CHAPTER 24 The Immune System Modules 24.3 – 24.9

The lymphatic system is a network of lymphatic vessels and organs 24.3 The lymphatic system becomes a crucial battleground during infection The lymphatic system is a network of lymphatic vessels and organs It returns tissue fluid to the circulatory system It fights infections

LYMPHATIC VESSEL Adenoid Tonsil VALVE Right lymphatic duct, entering vein Tissue cells Lymph nodes Interstitial fluid Blood capillary Thoracic duct, entering vein Thoracic duct Thymus Appendix LYMPHATIC CAPILLARY Spleen Masses of lymphocytes and macrophages Bone marrow Lymphatic vessels Figure 23.3

It will return it as lymph to the blood This lymphatic vessel is taking up fluid from tissue spaces in the skin It will return it as lymph to the blood Lymph contains less oxygen and fewer nutrients than interstitial fluid LYMPHATIC VESSEL VALVE Tissue cells Interstitial fluid Blood capillary LYMPHATIC CAPILLARY Figure 23.3B

Lymph nodes are key sites for fighting infection They are packed with lymphocytes and macrophages Masses of lymphocytes and macrophages Outer capsule of lymph node Macrophages Lymphocytes Figure 23.3C, D

24.4 The immune response counters specific invaders SPECIFIC IMMUNITY 24.4 The immune response counters specific invaders Our immune systems responds to foreign molecules called antigens Infection or vaccination triggers active immunity The immune system reacts to antigens and “remembers” an invader We can temporarily acquire passive immunity

24.5 Lymphocytes mount a dual defense Two kinds of lymphocytes carry out the immune response B cells secrete antibodies that attack antigens T cells attack cells infected with pathogens BONE MARROW Stem cell THYMUS Via blood Immature lymphocytes Antigen receptors T cell B cell CELL- MEDIATED IMMUNITY HUMORAL IMMUNITY Via blood Lymph nodes, spleen, and other lymphatic organs Final maturation of B and T cells in lymphatic organ OTHER PARTS OF THE LYMPHATIC SYSTEM Figure 24.5

24.6 Antigens have specific regions where antibodies bind to them Antigenic determinants are the molecules to which antibodies bind Antibody A molecules Antigen- binding sites Antigenic determinants Antigen Antibody B molecule Figure 24.6

24.7 Clonal selection musters defensive forces against specific antigens When an antigen enters the body, it activates only lymphocytes with complementary receptors B and T cells multiply into clones of specialized effector cells that defend against the triggering antigen This is called clonal selection

Antigen molecules Variety of B cells in a lymph node Antigen receptor (antibody on cell surface) Cell growth division, and differentiation Clone of many effector cells secreting antibodies Endoplasmic reticulum Antibody molecules Figure 24.7

24.8 The initial immune response results in a type of “memory” In the primary immune response, clonal selection produces memory cells These cells may confer lifelong immunity Figure 24.8A

When memory cells are activated by subsequent exposure to an antigen, they mount a more rapid and massive secondary immune response Unstimulated lymphocyte First exposure to antigen FIRST CLONE Memory cells Second exposure to antigen Effector cells SECOND CLONE More memory cells New effector cells Figure 24.8B

24.9 Overview: B cells are the main warriors of humoral immunity Triggered by a specific antigen, a B cell differentiates into an effector cell The effector cell is called a plasma cell The plasma cell secretes antibodies

PRIMARY RESPONSE (initial encounter with antigen) Antigen Antigen receptor on a B cell Antigen binding to a B cell Cell growth, division, and differentiation Clone of cells Memory B cell Plasma cell Antibody molecules Later exposure to same antigen SECONDARY RESPONSE (can be years later) Cell growth, division, and further differentiation Larger clone of cells Plasma cell Memory B cell Antibody molecules Figure 24.9