Schema What is an antigen? How do B cells and T cells (generally) work to fight antigens? (use page 936)

Formation of Lymphocytes

Surfaces of T cells and B cells Small portions of antigens called epitopes can bind to the antigen binding sites All antigen receptors on a lymphocyte are the same (identify the same epitope)

Antibodies Some B cells give rise to plasma cells that produce antibodies which also bind to antigens

B vs. T cells B cell receptors bind to any antigens – free or on surface of pathogen T cell receptors bind to antigens presented on surface of host cells

Host cells – MHC molecules Host cell engulfs pathogen – breaks up antigens and bind to MHC molecules in cell and push antigen to surface of cell Antigen presentation

T cells and B cells Once host cells present antigens, T cells can recognize antigens Cytotoxic T cells – use toxic gene products to kill infected cells B cells are antigen-presenting cells because they display antigens for recognition by Cytotoxic T cells or Helper T cells

How can acquired immune response be so effective? Body contains an enormous variety of antigen receptors and only a small fraction are specific to epitopes The binding of an antigen receptor to its specific antigen initiates events that activate the lymphocyte Activated B cells or T cells amplify response by dividing several times – Clonal selection Figure 43.14 Effector cells Memory cells

The presentation of an antigen to specific receptors on a lymphocyte leads to repeated rounds of cell divisions Thousands of cells are produced, all specific to that antigen

Primary vs. Secondary immune response What is the difference between the primary and secondary immune responses? Why does the secondary response happen so much more quickly in an individual?

Secondary Response Relies on Memory Cells

Schema activator 1) What is the difference between the humoral immune response and the cell-mediated immune response? 2) Observe Figure 43.16. What differences do you notice between the two responses? What is necessary for both to occur?

Acquired immunity Humoral immune response Activation and clonal selection of effector B cells (secrete antibodies) Cell-mediated immune response Activation and clonal selection of cytotoxic T cells (identify and destroy target cells) What is the role of the helper T cells?

Helper T Cells Helper T cells are activated when they come in contact with antigen presenting cells They reproduce and cells differentiate into activated helper T cells and memory helper T cells Activated helper T cells secrete cytokines that stimulate the activation of nearby B cells and cytotoxic T cells What are cytokines? Proteins that recruit and activate lymphocytes

T cells vs. B cells Observe Figures 43.18 and 43.19 How do cytotoxic T cells kill target cells? Do B cells kill cells? How do they respond to pathogens?

Cytotoxic T cells

B cell activation

Antibodies Activated B cells can produce thousands of plasma cells, which each secrete ~2,000 antibodies every second for 4-5 day life span Most antigens contain multiple epitopes, so multiple B cells are activated (producing different plasma cells  different antibodies) Classes of antibodies – Figure 43.20 How role of antibodies in immunity – Figure 43.21 Homework – How are monoclonal antibodies used in medicine?