B Cells

1.            Functions of B cells

a.         They differentiate into plasma cells and produce antibodies.

b.         They are antigen-presenting cells.

2.         Origin of B cells

a.            Embryogenesis:

i.          B cell precursors are recognized first in the fetal liver.

ii.          from there they migrate to the bone marrow, which is their main location during adult life.

iii          Pre-B cells lack surface immunoglobulins and light chains but do have m heavy chains in the cytoplasm.

b.            Maturation of B cells:

i.          antigen-independent phase consists of stem cells, pre-B cells, and B cells.

ii.          antigen-dependent phase consists of cells that arise subsequent to the interaction of antigen with the B cells, e.g. activated B cells and plasma cells.

c.         B cells display surface IgM, which is a pentamer.

d.            Location:

i.          B cells constitute about 30% of the recirculating pool of small lymphocytes, and their life span is short, i.e. days or weeks.

ii.          within lymph nodes, they are located in germinal centers.

iii.         within the spleen, they are found in the white pulp.

iv.         they are also found in the gut-associated lymphoid tissue, e.g. Peyer’s patches.

3.         Clonal Selection

a.         Each individual has a large pool of B lymphocytes.

b.         Each immunologically responsive B cell bears a surface receptor (either IgM or IgD) that react with one antigen.

c.         An antigen interacts with the B lymphocyte that shows the best ‘fit’ with its immunoglobulin surface receptor.

d.         After the antigen binds, the B cell is stimulated to proliferate and form a clone of cells.

e.         These selected B cells soon become plasma cells and secrete antibody specific for the antigen.

f.          Plasma cells synthesize the immunoglobulins with the same antigenic specificity.

4.            Activation of B cells

a.         Binding of antigen:

i.            multivalent antigen binds to surface IgM and cross-links adjacent immunoglobulin molecules.

ii.          the immunoglobulins aggregate to form ‘patches’ and eventually migrate to one pole of the cell to form a cap.

b.            Endocytosis of the capped material follows, the antigen is processed, and epitopes appear on the surface in conjunction with class II MHC proteins.

c.         Role of T helper cells:

i.          this complex is recognized by a T helper cell with a receptor for the antigen on its surface.

ii.          the T cell now produces various cytokines (IL-2, IL-4, and IL-5) that stimulate the growth and differentiation of the B cell.

iii.         many plasma cells that produce large amounts of immunoglobulins specific for the epitope are the end result.

d.            Memory cells:

i.          some activated B cells form memory cells, which can remain quiescent for long periods but are capable of being activated rapidly upon reexposure to antigen.

ii.          most memory B cells have surface IgG that serves as the antigen receptor, but some have IgM.

iii.            memory T cells secrete interleukins that enhance antibody production by memory B cells.

iv.         the presence of these cells explains the rapid appearance of antibody in the secondary response.