Many of the most important developments that result in a fully functio
ning vertebrate immune system take place in the developing fetus. From
a variety of gene segments there is assembled in B cells a congeries
of antibody combining sites, one to a cell, which form the greater par
t of the large repertoire of immunological specificities that characte
rise the system. This capability is further expanded later by somatic
mutations. Just as immunoglobulin isotypes are produced sequentially (
IgM, IgD, IgG, IgE, IgA) as they are read along the chromosome, so doe
s the fetus and neonate manifest immunological competence sequentially
to different antigens by employing variable region germline genes as
they appear along the chromosome. The generation of T cell receptor di
versity is accomplished by a similar mechanism of gene segment translo
cations. Each stage in the lineage of T and B cells is associated with
the appearance of unique combinations of surface molecular markers, w
hich in T cells characterise also the specialised functions of differe
nt subsets. If the immune system does not spring forth quite fully for
med from the evolved vertebrate genome, as Athena did from the forehea
d of Zeus, ontogenetic mechanisms have made it very nearly complete.