5 - Cell recognition and the immune system

• Infection


  An interaction between the pathogen and the body's various defence mechanisms
• Pathogen overwhelms the defences


  Individual dies
• Body's defence mechanisms overwhelm the pathogen


  Individual recovers from the disease
• Immunity


  The body's defences being better prepared for a second infection from the same pathogen and can kill it before it causes any harm
• Defence mechanisms


  • Cell-mediated responses involving T lymphocytes
  • Humoral responses involving B lymphocytes
• Cell recognition


  Each type of cell, self or non-self, has specific molecules on its surface that identify it
• Molecules that allow the immune system to identify

  • Pathogens
  • Non-self-material
  • Toxins
  • Abnormal body cells
• Tissue or organ transplants are recognised as non-self by the immune system and attempts to destroy them
• Clonal selection


  The process where the one type of lymphocyte already present that has the complementary proteins to those of the pathogen is stimulated to divide to build up its numbers to a level where it can be effective in destroying it
• There are probably around 10 million different lymphocytes present at any time, each capable of recognising a different chemical shape
• In the foetus, lymphocytes are constantly colliding with other cells, but infection is rare because it is protected from the outside world by the mother and the placenta
• Lymphocytes that show an immune response to self-antigens undergo programmed cell death before they can differentiate into mature lymphocytes, leaving only those that might respond to non-self-antigens
• The response is immediate in non-specific but slower in specific. The non-specific is the same for all pathogens but the specific is specific to each pathogen
• Phagocytosis


  The process where large particles, such as some types of bacteria, are engulfed by cells in the vesicles formed from the cell-surface membrane
• Phagocytes


  • Types of white blood cells that carry out phagocytosis
  • Provide an important defence against pathogens that manage to enter the body
  • Some phagocytes travel in the blood but can move out of the blood vessels into other tissues
• Phagocytosis


  1. Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes to move towards the pathogen

  2. Phagocytes have several receptors on their cell-surface membrane that recognise, and attach to, chemicals on the surface of the pathogen

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

  4. Lysosomes move towards the vesicle and fuse with it

  5. Enzymes called lysozymes within the lysosome destroy ingested bacteria by hydrolysis of their cell walls

  6. The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte
• If a pathogen is to infect the body, it must first gain entry
• The body's first line of defence is to form a physical or chemical barrier to entry
• Should the physical or chemical barrier fail, the next line of defence is the white blood cells
• Types of white blood cells

  • Phagocytes
  • Lymphocytes
• Phagocytes ingest and destroy the pathogen by a process called phagocytosis before it can cause harm
• Lymphocytes are involved in immune responses
• Despite various barriers, pathogens still frequently gain entry and the next line of defence is then phagocytosis
• Antigen


  Any part of an organism or substance that is recognised as foreign in the immune system and stimulates an immune response
• Antigens


  • Usually proteins that are part of the cell surface membranes or cell walls of invading cells, such as microorganisms, or abnormal body cells, such as cancer cells
  • The presence of an antigen triggers the production of an antibody as part of the body's defence system
• Lymphocytes


  A type of white blood cell that is produced by stem cells in the bone marrow and is involved in the specific immune response
• Types of lymphocytes


  • B lymphocytes (B cells)
  • T lymphocytes (T cells)
• B lymphocytes (B cells)


  Mature in the bone marrow and are associated with humoral immunity, that is immunity involving antibodies that are present in body fluids
• T lymphocytes (T cells)


  Mature in the thymus gland and are associated with cell-mediated immunity, that is immunity involving body cells
• Humoral immunity


  Involves antibodies, and antibodies are soluble in the blood and tissue fluid of the body
• Cell-mediated immunity


  1. Lymphocytes respond to an organism's old cells that have been infected by non-self-material from a different species

  2. Lymphocytes respond to cells from other individuals of the same species because these are genetically different and have different antigens on their cell surface membrane

  3. Phagocytes that have engulfed and hydrolysed a pathogen present some of the pathogen's antigens on their own surface membrane

  4. Body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane

  5. Transplanted cells from individuals of the same species have different antigens on their cell-surface membrane

  6. Cancer cells are different from normal body cells and present antigens on their cell-surface membranes
• An old-fashioned word for body fluids is 'humour'
• B cells


  • There are many different types, possibly as many as 10 million
  • Each B cell starts to produce a specific antibody that responds to one specific antigen
• Antigen-presenting cells


  Cells that display foreign antigens on their surface and can present antigens of other cells on their own surface membrane
• Antigen entry and B cell response

  1. Antigen enters blood or tissue fluid

  2. One B cell has an antibody on its surface whose shape exactly fits the antigen

  3. Antibody attaches to complementary antigen

  4. Antigen enters B cell by endocytosis and gets presented on its surface

  5. Specific helper T cell binds to processed antigens and stimulates B cell to divide by mitosis to form a clone of identical B cells

  6. Clone produces antibody specific to foreign antigen
• Response of T lymphocytes to infection by a pathogen

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

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

  3. Receptors on a specific helper T cell fit exactly onto these antigens

  4. This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells

  5. The cloned T cells develop into memory cells that enable a rapid response to future infections by the same pathogen, stimulate phagocytes to engulf pathogens by phagocytosis, stimulate B cells to divide and secrete their antibody, and activate cytotoxic T cells
• Clonal selection


  Accounts for the body's ability to respond rapidly to any of a vast number of antigens
• A typical pathogen has many different proteins on its surface, all of which act as antigens
• How cytotoxic T cells kill infected cells

  1. Cytotoxic T cells produce a protein called perforin that makes holes in the cell-surface membrane

  2. The holes mean the cell membrane becomes freely permeable to all substances and the cell dies as a result
• Some pathogens also produce toxins, and each toxin molecule also acts as an antigen