• B cells
  • Humoral immunity (against extracellular pathogens).
  • Recognize antigen—undergo somatic hypermutation to optimize antigen specificity.
  • Produce antibody—differentiate into plasma cells to secrete specific immunoglobulins.
  • Maintain immunologic memory—memory B cells persist and accelerate future response to antigen.
• T cells
  • Cell-mediated immunity (against intracellular pathogens).
  • CD4+/helper T cells help B cells make antibodies and produce cytokines to recruit phagocytes and activate other leukocytes – T helper cells call for help!
  • CD8+/cytotoxic T cells directly kill virus-infected cells.
  • Delayed cell-mediated hypersensitivity (type IV).
  • Acute and chronic cellular organ rejection.
  • Rule of 8: MHC II × CD4 = 8; MHC I × CD8 = 8.