Chapter 7 - Biology of the B Lymphocyte

Figure  7.1 describes the stages of B-cell differentiation

• This takes place in the bone marrow
• Note that the pre-B-cell has mu heavy chains and surrogate light chains
• Real light chains are the hallmark of immature B-cells, these cells are easily “tolerized” by self-antigen
• Mature B-cells make both IgM and IgD (remember variable splicing of mRNA)
• Signal transduction molecules are Ig-alpha and Ig-beta transmembrane proteins
• Under the direction of T-helper cells, B-cells may switch isotypes but keep their specificity.  This coordinated response occurs is usual for most antigens.  Such antigens are called T-dependent antigens
• There is a class of antigens which trigger antibody production without T-cell help.  The antibody produced is IgM only and the B-cells involved are usually called B-1 cells.  The antigens triggering such a response are called T-independent antigens.

Figure 7.5 describes the various integral membrane proteins found on B-cells:

•  
• Note that a group of proteins are called the B-cell co-receptor – if these get engaged by binding to something then the B-cell becomes more easily activated.
• CD 21 is part of the co-receptor – it binds to activated complement components (C3d) and to Epstein-Barr virus (EBV).
• There is a negative-feedback receptor called CD 32 – the low affinity IgG receptor (FcgRII)
• B-cells also have MHC class II proteins – this means that they can present to T_H cells
• B-cells have CD40 which binds to CD40L (CD 154) on T-cells – this interaction is important for class switch.