Immunology Chapter 16, Lecture 2 • • • • • • Richard L. Myers, Ph.D. Department of Biology Southwest Missouri State Temple Hall 227 Telephone: 417-836-5307 Email: rlm967f@mail.smsu.edu The Humoral Response • Used for eliminating extracellular pathogens – produces many different antibody molecules – each specific for a certain epitope – may produce 1011 different antibodies • In addition, the constant portion of the antibody may account for biological effector functions • Humoral process requires participation of other cells – macrophages – B cells – also important is the interaction between TH and antigen-class II MHC complex • B cells are the principle cell in humoral immunity – they interact with antigen via a BCR – proceeds with receptor-mediated endocytosis • unlike macrophage which phagocytizes anything – then antigen presented with a class II MHC on the membrane Humoral effector functions • • • • • • Activate complement system Enhance phagocytosis via opsonins Neutralize bacterial toxins Neutralize viruses Prevent colonization at mucosal surfaces Involved in ADCC Basic facts • Immunocompetent B cells possess IgM and IgD membrane bound antibodies • Clonal proliferation and differentiation occur after activation • B cells have average cell cycle of 15 hr • Unless activated by antigen, they will die in a few days (usually 90% will die) • Marrow produces about 107 B cells/day General response to antigen • The response is characterized by the 1) production of antibody-secreting cells and 2) memory B cells – during the lag phase cells undergo clonal selection – then the logarithmic phase occurs • increase in antibody; it eventually declines – for example, with SRBCs, lag phase lasts 4 days; peak plasma cell levels within 5 days; peak antibody within 7 days – IgM secreted initially, followed by IgG • Referred to as the primary response • Primary response with formation of antibodies differs depending upon – – – – nature of the antigen route of antigen administration presence of adjuvants species or strain • Secondary response different from primary – response is more rapid – produces more antibody – lasts for a longer time • maybe 1,000 times more antibody produced • Secondary response occurs with second exposure to the antigen • Depends upon the existence of memory B cells and memory T cells Plasma cell Hemolytic plaque assay • Assay to measure plasma cell numbers in mice primed with SRBCs – many modifications • Assay can be used to quantitate plasma cells secreting antibodies specific for any antigen • First, immunize mice with SRBCs • Prepare a spleen cell suspension from a primed mouse • Mix in warm, melted agar to which SRBCs have been added • Prepare a petri dish with a layer of hard agar • Overlay with mixture above • Allow to cool and solidify • Incubate for 1 hr at 37oC • During incubation, antibodies diffuse into agar and binds to the SRBC • Guinea pig serum containing complement is added • Complement reacts with the bound antibody – mediates lysis • Lysis is indicated by a plasma cell surrounded by a clear plaque devoid of cells • Plaques can be counted – referred to as direct plaqueforming cells (PFC) Elispot assay • • • • Plasma cells quantitated without SRBCs Use antigen-primed splenocytes Plate in agar containing antigen Plasma cells secrete antibody which binds to the antigen • Remove cells • Visualize bound antibody with ELISA Associative (linked) recognition • This is a process where TH and B cells must see peptides on the same molecule for B cell activation to occur • In the following example the epitope is a viral coat (spike) protein • T cells recognize internal protein which allows B cells to make antibody to coat protein • The activated TH cell recognizes the processed peptide together with the class II MHC molecule • Antibodies can then be produced to the peptide • Binding of antibody to virus occurs • There is also localized release of cytokines • Cytokines allow B cell to proliferate/differentiate • There are other membrane receptors involved – LFA-1 and CD4 are involved in cellular adhesion • Once in contact a signal generates the expression of CD40L on the T cell • This interacts with CD40 on the B cell membrane • This causes induction of cytokine receptors • Results in fully activated B cells – these can proliferate Assignment • Read Chapter 17, Hypersensitivity Reactions • Review question 3 (pg 439)