Lymphokine-Secreting T Cells

Chapter 7 The third line of defense in the human body is adaptive immunity, which consists of lymphocytes and antibodies. Unlike innate immunity and inflammation, adaptive immunity is inducible and develops more slowly. It is highly specific to the infecting microbe and provides long-term protection through memory. The innate and adaptive immune systems are interactive and complementary, working together for complete protection against infectious disease. Bullet points Lymphokine-Secreting T Cells  Th1 cells secrete cytokines like interferon-y (IFN-y) that activate M1 macrophages for increased phagocytic and microbial killing functions  Th2 cells secrete cytokines like IL-4, IL-13 that activate M2 macrophages for healing and repair of damaged tissue  Th17 cells secrete a different set of cytokines with their own functions Functions of T cytotoxic cells  T cytotoxic cells play a role in killing infected cells and tumor cells Differences between NK cells and T cytotoxic cells  NK cells are part of the innate immune system, while T cytotoxic cells are part of the adaptive immune system  NK cells can directly kill infected or cancerous cells without prior stimulation, while T cytotoxic cells require activation Role of Treg cells  Treg cells help regulate the immune response and maintain tolerance against self-antigens  They suppress the maturation of autoreactive T cells in the thymus and can be derived from Th cells in response to excessive inflammation and autoimmunity  Treg cells produce immunosuppressive cytokines TGF-B and IL-10, which decrease activity of Th1 and Th2 cells Age-related mechanisms of self-defense  Newborns have deficient antibody production, phagocytic activity, and complement activity compared to adults  Their immune system is reliant on maternal antibodies for protection, with IgG levels increasing after birth  Elderly individuals experience diminished T-cell function and reduced antibody responses to challenges  The thymus, which plays a role in T-cell differentiation, undergoes involution with age Overview of Adaptive Immunity  Adaptive immunity is a state of protection against infectious agents  It is slower to develop, more specific, and has memory compared to inflammation  Cells of the innate immune system initiate the adaptive immune response by presenting antigens to lymphocytes  There are two types of lymphocytes involved - B lymphocytes and T lymphocytes Types of immunity  Humoral immunity is mediated by circulating antibodies and is responsible for fighting extracellular pathogens  Cell-mediated immunity is mediated by T cells and is responsible for killing targets directly or stimulating the activity of other leukocytes Active and passive immunity  Active immunity is when the immune response components originate in the host  Passive immunity is when the immune response components come from a donor Antigens and immunogens  Antigens are molecules that react with components of the immune response such as antibodies and receptors on B and T cells  Immunogens are antigens that induce an immune response  Not all antigens are immunogens  Pathogens can mimic "self" antigens to avoid initiating an immune response Factors affecting antigen immunogenicity  Large molecules like proteins, polysaccharides, and nucleic acids are most immunogenic  Haptens are antigens too small to be immunogens on their own, but become immunogenic when combined with larger molecules BCR and TCR generation  Clonal diversity is the production of B and T lymphocytes with receptors against millions of antigens  BCR recognition of antigens and communication with the cell's nucleus is their main role  BCR specificity is made possible by rearrangement of DNA during B-cell development, called somatic recombination  The development of T-cell proliferation and differentiation is similar to B cells, occurring in the thymus Central tolerance  Clonal deletion or central tolerance eliminates "autoreactive" B cells that recognize the individual's own antigens  Most developing B and T cells undergo apoptosis during this process Development of T cells  T-helper cells (Th cells) and CD 8 cells become T-cytotoxic cells (Tc cells)  Other mature T cells include T-regulatory cells (Treg cells) and memory cells Clonal diversity and migration of T and B cells  Clonal diversity concludes when immunocompetent T and B cells migrate from primary lymphoid organs to circulation and secondary lymphoid organs Induction of immune response  Immune response begins when antigen enters the body  Most antigens interact with antigen-presenting cells (APCs) Antigen presentation and MHC molecules  APCs present antigens to Th cells for optimal immune response  MHC class II molecules present antigen to Th cells  Tc cells require antigen presentation by MHC class I molecules Interaction between T cells and antigens  T cells recognize antigen through TCR and accessory molecules CD4 or CD8  CD4 is found on Th cells and reacts with MHC class II; CD8 is found on Tc cells and reacts with MHC class I Functions of Th cells and Tc cells  Th cells help Tc cells respond to antigen; Th2 cells help B cells develop into plasma cells; Th17 cells help activate macrophages  Tc cells bind to and kill cellular targets, such as virus-infected or cancer cells Humoral immunity (Antibodies)  The humoral immune response involves antibodies produced by B cells  Antibodies can be classified as IgG, IgM, IgA, IgE, or IgD  An antibody molecule has two Fab portions for antigen binding and an Fc portion for complement activation or interaction with cell receptors  Antibodies can directly or indirectly protect the body