6.2.3 Comparative Biochemistry of Immunoglobulins
225
structures and antigen specificity. The hypervariability of the CDRs is related to their extremely
high rates of evolution. Substitution rates of
3.14 . 1~, 3.72 . 10--9 and 7.47 . 1~ per nucleotide per year have been calculated for the three
codon positions of human and murine CDRs;
thus in this case there is no effective selection
pressure [58]. The sections of the V region outside
of the hypervariable CDRs are much less variable
and evolve so slowly that common molecular
structures may be detected immunologically even
when the V H chains of agnathans or cartilaginous
fish and the mammals are compared [153]. One of
the V H genes of the clawed frog Xenopus laevis is
identical in 61 % of its nucleotides to the V-T15
gene of the mouse [199]. The different Hand L
chains can be freely combined; thus, a million different immunoglobulins would result from a
thousand each of Hand L variants. The actual
total number of V sequences in mammals is
unknown but could well be of the above order;
there is probably considerable fluctuation according to chain type and species.
The large variety of immunoglobulins has its
origin in the construction of the coding sequences
of the V regions from two or three gene segments
during lymphocyte development (somatic rearrangement): the heavy chains arise from VH
(variable) with 95-100 amino acid codons, D
(diversity) with 13-17 codons, and IH Goining)
with just a few codons; several (in the case of D
and I H ) or many (VH) variants of these segment
types exist. The V regions of the light chains are
also made up from V Land h segments. The gene
rearrangement involves the recognition of recombination signals which are conservative nonamer
and heptamer sequences flanking each V, I and D
segment on both sides. The sequences lying in
front of the h segments and behind the V L segments are complementary and can recombine. The
same is true for DNH and IHID. The variety of the
Hand L genes is further increased by variability in
the regions joining the gene segments and by
somatic point mutations (nucleotide substitutions)
which may occur in the genes themselves [10, 74].
The following terms are used to describe the
diversity of antibodies: the variants present in all
individuals of a species, e.g. the Ig classes and
subclasses, are known as isotypes. AIlotypes are
allelic variants which differ particularly in the C
region of the heavy chain. Idiotypes show differences in the structure of the antigen-binding site,
i.e. in the hypervariable segment of the V region.
The general number of gene segments available
for combination is known for only a few species
(Fig. 6.3): the A locus of the mouse contains two
Vi.. segments, each of which is coupled to two II.
and two CI. segments, whereas the rat A locus has
only one VI. and two CI. genes [177]. The human A
locus is considerably more complicated. The x
locus in man, as in the mouse, consists of many
V"' five I" and one c,.; the murine H locus is
made up of about 1000 VH , 15 D, 5 IH and 8 CH ,
whereas that of man has at least 200 VH , more
than 20 D, 6 IH and 9 functional CH [25]. Multiple
C genes of the same type define different Ig subclasses (isotypes), e.g. the 4 Cy genes of the
mouse and man, the two human Ca genes
(Fig.6.3b and Table 6.1), or the 13 Ca genes of
the rabbit [22].
Each B lymphocyte expresses only one Hand
one L gene at anyone time (allelic exclusion);
this is of biological advantage in that simultaneous expression of further loci would lead to a
reduction in the density on the cell surface of
receptors with a particular specificity. The V
sequence combination arising during lymphocyte
development is linked to a C sequence to give a
complete H or L gene. In the case of an H gene,
C", initially lies nearest to I H ; the lymphocyte thus
produces an IgM which is specific for a particular
antigen. Stimulation by this antigen causes the B
lymphocytes to proliferate and to differentiate
into Ig-secreting plasma cells. The secretory form
of Ig is now produced instead of the membranebound form, and this requires a gene rearrangement in the region of the H chain C-terminus
(Fig. 6.4). The first cells to be formed still produce IgM; the specific VH sequence is later translocated to one of the Cy or Ca sequences (class
switch) and IgG or IgA is formed. The class
switch involves recombination in the area of the S
regions which lies in front of all CH genes except
CII (Fig. 6.3a). The S regions are built up of repetitive 49-bp elements and, for example in the
mouse, are all very similar. Hybridization experiments have shown comparable sequences not
only in humans and the clawed frog Xenopus laevis but also in Drosophila and sea urchins, suggesting that they originally had other functions.
Rearrangements, gene switching and allelic
exclusion are all molecular biological specialities
of the immune system [74].
6.2.3 Comparative Biochemistry
of Immunoglobulins
Most information about the immunoglobulins
and their genes comes from studies of man and
225
structures and antigen specificity. The hypervariability of the CDRs is related to their extremely
high rates of evolution. Substitution rates of
3.14 . 1~, 3.72 . 10--9 and 7.47 . 1~ per nucleotide per year have been calculated for the three
codon positions of human and murine CDRs;
thus in this case there is no effective selection
pressure [58]. The sections of the V region outside
of the hypervariable CDRs are much less variable
and evolve so slowly that common molecular
structures may be detected immunologically even
when the V H chains of agnathans or cartilaginous
fish and the mammals are compared [153]. One of
the V H genes of the clawed frog Xenopus laevis is
identical in 61 % of its nucleotides to the V-T15
gene of the mouse [199]. The different Hand L
chains can be freely combined; thus, a million different immunoglobulins would result from a
thousand each of Hand L variants. The actual
total number of V sequences in mammals is
unknown but could well be of the above order;
there is probably considerable fluctuation according to chain type and species.
The large variety of immunoglobulins has its
origin in the construction of the coding sequences
of the V regions from two or three gene segments
during lymphocyte development (somatic rearrangement): the heavy chains arise from VH
(variable) with 95-100 amino acid codons, D
(diversity) with 13-17 codons, and IH Goining)
with just a few codons; several (in the case of D
and I H ) or many (VH) variants of these segment
types exist. The V regions of the light chains are
also made up from V Land h segments. The gene
rearrangement involves the recognition of recombination signals which are conservative nonamer
and heptamer sequences flanking each V, I and D
segment on both sides. The sequences lying in
front of the h segments and behind the V L segments are complementary and can recombine. The
same is true for DNH and IHID. The variety of the
Hand L genes is further increased by variability in
the regions joining the gene segments and by
somatic point mutations (nucleotide substitutions)
which may occur in the genes themselves [10, 74].
The following terms are used to describe the
diversity of antibodies: the variants present in all
individuals of a species, e.g. the Ig classes and
subclasses, are known as isotypes. AIlotypes are
allelic variants which differ particularly in the C
region of the heavy chain. Idiotypes show differences in the structure of the antigen-binding site,
i.e. in the hypervariable segment of the V region.
The general number of gene segments available
for combination is known for only a few species
(Fig. 6.3): the A locus of the mouse contains two
Vi.. segments, each of which is coupled to two II.
and two CI. segments, whereas the rat A locus has
only one VI. and two CI. genes [177]. The human A
locus is considerably more complicated. The x
locus in man, as in the mouse, consists of many
V"' five I" and one c,.; the murine H locus is
made up of about 1000 VH , 15 D, 5 IH and 8 CH ,
whereas that of man has at least 200 VH , more
than 20 D, 6 IH and 9 functional CH [25]. Multiple
C genes of the same type define different Ig subclasses (isotypes), e.g. the 4 Cy genes of the
mouse and man, the two human Ca genes
(Fig.6.3b and Table 6.1), or the 13 Ca genes of
the rabbit [22].
Each B lymphocyte expresses only one Hand
one L gene at anyone time (allelic exclusion);
this is of biological advantage in that simultaneous expression of further loci would lead to a
reduction in the density on the cell surface of
receptors with a particular specificity. The V
sequence combination arising during lymphocyte
development is linked to a C sequence to give a
complete H or L gene. In the case of an H gene,
C", initially lies nearest to I H ; the lymphocyte thus
produces an IgM which is specific for a particular
antigen. Stimulation by this antigen causes the B
lymphocytes to proliferate and to differentiate
into Ig-secreting plasma cells. The secretory form
of Ig is now produced instead of the membranebound form, and this requires a gene rearrangement in the region of the H chain C-terminus
(Fig. 6.4). The first cells to be formed still produce IgM; the specific VH sequence is later translocated to one of the Cy or Ca sequences (class
switch) and IgG or IgA is formed. The class
switch involves recombination in the area of the S
regions which lies in front of all CH genes except
CII (Fig. 6.3a). The S regions are built up of repetitive 49-bp elements and, for example in the
mouse, are all very similar. Hybridization experiments have shown comparable sequences not
only in humans and the clawed frog Xenopus laevis but also in Drosophila and sea urchins, suggesting that they originally had other functions.
Rearrangements, gene switching and allelic
exclusion are all molecular biological specialities
of the immune system [74].
6.2.3 Comparative Biochemistry
of Immunoglobulins
Most information about the immunoglobulins
and their genes comes from studies of man and