Adaptive Immunity

• Adaptive Immunity


  Immune response that is activated by exposure to an antigen
• Genetic Recombination


  Process of rearranging gene segments to generate diverse T-cell and B-cell receptors
• Cells of Adaptive Immune Response

  • T-cells
  • B-cells
• 1. cells and B-cells


  • Originate from stem cells in the bone marrow
  • T-cells mature in the thymus
  • B-cells complete maturation in the bone marrow
• Primary/Central Lymphoid Organs


  Thymus and bone marrow
• Antigen Receptors


  • Immunoglobulins on B-cell surface
  • T-cell receptors (TCRs) on T-cell surface
• Immunoglobulin Receptors


  • Only recognise and bind to one specific antigen
• 1. cell Receptors (TCRs)


  • Only recognise and bind to one specific antigen
• Naïve T-cells and B-cells

  Lymphocytes leaving primary lymphoid organs that have never encountered their specific antigen
• Clonal Selection


  1. Single antigen stimulates B and T-cells to multiply and produce clones

  2. Clones are populations of lymphocytes derived from same progenitor cell, all recognising same antigen
• CD4+ Helper T-cells


  Influence B-cell development via cytokines that help B-cells make antibody
• 1. cell Development


  Develop into either memory cells or antibody-producing plasma cells
• CD8+ T-cells


  • Cytotoxic, kill other cells
  • Twice as many CD4 T-cells as CD8 T-cells, together account for 85% of lymphocytes
• Plasma Cells


  Main antibody producing cells of the body
• Memory B-cells


  Long-lived, circulate as quiescent cells, can rapidly produce antibodies when re-exposed to same antigen
• Clonal Selection Hypothesis


  • Lymphocytes activated by antigen give rise to clones of antigen-specific cells
  • Each naïve lymphocyte carries receptors of single specificity
  • Genetic recombination generates diversity of antigen receptor molecules
  • Only lymphocytes with receptors that bind antigen are activated
• Clonal Selection Hypothesis


  1. Single progenitor cell gives rise to lymphocytes with different specificities

  2. Self-reactive immature lymphocytes removed by clonal deletion/negative selection

  3. Mature, naïve cells recognise and bind foreign antigen

  4. Activated cells proliferate and differentiate into effector cells
• 1. cell Receptor (TCR)


  • Heterodimer of alpha and beta chains
  • External part has variable and constant domains
  • Antigen-recognition site made of variable domains
  • Variable regions coded by rearranging VDJ gene segments
• Possible combinations of TCR alpha and beta chains is ~11.7 x 10^6
• Gene Rearrangement


  • Enables limited gene segments to generate diverse proteins
  • Each cell expresses unique receptor
  • Irreversible change, all daughter cells inherit same receptor