B5 immunity

• the human body has a range of defence mechanisms to protect itself from pathogens, they can be split into specific and non-specific
• non-specific defence mechanisms are immediate responses, they are the same for all pathogens, examples are physical barriers and phagocytosis
• specific defence mechanisms are slower responses, they are specialised to the pathogen, examples are the cell-mediated response and the humoral response
• to defend the body from pathogens, lymphocytes need to be able to identify body cells and foreign material, otherwise they would destroy the body’s tissues
• all cells have specific molecules on their surface which identify them

  lymphocytes use these specific molecules to distinguish between pathogens, non-self material from other organisms of the same species, toxins and abnormal body cells
• phagocytes are a type of white blood cell which ingest and destroy pathogens in a process called phagocytosis
• the process of phagocytosis:

  • chemoattractance, chemicals released by the pathogen or by dead cells attract the phagocyte to the pathogen
  • phagocyte receptors on the cell-surface membrane recognise and attach to the pathogen
  • the phagocyte engulfs the pathogen, forming a vesicle called a phagosome
  • lysosomes move towards the phagosome and fuse with it
  • lysosomes release hydrolytic enzymes called lysozymes into the phagosome, they hydrolyse the pathogen, destroying it
  • soluble hydrolysis products are absorbed into the cytoplasm of the phagocyte
• an antigen is any part of an organism or substance that is recognised as foreign by the immune system, stimulating an immune response
• usually antigens are proteins on the cell-surface membrane or cell wall of invading cells
• lymphocytes are white blood cells which are involved in immune responses designed to produce long-term immunity
• there are two types of lymphocyte:

  • B lymphocyte = mature in the bone marrow, associated with humoral immunity
  • T lymphocyte = mature in the thymus gland, associated with cell-mediated immunity
• cell-mediated immunity process:

  • pathogens invade body cells or are engulfed by phagocytes
  • body cells/phagocytes present the antigens on their surface
  • receptors on a specific T helper cell are complementary to the antigens
  • the T helper cell receptors bind to the antigen
  • this activates the T helper cell to divide rapidly by mitosis, forming genetically identical clones
  • the cloned T helper cells become T cytotoxic cells, T memory cells, or more T helper cells
• clonal expansion = the process of many genetically identical daughter cells being produced by cell division of B or T lymphocytes once they are activated
• clonal selection = the process of matching the antigens on an antigen-presenting cell with receptors on B or T lymphocytes
• T helper cells bind to antigens and when activated, divide rapidly through clonal expansion to produce T cytotixic cells, T memory cells, or T helper cells, and stimulate B cells to divide
• T cytotoxic cells produce a protein called perforin which makes holes in the cell-surface membrane of body cells infected by pathogens, leading to the death of the cell
• T memory cells provide long-term immunity by remaining in the body and rapidly dividing into plasma cells and memory cells if the body is reexposed, leading to a much faster secondary response
• B cells can be stimulated to divide by T helper cells
• plasma cells produce antibodies, which are complementary to a specific antigen
• B memory cells provide long-term immunity by remaining in the body and rapidly dividing into plasma cells and memory cells if the body is reexposed, leading to a much faster secondary response
• humoral immunity process:

  • B cells have antibodies on their cell-surface membrane
  • B cells take up complementary antigens, process them and present them
  • T helper cells attach to the antigens on the B cells, activating the B cells
  • B cells divide by mitosis into B memory cells and plasma cells
  • plasma cells secrete complementary antibodies
  • memory cells remain in the body until reexposure
• antibodies are proteins produced by plasma cells which are complementary to specific antigens
• an antibody can bind to its complementary antigen, forming an antibody-antigen complex
• antibodies are made up of four polypeptide chains forming a Y-shape structure, two heavy chains and two light chains
• each antibody has a specific binding site which is complementary to the antigen, but this is different for different antibodies so is called the variable region

  the rest of the antibody is called the constant region
• antibodies can lead to the destruction of a pathogen in two ways:

  • by causing agglutination of the pathogen, one antibody binds to two pathogens, grouping them together so they are easier to find
  • by binding to the antigens on a pathogen acting as markers which stimulate phagocytes to engulf the pathogen they are attached to
• monoclonal antibodies are produced when a single type of antibody is cloned, they are useful in medicine for targeting medication to specific cells, medical diagnosis, and pregnancy testing
• monoclonal antibodies are useful for targeting medication to specific cells as if some are produced that are specific to receptors on a cell type, these are given to a patient, they attach to the receptors and act as markers, or for cancer cells block signals that stimulate uncontrolled growth
• monoclonal antibodies are useful for medical diagnosis as some diseases cause elevated levels of specific antigens in the body, so adding monoclonal antibodies to a sample of blood from someone can test the levels of these antigens
• monoclonal antibodies are useful for pregnancy testing as pregnancy tests contain monoclonal antibodies linked to coloured particles, if a hormone called hCG which is present in pregnancy is present, it binds to these antibodies, the complex moves along the strip until it is trapped by a different antibody, creating a coloured line
• ethical issues with monoclonal antibodies:

  • production involves deliberating inducing cancer in animals, typically mice, suffering is minimised but some people disagree with their use at all
  • many diseases have been successfully treated, but patients need to fully know and understand the risks before they can be treated
  • testing presents dangers as side effects can be harmful, even when testing on humans it can go unexpectedly wrong
• immunity is the ability of an organism to resist infection
• the two types of immunity are active and passive
• passive immunity is produced by the introduction of antibodies into organisms from an outside source, no direct contact with the pathogen is required to induce immunity, immunity is acquired immediately, but the antibodies are not replaced when broken down and no memory cells are formed so the immunity is not lasting
• active immunity is produced by stimulating production of antibodies by the organism’s own immune system, direct contact with the pathogen is required to induce immunity, immunity takes time to develop
• there are two types of active immunity, natural and artificial
• natural active immunity is when an organism is infected with a disease under normal circumstances and produces its own antibodies
• artificial active immunity is when an immune response is induced in an organism without them suffering the symptoms of the disease, such as vaccination
• vaccination is the introduction of dead, weak or inactive pathogens into an organism, either by injection or by mouth, in order to stimulate a response against a particular disease
• a vaccine contains dead, weak or inactive pathogens