Humoral Immunity - Antibody Response II - B-cell response

Primary Antibody Response
1. 1. Clonal Selection
  1. antigen enters + localized in secondary lymphoid tissue
    1. here: attaches to specific B cell antigen receptors
      1. T-independent (polysaccharide) antigens directly activate B cells to make IgM antibodies
      2. T-dependent protein antigens only activate B cells with help of Th cells
        to activate Th cells , antigen is processed by APCs and presented to Th cells on MHCII
        Hence Clonal selection - antigen selects specific B cells to take part in antibody response
        selected B cells do rapid clonal proliferation = increase no. antigen-specific cells to deal w. pathogen
        some become memory cells
        many become plasma cells
2. Time taken for process
  1. I) Lag period before detectable antibody levels
    1. cellular events take time
  2. ii) Log phase
    1. increasing antibody amounts
      1. as more cells divide = more plasma cells
  3. iii) Plateau phase
    1. bound antigen is removed
      1. lymphocytes no longer stimulated
        antibody decline in serum
        most plasma cells die after completing function
Affinity Maturation
1. Germinal centres
  1. made after B cell activation
  2. in secondary lymphoid tissue
  3. where B cells undergo
    1. class switching
    2. clonal expansion
    3. somatic hypermutation of variable region genes
      1. espec. Complementary Determining Regions (part that makes contact with antigen)
      2. random mutations = occasional cells produced with higher affinity receptor
        rescued through recognition of antigen that are on the immune complex on FDC
        these B cells proliferates
        some become memory cellos
        o/ become plasma cell precursors
        leave germinal centres
        become plasma cells in lymph nodes' medulla or spleen's red pulp
        others bind less well and can't stimulate b cells
        these die by apoptosis in germinal centre
        removed by phagocytic macrophages
    4. selection of high affinity clones leading to affinity maturation of antibody response
Class Switching
1. 1st antibodies made in 1ary response - IgM
  1. as IgM B cells stimulated to divide = plasma cells secreting IgM
2. Cytokines - from Th cells
  1. influence some B cells
    1. to switch to IgG, IgA or IgE during proliferation
      1. some develop into memory cells
      2. e.g. cytokin IL-4: cause switch to make IgE
    2. driving proliferation + plasma cell differentiation
Polyclonal
1. bc microbe has many diff. antigens
  1. each w/ several epitopes
Immunoloical Memory
1. adapative immune response
  1. many memory cells remain after primary response
    1. can respond to same antigen
      1. so secondary immune response = bigger and faster
2. none/little w/out Th cells

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Humoral Immunity - Antibody Response II - B-cell response

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