Phagocytes and b cells

• What is an antigen 


  Foreign protein that stimulates an immune response and production of antibodies
• Describe how bacteria are destroyed by phagocytes


  Phagocyte recognises the bacteria as foreign. phagocyte engulfs the bacteria to form vesicle called a phagasome. Lysosome fuses with phagosome to form a phagolysome, lysozyme digest and hydrolyse bacteria. The antigens are displayed on cell surface membrane of phagocyte
• Describe how phagocytosis of a virus leads to presentation of its antigens 


  Phagosome fuses with lysosome; The Virus is destroyed by lysozymes; antigens from the virus are displayed on the cell membrane;
• Phagocytes and lysosomes are involved in destroying microorganism. Describe how

  phagocytes engulf pathogens; Enclosed in a vesicle called a phagosome; Lysosomes have enzymes; That digest the microorganism by hydrolysing molecules such as proteins or lipids;
• Describe how presentation of a virus antigen leads to secretion of an antibody against this virus antigen

  Helper T cell binds to the antigen on the antigen-presenting cell;

  This helper T cell stimulates a specific B cell; B cell clones by mitosis; These form plasma cells that release antibodies;
• what is an antibody 


  A quaternary protein specific to an antigen produced and secreted by B plasma cells
• Explain why the antibody will only detect a specific antigen

  The variable region of the antibody has specific amino acid sequence; The tertiary structure of the binding site is complementary to, and binds with these antigens; This forms an antibody-antigen complex.
• describe the cellular response 


  T-helper cell binds to specific antigen on the surface of an antigen presenting cell. T-helper cell divides by mitosis to form clones. The helper T-cells activate B-cells, activate cytotoxic T-cells and stimulate phagocytes. Some differentiate into T memory cells
• describe the humoral response 


  B-cell binds to specific antigen AND is activated by T-helper cells. Activated B-cells divide by mitosis to form clones; they produce plasma cells; and the plasma cells make specific antibodies; B-cells also produce memory cells;
• How do antibody lead to the destrunction of antigens

  As antibodies have 2 binding sites, they can cause the agglutination of bacterial cells so that it is easier for phagocytes to engulf them. The binding of an antibody to a pathogen can act as a chemical marker and attracts phagocytes. The binding of an antibody to a pathogen or toxin can neutralise them because they are unable to find to cells when they have antibody attached.