Dr Nirmala Limbu, MD Assistant Professor Department of Physiology BPKIHS, Dharan Learning Objectives Introduction to humoral immunity Development of immune system Activation, proliferation & differentiation of B cells Actions of antibodies Primary vs. secondary response Genetic basis of diversity in immune system Acquired Immunity Specificity Memory Self/Non-self recognition Introduction to Acquired Immunity- Humoral (Antibody-mediated) Mediated by circulated immunoglobin antibodies in γ-globulin fraction of plasma proteins Introduction to Acquired Immunity- Humoral (Antibody-mediated) continue……. Plasma cells (differentiated forms of B lymphocytes) – secrete Ig- activate complement system, attack & neutralize antigens Major defense against bacteria, bacterial toxins & viruses that circulate in body fluids Development of Immune system Development of Immune system continue…… Before T cells leave thymus & B cells leave red bone marrow- they make distinctive proteins- insert into plasma membrane as antigen receptors, in T cells- CD4 & CD8 Development of Immune system continue…… Development of Immune system continue…… B Lymphocytes differ from T lymphocytes in 2 ways: - B lymphocyte secretes antibody develop reactivity against antigen instead of whole cell as in T cells - B lymphocyte- greater diversity than T lymphocytes Humoral Immunity (Antibody-mediated) Activation, Proliferation & differentiation of B cells Antigen can bind directly to appropriate receptors on B cells. However, full antibody response requires that B cell contact with helper T cell Depending on antigen, antibody synthesis is induced by thymus independent or thymus dependent mechanisms Activation, Proliferation & differentiation of B cells continue….. Thymus independent antigen – do not require cooperation of T cells for activating B cells • Antigens are mainly polysaccharides or lipopolysaccharides with repeating subunits (bacterial capsules). • Weaker immune response than for T- dependent antigens. • produce Ig M type of Antibody Thymus dependent antigen- requires cooperation of T cells for B cell activation Activation, Proliferation & differentiation of B cells continue….. • Antigen is presented to specific B cells by Antigen presenting cells (macrophage) Major Histocompatibility Complex (MHC) Transmembrane glycoproteins located in plasma membrane of cells- “Self-Antigen” MHC antigens are unique unless identical twins 2 classes- MHC I- present in all nucleated cells except RBC; MHC II- surface of APC, B cells Activation, Proliferation & differentiation of B cells continue….. MHC II appear on cell membrane only when cell is processing antigens. Activation, Proliferation & differentiation of B cells continue….. B cells of particular clone- capable of secreting only one kind of antibody- identical in specificity in antigen receptor displayed by B cells that 1st responded to antigen Antibody forms antigen-antibody complex n initiate their actions. Immunoglobins Antibodies circulate in globin fraction of plasma - Immunoglobins Immunoglobins continue…… Classes of antibodies- five general classes- IgM, IgG, IgA, IgD & IgE Basic component of each- symmetric unit- 4 polypeptide chains (2 heavy & 2 light) Immunoglobins continue…… Variable portion Constant portion Action of Antibodies Agglutination of antigen-bearing particles Neutralization-blocks viral binding sites, coats bacteria and/or opsonization Action of Antibodies continues….. Precipitation of soluble antigens Opsonization- coat antigens All these enhances phagocytosis Action of Antibodies continues….. Action of Antibodies continues….. Activating complement system- amplify the action of antibodies Primary vs. Secondary immune response Primary vs. Secondary immune response continue….. Primary vs. Secondary immune response continue….. Genetic basis of diversity of immune system Epitopes: Antigen Regions that Interact with Antibodies Genetic basis of diversity of immune system continue….. Due to Genetic recombination During preprocessing (T- & B- lymphocytes)- gene segments mix with one another in random combination & form one specific whole gene i.e. code for only single antigen specificity Genetic basis of diversity of immune system continue….. ? Diverse antibody- during B cell development, coding region of each V, D & J regions are selected at random & recombine to form gene- produce specific variable portion THE END…!!!