Warm Up 2/12

1. Describe the steps of blood clotting
Clotting is a mechanism that prevents the loss of blood from broken vessels.  Damaged cells and platelets release helical signals called clotting factors which trigger a coagulation cascade.  Clotting factors convert the inactive zymogen prothrombin into the activated enzyme thrombin.  Thrombin catalyses the conversion of the soluble plasma protein fibrinogen into an insoluble form (fibrin).  Fibrin forms an insoluble mesh of fibers that trap blood cells at the site of damage.  Clotting factors also cause platelets to become sticky, which then adhere to the damaged region to form a solid plug called a clot.  The clot prevents further blood loss and blocks entry to foreign pathogens

2. Outline the immune response to an antigen
An antigen is a virus or bacteria.  Surface molecules called epitopes are recognized by leukocytes and trigger the immune response.  The antigens' epitope is detected by a large white blood cell called a macrophage.  The macrophage engulfs the antigen, digests it and then incorporates the epitope into its own cell membrane.  Then the macrophage migrates to the lymph nodes.  The macrophage presents the antigens' epitope and selects a B lymphocyte that has a complementary surface epitope.  As the macrophage may have a number of molecules in its membrane, it is possible for multiple B Lymphocytes to be selected (polyclonal selection).  Corresponding B or T lymphocytes are selected.  A Helpter T cell with the corresponding epitope is also selected.  The Helper T cell works with the B and T lymphocytes to stimulate the B and T lymphocyte to divide rapidly.  B lymphocytes rapidly divide to form a clone of Plasma cells and B memory cells.  The T lymphocytes also rapidly clone and produce other cells called T Killer cells and T Killer memory cells.

3. Discuss Active vs. Passive Immunity
Active immunity is immunity due to the production of antibodies by the organism after the body's defense mechanisms are stimulated by antigens.  Passive immunity is immunity due to the acquisition of antibodies from another organism in which active immunity has been stimulated.  This passive acquisition of antibodies can be achieved via the placenta, colostrum or by injection (ex: blood transfusions).