B cell

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  • The B stands for Bursa of Fabricius and not bone marrow
  • Expression of B-cell receptor genes
  • Membrane-bound immunoglobulins serve the receptor functions
  • Some B cells will become memory cells after any given encounter
  • The memory cells will then escalate to the next immunoglobulin type the next time they are exposed to the same antigen

B Cell receptors

  • aka BCR
  • B Cells recognize antigens in their natural form through the surface antigen receptors known as B cell receptors
  • Each receptor consists of a recognition, signaling, and coreceptor unit

Recognition unit

Signaling unit

  • Includes at least two types of chains: Igα and Igβ

Coreceptor unit

  • A molecular complex formed by CR2 (binds complement components), CD19 (binds intracellular signaling molecules), and TAPA-1 (function unknown) molecules
Light and heavy chain construction highlighting Ig diversity generation


  • The construction of the heavy and light chains always happens sequentially (Heavy, then light).
  • This is done so that the molecule ends up with only one specificity
  • This is called allelic exclusion -- making sure that only one specificity is expressed
  • Also see Janeway Figures 7.1, 7.3, 7.5, 7.14, 7.18, 7.25, 7.26, 7.37, 7.40, 7.45

Heavy Chain

  • First part of cell development
  • Joins VH - DH - JHand then tests to see if the strands check out.
  • Once joined, VHDHJH is then joined to Cμ, which is then also tested.

Light Chain

  • Once the heavy chain is validated, the light chain is formed
  • Joins a Vlightκ to Jlightκ, then to Lκ
  • Once the κ light chain is constructed, then the heavy λ chain will be constructed
  • This is why there is always more variation in κ light chains than in λ light chains
  • There is always a number of cells that express antibodies against self -- a screw-up in development
  • Somatic hypermutation - aims to increase the affinity of immunoglobulins.
  • Called somatic because it is not in the germ line (e.g. does not occur during development)
  • If a B cell does not see an antigen in three days, then the B cell dies
  • If it does see an antigen, then clonal selection begins