The Immune Response

• Non-specific defence mechanisms are immediate responses to pathogens that do not require specific recognition of the pathogen. For example, the skin acts as a physical barrier against all antigens, or phagocytosis can occur with all foreign molecules.
• Specific defence mechanisms are slower responses specific to each pathogen/antigen. These can be cell-mediated responses (T-lymphocytes) or humoral responses (B-lymphocytes).
• The body has evolved to keep microbes out via:

  • physical barriers (non-specific)
  • the immune system (specific)
• Red blood cells are referred to as erythrocytes and white blood cells are referred to as leukocytes.
• White blood cells (AKA leukocytes) include phagocytes and lymphocytes (T- and B-).
• The lymphatic system is a network of vessels that transports lymph - a clear fluid that contains white blood cells -from the tissues to the blood.
• The lymphatic system circulates patrolling lymphocytes which activate upon interaction with APCs (antigen presenting cells).
• Activated T-cells can activate B-cells, causing them to divide by mitosis, and this is the start of the humoral (body fluid) response.
• 𝙏𝙃𝙀 𝙎𝙋𝙀𝘾𝙄𝙁𝙄𝘾 𝙄𝙈𝙈𝙐𝙉𝙀 𝙍𝙀𝙎𝙋𝙊𝙉𝙎𝙀:

  1. CELL-MEDIATED RESPONSE: involves T-lymphocytes, which only respond to APCs. This could be phagocytes, own body cells, cancer cells or transplanted cells.
  2. HUMORAL RESPONSE: involves B-lymphocytes, which target antigens with antibodies. Antibodies are soluble and therefore found in the humours (body fluids).
• "Cyto" = cells

  "Toxic" = toxic/deadly

  "Cytotoxic" = toxic to cells (kills them).
• Explain the role of B-lymphocytes and T-lymphocytes against virus infection.

  1. B lymphocytes recognise antigen
  2. Produce plasma cells which make antibodies
  3. Antibodies agglutinate (clump together) virus
  4. T lymphocytes helpers produce chemicals
  5. Encourages phagocytes to engulf clumped virus
  6. Killer (cytotoxic) T cells kill infected cells
• Describe how memory cells protect the body from disease.

  1. Recognise antigen
  2. Stimulates greater production of antibodies
  3. Faster immune response
• What is an antibody?

  A glycoprotein molecule that stimulates an immune response
• Explain the importance of memory cells.

  1. Carried memory of specific antigen
  2. Greater rapid production of antibodies
• People who do not have the specific receptor protein in their cell-surface membranes may be infected with the Ebola virus but do not develop the disease.

  Explain why they do not develop the disease.

  1. Ebola antigen doesn't bind with receptor
  2. Can't enter the cell and reproduce
• Explain the increase in specific plasma cells and antibodies in people infected with the Ebola virus.

  1. Ebola antigen binds to B-cell
  2. Helper T-cells stimulate B-cells to replicate producing antibodies by plasma cells
• Describe how the B-lymphocytes of a frog would respond to vaccination against Ranavirus. You can assume that the B-lymphocytes of a frog respond in the same way as B-lymphocytes in humans.

  The B-cell binds to the complementary antigen and forms an antigen-antibody complex.

  The B-cells clone and plasma cells produce monoclonal antibodies against the virus.

  Memory cells are also produced so that a secondary immune response will occur faster.
• Lymphocytes are white blood cells that respond to specific antigens.
• Two types of lymphocytes:

  • T lymphocytes (T-cells) perform the cellular response
  • B lymphocytes (B-cells) perform the humoral response
• Each type of lymphocyte (T- and B-) exists in a huge number of varieties. Each cell carries specific receptors on its cell-surface membrane, which can only bind to specific antigens.
• When a macrophage engulfs a pathogen, it breaks the pathogen apart and places some of its antigens in its own cell-surface membrane, where they can be encountered by T lymphocytes.

  The display of antigens by the macrophage is called antigen presentation, and the macrophage is said to be an antigen-presenting cell (APC).
• Cells other than macrophages can also be APCs, as any cell that has been invaded by a virus will have that virus's antigens in its cell-surface membrane.

  This effectively 'labels' the cell as being infected, sending out a signal that it should be destroyed.
• Each type of T lymphocyte in the blood has its own unique set of protein and glycoprotein molecules in its cell-surface membrane, which act as receptors.
• If a T lymphocyte encounters an antigen-presenting cell displaying the antigen that has a complementary shape to the specific receptors on the T-cell's surface, it is stimulated into action.
• 𝙏-𝘾𝙀𝙇𝙇 𝘼𝘾𝙏𝙄𝙊𝙉:

  • the T-cell responds by dividing by mitosis to form a clone of identical T-cells (clonal selection).
  • if the original T-cell was cytotoxic, the daughter cells will bind to any infected cells they find and destroy them.
  • if the original T-cell was a helper, the clone of daughter cells releases cytokines, stimulating macrophages to carry out phagocytosis as well as B-cells and other T-cells.
• Clonal selection occurs when stimulated T-cells divide by mitosis to produce a clone of identical daughter T-cells.
• Only a T-cell carrying a receptor that specifically binds to the detected antigen is stimulated to divide and form a clone.

  All of the daughter cells produced by the division carry the same receptor in their cell-surface membranes, so all of them can detect and respond to the specific antigen that stimulated the response.
• Cytotoxic T-cells attach themselves to any cell displaying copies of the specific antigen the T-cell has the receptor for and then destroy it.

  It may do this by secreting toxic substances into the cell, such as hydrogen peroxide, which perforates (pierces holes in) the cell's surface membrane.
• Cytotoxic T-cells destroy cells that have been infected by the pathogen that carries the specific antigen to which that T-cell can respond.
• Helper T-cells do not attack and destroy infected cells; instead, they release substances called cytokines to stimulate macrophages to carry out phagocytosis of infected cells.

  Cytokines also stimulate B-cells and other T-cells to respond to the invasion.
• Cytokines are small proteins released by helper T cells that are crucial in controlling the activity of other immune system cells. When released, they signal the immune system to do its job.
• Describe the role of macrophages in stimulating B-lymphocytes.

  They present antigens in their membranes to lymphocytes and produce cytokines.
• What changes occur in a B-lymphocyte after stimulation?

  • more mitochondria produce more ATP
  • more RER/ribosomes synthesise more proteins
  • more Golgi body modifies and packages more proteins
  • the B-lymphocyte produces antibodies quickly and in large numbers
• Why is the cell-mediated response necessary?

  Phagocytosis is a non-specific response and is sometimes not enough to cope with large numbers of pathogens (e.g. the flu virus).
• APCs activate helper T-cells, which stimulate and recruit more phagocytes and T-cells to assist in the immune response. This includes cytotoxic T-cells which destroy infected/foreign cells by releasing chemicals (e.g. perforin) into the invaded cell.
• Activated T-cells can also activate B-cells and cause them to divide by mitosis (the humoral response).
• THE CELL-MEDIATED RESPONSE:

  T-lymphocytes respond to an APC.
• THE HUMORAL RESPONSE:

  B-lymphocytes target antigens with antibodies, which are soluble and found in the humours.
• 𝙏-𝙃𝙀𝙇𝙋𝙀𝙍 𝘾𝙀𝙇𝙇𝙎:

  • activate other cells via release of cytokines
  • have surface receptors specific to one antigen
  • small number of each type
  • when the correct type is activated it must divide by mitosis to produce many clones
• 𝘾𝙔𝙏𝙊𝙏𝙊𝙓𝙄𝘾 𝙏-𝘾𝙀𝙇𝙇𝙎:

  • specific
  • kill infected body cells
  • a different cell is needed for each antigen
  • each cell has a specific receptor and will bind to one specifically shaped antigen
  • releases the chemical perforin to perforate (make holes in) the infected cell's membrane to kill it so that it can no longer make viruses