(4) B lymphocytes and humoral immunity

• first phase of the specific response to infection is the mitotic division of specific T lymphocytes to form a clone of the relevant T cells to build up their numbers
• T helper cells produce cytokines that stimulate B cells to divide
• humoral response involves B cells - antibodies soluble in blood
• there are many different types of B cells
• each B cell is covered in antibodies
• antibodies = proteins that bind to antigens to form an antigen-antibody complex
• antigen-antibody complex = antibody binded to antigen
• when an antigen enters the blood or tissue fluid, there will be one B cell that has a specific antibody on its surface - its shape fits the antigen
• when the antibody attaches to the complementary antigen, the antigen enters the B cell by endocytosis
• after endocytosis, the antigen gets presented on the surface of the B cell
• endocytosis = antibody attaching to antigen and the antigen entering the B cell
• antigens of cells that have a complementary B cell :

  • pathogens
  • foreign cells
  • toxins
  • damaged or abnormal cells
• T helper cells bind to processed antigens and so stimulate B cells to divide by mitosis to form clones of identical B cells
• all the clones of the B cell produce antibodies specific to the foreign antigen
• clonal selection accounts for the body's ability to respond rapidly to a vast number of antigens
• a typical pathogen has many different proteins on its surface, all of which act as antigens
• some pathogens such as the bacterium that causes cholera also produce toxins
• each toxin molecule released by a bacterium pathogen also acts as an antigen
• • many different B cells make clones each of which produce its own type of antibody
  • each clone produces one specific antibody
  • these antibodies are referred to as monoclonal antibodies
• In each clone of B cells, they are either developed into :

  • Plasma cells
  • Memory cells
• Plasma Cells :

  • secrete antibodies usually into blood plasma
  • these antibodies lead to the destruction of the antigen
  • so responsible for the immediate defence of the body against infection
  • production of antibodies and memory cells is known as te primary immune response
• production of plasma and memory cells is known as the primary immune response
• plasma cells are responsible for the immediate defence of the body against infection
• antibodies made by plasma cells produce per second
• primary immune response = slower, person shows symptoms
• Memory cells :

  • responsible for secondary immune response
  • cells do not produce antibodies directly but circulate in the blood and tissue fluid
  • when they encounter the same antigen at a later date, they divide rapidly into plasma cells and more memory cells
  • provide long term immunity against original infection
• in secondary immune response, theres an increased quantity of antibodies is secreted at a faster rate than in the primary immune response
• memory cells ensure that a new infection is destroyed before it can cause any harm - individuals are often unaware that they have been infected
• when encountering the pathogen again, memory cells rapidly develop into plasma cells to produce antibodies and more memory cells
• memory cells provide long term immunity against the original infection
• Primary Immune Response :

  • one B cell has an antibody receptor specific to the shape of the antigen
  • the activated B cell divides by mitosis, the daughter cells become plasma and memory cells
  • plasma cells secrete antibodies that specifically combine with the antigen that has entered the body
• B cell Response :

  1. surface antigens of an invading pathogen are taken up by a B cell
  2. B cell processes the antigens and presents them on its surface
  3. helper T cells attach to processed antigens on B cell-activates B cell
  4. the B cell is activated to divide by mitosis to give a clone of plasma cells
  5. the clones plasma cells produce and secrete the specific antibodies that fit the antigen on the pathogens surface
  6. the antobody attacks the antigens and destorys them
  7. some B cell develop into memory cells - respond to future infections by same pathogen-divide rapidly into plasma cells