Common Characteristics of B and T Lymphocytes Graphics are used with permission of Pearson Education Inc., publishing as Benjamin Cummings (http://www.aw-bc.com). Page 1: Introduction While B and T lymphocytes have different functions in the immune system, they share some common properties, which are the focus of this Topic. Page 2: Goals To outline the shared features of B and T lymphocytes. To describe the structure, function, and sources of diversity of B and T cell antigen receptors. To understand how immune reactions to self-antigens can be avoided. To explain why lymphocyte recirculation is necessary for effective host defense. To distinguish between effector cells and memory cells. Page 3: Overview B and T cells demonstrate unique as well as shared features. Shared features of B and T cells explained in this Topic include: o Specificity of receptors o Diversity of receptors o Regulation of activation o Memory Page 4: Antigens: Self-Antigens The body has a vast number of its own antigens called self-antigens. These normally do not trigger immune responses. Page 5: Antigen Receptors: Specificity The antigen specificity of B and T cells is based on the shape of their antigen receptors. Each B and T cell expresses unique antigen receptor specificity unlike the receptor specificity of any other B or T cell. Each B or T lymphocyte expresses 10 4-10 5 identical antigen receptors on its plasma membrane. Each B or T lymphocyte binds optimally to only one antigenic determinant. Page 6: Antigen Receptors: Shared Properties The B cell antigen receptor is a molecule of membrane-bound antibody. The T cell antigen receptor, while not an antibody, shares some structural and functional properties with antibodies. The T cell antigen receptor binds to protein antigens that have been digested into short peptides and are displayed by proteins called major histocompatiblity complex (MHC) proteins on the membranes of antigen-presenting cells. Page 7: Antigen Receptors: Clonal Selection 1
The immune system generates a vast array of unique lymphocyte antigen receptors that exist before their foreign antigens ever enter the body. When an antigen encounters its receptor, it causes a clone of lymphocytes to form. This mechanism is called clonal selection. Page 8: Antigen Receptors: Diversity Diversity in the immune system is due to the enormous number of B and T cells with unique antigen receptor specificities. It is estimated that an individual s immune system makes about 100 million different types of lymphocyte antigen receptor. Page 9: Antigen Receptors: Generation of Diversity Lymphocyte antigen receptors are generated by randomly combining and editing blocks of genetic information to create a single effective gene. Page 10: Education: Immunocompetence, Self-Tolerance A lymphocyte becomes immunocompetent when it has a viable lymphocyte antigen receptor. Most mature B and T cells exhibit tolerance to self-antigens, a characteristic established as the cells developed in the primary lymphoid organs. Page 11: Education: Positive and Negative Selection Although this Topic focuses on T cells, developing B cells also go through a selection process to eliminate self-reactive cells. T cells are educated in the thymus to be tolerant of self-proteins. The T cell receptors of developing T cells interact with cells of the thymus that display selfantigens on MHC proteins. Developing T cells expressing functional antigen receptors are allowed to mature (positive selection), but those expressing nonfunctional antigen receptors, or receptors that interact with self-antigens, are killed (negative selection). Page 12: Education: Autoimmune Diseases Some self-reactive B or T cells survive negative selection and may cause autoimmune diseases later in life. Page 13: Lymphocyte Circulation B and T cells continually circulate throughout the body, searching for their specific antigens. This process is called lymphocyte recirculation. Page 14: Activation and Clonal Expansion Following appropriate interaction with antigen, B and T cells undergo activation, which is called clonal selection, and proliferation, which is called clonal expansion. Two functionally different cell types arise through clonal expansion: effector cells and memory cells. 2
Effector cells carry out the function of the antigen-activated cell (antibody secretion, cytokine secretion, cytotoxicity); memory cells circulate throughout the body to respond to re-exposure to the same antigen. Page 15: Primary Immune Responses Primary immune responses begin with activation of a naive lymphocyte. The time required for clonal expansion is responsible for much of the delay before effector cells can start their work. Page 16: Secondary Immune Responses Secondary immune responses are generated when individuals are exposed to a previously encountered antigen. Appropriate antigen activation of memory cells leads to further clonal expansion with the production of effector cells and memory cells. Secondary immune responses are generated more rapidly, of longer duration, and greater magnitude than primary immune responses. Page 17: Summary Each lymphocyte has multiple identical copies of an antigen receptor, but receptors on different lymphocytes are unique. Antigen receptor diversity is produced by random recombination of gene segments. During development in primary lymphoid organs, lymphocytes become immunocompetent and self-tolerant. Lymphocytes continually circulate throughout the body in search of their one specific antigen. Binding of antigen to a naive lymphocyte s antigen receptor causes clonal selection and clonal expansion, resulting in the production of effector cells and memory cells. Secondary immune responses are faster and of greater magnitude than primary immune responses. Study Questions 1. (Goal 1, Page 3) List four features that are common to B and T lymphocytes. 2. (Goal 2, Page 5) What determines a lymphocyte s ability to recognize a specific antigen? 3. (Goal 2, Page 5) How many antigens does each individual B cell or T cell recognize? 4. (Goal 2, Page 5) How many identical antigen receptors are present on the B or T cell plasma membrane? 5. (Goal 2, Page 5) How many antigenic determinants does one B or T lymphocyte receptor bind to? 6. (Goal 2, Page 6) Compare and contrast a B cell antigenic determinant to a T cell antigenic determinant. Label the B cell and T cell in the diagram. 3
7. (Goal 2, Page 6) Describe the presentation of an antigen to a T cell. 8. (Goal 2, Page 7) When does the immune system generate antigen receptors? 9. (Goal 2, Page 7) Explain what happens when an antigen receptor binds to its antigen. What is the process called? 10. (Goal 2, Page 8) What factor allows the immune system to respond to so many diverse antigens? 11. (Goal 2, Page 8) How many different types of antigenic receptors are estimated to make up an individual s immune system? 12. (Goal 2, Page 9) Describe how lymphocyte antigen receptor specificity occurs. Is it a random or systematic process? 13. (Goal 3, Page 10) Where do lymphocytes develop antigen receptors and learn tolerance to self-antigens? 14. (Goal 3, Page 11) What is the goal of B cell and T cell selection process? Where does the educational process occur for B cells and T cells? 15. (Goal 3, Page 11) What role do the thymus cells play in the educational process? 16. (Goal 3, Page 11) Explain positive and negative selection. 17. (Goal 3, Page 12) Relate what may happen if some self-reactive B or T cells survive negative selection. 4
18. (Goal 4, Page 13) Why do B and T cells circulate, and what is the process called? 19. (Goal 5, Page 14) What happens to a B or T cell that interacts with its antigen? 20. (Goal 5, Page 14) Define clonal selection and clonal expansion. 21. (Goal 5, Page 14) Name the two functionally different cell types that appear after activation has occurred and describe their respective roles. 22. (Goal 5, Page 15) What triggers a primary immune response? Name the two cell types that are produced as a result of the response. 23. (Goal 5, Page 16) What triggers a secondary immune response? 24. (Goal 5, Page 16) Compare and contrast a primary immune response with a secondary immune response. Summary Each lymphocyte has multiple copies of an antigen receptor, but receptors on different lymphocytes are. Antigen receptor diversity is produced by random recombination of segments. During development in organs, lymphocytes become and self-tolerant. Lymphocytes continually circulate throughout the body in search of their one specific. Binding of antigen to a naive lymphocyte s antigen receptor causes selection and expansion, resulting in production of cells and cells. Secondary immune responses are and of greater magnitude than immune responses. 5