4. lysosomes move towards phagosome and fuse with it.
  5. the enzymes in the lysosomes , called lysozymes , break down and digest the pathogen into small soluble molecules. These are then absorb by the phagocyte.
• Antigens of a pathogen can develop variation that can lead to new strands arising in a population.
• The structure and shape of the antigen can change due to random mutations in the genetic code of the pathogen.
• if pathogens with new antigens multiply and spread new strains of the pathogen can emerge through natural selection.
• Pathogens invade the body and are engulfed by phagocytes
• A special type of phagocyte becomes an antigen presenting cell (APC) by inserting the pathogens antigens into its cell surface membrane
• The APC presents the antigens to t-helper cells
• The antigen binds to complementary cell receptor on t-helper cells
• The t-helper cell is activated and stimulates other t cells causing rapid division
• activation of t-helpers cells can cause:

  • Stimulates phagocytes to engulf more pathogens
  • Stimulates B cells
  • May activate cytotoxic T cells which kill infected cells by making holes in their surface membranes
• The humoral response part one:

  1. the surface antigens of the pathogen in the blood are taken up by b cells.
  2. the b cells process and present the antigens on their cell surface membrane.
  3. T helper cells bind to the processed antigens and activate the b cells.
  4. The activated B cells divide by mitosis and these differentiate into short-lived plasma cells and long-lived b memory cells (clonal selection)
• The humoral response part two:

  1. (5.) the short-lived plasma cells produce antibodies complementary to the antigens.
  2. (6.) the antibodies attach to the antigens on the pathogen a, which leads to their destruction via agglutination and phagocytosis. This is the primary immune response.
  3. (7.) some of the remaining B cells develop into memory cells that stay in the bloodstream. when they come into contact with the antigen again they divide rapidly to from b plasma cells and b memory cells. This is the secondary immune response.
• During primary response plasma cells produce antibodies but these only survive in the bloodstream for a few days.
• the division of memory cells to produce plasma cells and memory cells is the secondary immune response.
• agglutination is the process of clumping bacteria cells together making them easier to locate and be engulfed by phagocytes.
• Antibody-antigen complexes can act as markers that stimulate phagocytosis.
• Antibodies are globular proteins produced by a plasma cell in response to the presence of a specific antigen.
• Antibodies structure:

  • made of four polypeptide chains , two heavy chains and two light chains held together by disulfide bonds.
  • the tips of the antibodies y shape form the antigen binding site, each tip has a variable region formed by unique amino acid sequences.
  • all antibodies have identical constant regions.
  • contains a receptor binding site that enable antibodies to attach to the cell surface of a lymphocyte or other antibodies.
• For an antibody to bind with an antigen the shape of the variable region must be complementary to to the antigen that the antibody is targeting.
• Vaccination is the introduction of a substance containing appropriate antigens into the body to stimulate active immunity against a disease.
• vaccination can be given orally or by injection
• Vaccinations stimulate a faster immune response against a particular pathogen than the response would have been without a vaccine , this is called active immunity.
• passive immunity is where already made antibodies are introduced to the body , usually by breastmilk.
• passive immunity has a shorter time period than active as the antibodies are usually broken down.
• HIV is a retrovirus , a retrovirus contains reverse transcriptase which is able to synthesize single-stranded DNA from an RNA template.
• HIV weakens the immune system by infecting and destroying t-cells. A sufferer may develop AIDS which is when they are susceptible to opportunistic infections such as pneumonia.
• HIV replication in T-Helper cells:

  1. HIV attaches to CD4 surface receptors on a Th lymphocyte and injects reverse transcriptase and RNA into the cell.
  2. Reverse transcriptase uses the viral RNA as a template to make a DNA copy which gets inserted into the host chromosome.
  3. The viral DNA is transcribed to make viral mRNA and translated to make proteins.
  4. Viral proteins and RNA form new HIV particles which burst out of host cell to infect more Th cells.
  5. this destroys t helper cells , eventually leading to dramatic reduction in the immune capability of the host.
• A monoclonal antibody is an antibody that has been isolated and cloned.
• monoclonal antibodies are used in science and medicine for example:

  • delivering medication to specific cell types
  • medical diagnosis , such as pregnancy tests
  • the ELISA tests.