Cell recognition and the immune system

• What is the definition of a pathogen?

  A pathogen refers to any organism which can cause disease. There are many forms of pathogens: such as viruses, bacteria, protists, fungi.
• What are physical defences against disease?

  Skin, stomach acid, mucus, blood clotting, inflammation.
• How is the skin a physical barrier against pathogens?

  It is a physical barrier made of dead cells stopping pathogens enter the blood. Keratin strengthens the barrier and sebum is secreted, which is an antimicrobial fluid.
• How does stomach acid prevent infection by pathogens?

  Stomach acid is very acidic, so helps destroy any pathogens that have been ingested. Usually pH 1.5-2.
• How is mucus a physical defence against pathogens?

  Goblet cells secret mucus, which traps pathogens then ciliated cells sweep them out. The trapped pathogens can be removed from the body by expulsive reflexes e.g. coughing, sneezing and vomiting.
• How is blood clotting an important physical barrier against pathogens?

  It prevents excess blood loss, a clot is formed by the aggregation of platelets. They form clumps with each other and form a mesh of fibres along with strands of protein called fibrin. AS the wound is sealed it prevents pathogens getting into the wound.
• How is inflammation a physical barrier against pathogens?

  Damaged cells release molecules that cause vasodilation and increased vascular permeability. This helps bring white blood cells to the affected site quickly, so that any pathogen can be fought off. Inflammation: redness, heat, swelling, pain.
• What is the definition of an antigen?

  A foreign protein that stimulates an immune response, the body recognises it as non-self.
• How do bacteria and viruses make you ill?

  Bacteria release toxins which are poisons causing illness. Viruses invade host cells, replicate causing cell lysis.
• What are the 4 types of antigen presenting cell?

  Phagocyte - that is presenting digested pathogens antigens on its surface.

  Cancerous cell - has different antigens to a non-cancerous cell.

  Transplanted cell - will have different antigens

  Pathogen - cell infected by a virus.
• What are the 2 types of immune response?

  Specific - cell-mediated response and humoral response.

  Non-specific - physical barriers and phagocytosis.
• What are the differences between specific and non-specific?

  Specific: slower and specific to each pathogen.

  Non-specific: immediate and same for all pathogens
• What is the process phagocytosis?

  1.Phagocyte attracted to an antigen and moves towards it.

  2. Phagocytes have several receptors on their surface membrane that recognise and attach to antigens on the surface of the pathogen.

  3. They engulf the pathogen to form a vesicle, known as a phagosome.

  4. Lysosomes fuse with the phagosome.

  5. The lysosome releases lysozymes which are hydrolytic enzymes that break down the pathogen.

  6. Antigens from the pathogen are displayed on the cell membrane.

  7. Soluble products from breakdown absorbed into cytoplasm.
• What is a diagram of phagocytosis?

  As shown:

  
• What are the 2 types of lymphocytes and where are they produced?

  They are both produced by stem cells in the bone marrow.

  B-lymphocytes, T-lymphocytes
• What are B-lymphocytes associated with?

  B-lymphocytes: mature in the bone marrow, associated with humoral immunity, which is immunity involving antibodies that are present in bodily fluids.
• What are T-lymphocytes associated with?

  T-lymphocytes: mature in the thymus gland, associated iwth cell-mediated immunity, immunity involving body cells.
• What is cell-mediated immunity?

  T-lymphocytes responds to an organisms own cells that have been infected by non-self material, also transplanted cells.
• How many different types of T-cells are there?

  Many - the receptors on T cells respond to a single antigen, meaning there is a vast number of different T cells each responding to a different antigen.
• What type of cell will T-cells only respond to?


  They will only respond to antigen-presenting body cells, rather then to antigens within the bodily fluids.
• What are the stages of cell-mediated immunity?

  1.Pathogens invade body cells or are taken in by phagocytes.

  2. The phagocyte presents antigens from the pathogen on its cell-surface membrane.

  3. Receptors on specific helper T cells are complementary to the antigens.

  4. This attachment activates the T cell to divide rapidly by mitosis and form cloned T cells.
• What are the 4 things cloned T cells do when produced by mitosis?

  They include:

  • develop into memory cells that enable rapid response to future infections
  • stimulate phagocytes to engulf pathogens by phagocytosis
  • stimulate B cells to divide and secrete their antibody, by releasing cytokines which stimulate maturation.
  • activate cytotoxic T cells
• How do cytotoxic T cells play a role in cell-mediated immunity?

  They kill antigen-presenting body cells, by producing a protein called perforin. Perforin makes holes in the cell membrane, making it freely permeable to all substances. This causes the cell to die.
• Which pathogen are cytotoxic T cells most effective towards?

  Viruses as they replicate inside body cells, the sacrifice of the body cells prevents viruses multiplying and infecting more cells.
• What is the definition of an antibody?

  A protein/immunoglobulin that is specific to an antigen, they are produced by B cells and secreted by plasma cells.
• What is the structure of an antibody?

  Composed of 4 polypeptides, it has a unique variable region that is responsible for antigen detection and specificity.

  Constant region - the same for all antibodies, this region binds to receptors on cells such as B cells.

  Variable region - specific binding site that is specific to an antigen to form an antigen-antibody complex. Due to tertiary structure and sequence of amino acids.

  One pair is long heavy chains and one pair is light short chains.
• What is the diagram of the structure of an antibody?

  The black lines connecting the polypeptides are disulphide bridges.

  There is one connecting heavy and light chains.

  There are two connecting 2 heavy chains.

  
• How do antibodies work?

  They form antigen-antibody complexes and agglutinate(clump together). agglutination means more pathogens are engulfed by phagocytosis. It occurs due to antibodies having 2 antigen-binding sites. They serve as markers that stimulate phagocytes to engulf.
• What is humoral immunity?

  Humoral immunity - the immune response that involves antibodies which are soluble in bodily fluids (humours).
• How do B cells divide by clonal selection?

  1.The B cell has an antibody on its surface that is complementary to the antigen, an antigen-antibody complex formed.

  2. The antigen enters the B cell by endocytosis and gets presented on its surface.

  3. Helper T cells bind to the expose antigens, releasing cytokines.

  4. This stimulates to the B cell to divide by mitosis, the clones then differentiate producing plasma and memory B cells (clonal selection).
• What are plasma cells?

  They secrete antibodies that are specific to an antigen, the plasma cells do not live long. However they secrete around 2000 antibodies every second in their lifespan. The antibodies secreted lead to the destruction of the antigen.
• What are memory cells?

  Memory cells - they live a lot longer then plasma cells, when they encounter the same antigen at a later date they divide and differentiate into plasma cells and more memory cells. Memory cells provide long-term immunity, and an increased quantity of antibodies is secreted at a faster rate than in the primary response.
• What is the graph showing antibody production in both the primary and secondary response to an antigen?

  Primary - antibody secretion by plasma cells and production of memory cells.

  Secondary - memory cells encounter antigen and divide and differentiate into plasma and memory cells, rapidly secreting antibodies.

  
• What is antigenic variability?

  The ability of pathogens to mutate meaning the tertiary structure of the antigens is slightly different. This means upon secondary exposure to the antigen the antibodies produced on primary exposure will no longer be complementary to the antigen. Meaning the person will get ill from the pathogen 2 times. This is why people need to get vaccinated for the flu every year as it mutates frequently.
• What is passive immunity?

  Immunity acquired by the introduction of antibodies from an outside force, there is no direct contact with antigen required, the immunity is acquired immediately. However it does not last as no memory cells are formed, and antibodies are not replaced when broken down. Examples: anti-venom/monoclonal antibodies and antibodies passed from placenta to the fetus.
• What is active immunity and what are the 2 different types?

  Immunity acquired by stimulating the production of antibodies by the individuals' own immune system, there is direct contact with the antigen and immunity is long lasting and takes time to develop. The 2 types are natural and artificial.
• Describe the two different types of active immunity.

  Natural active immunity - results from an individual becoming infected with a disease under normal circumstances. The body produces its own antibodies and may continue to do so for many years.

  Artificial active immunity - forms the basis of vaccination, it involves inducing an immune response in an individual without them suffering the symptoms of the disease.
• What are the features of antigens found in vaccines?

  Vaccines always contain antigens, they can be free or attached to a dead or attenuated pathogen, vaccines may be injected or taken orally, the result of vaccines is the production of memory cells.
• What is the disadvantage of orally taking vaccines?

  May be broken down by enzymes(protease), as antigens are proteins, the molecules may be too large to be absorbed into the blood.
• What are the features of a successful vaccination programme?

  Features:

  • economically available in sufficient quantities to immunise vulnerable
  • few side-effects, as they may discourage individuals from being vaccinated
  • means of producing, storing and transporting - hygienic conditions and refrigerated transport
  • training staff at different centres throughout population
  • must be possible to vaccinate the vast majority of the vulnerable to produce herd immunity