Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1316 Chapter 24: The Innate and Adaptive Immune Systems
them in its plasma membrane. Each BCR is stably associated with invariant transmembrane
proteins that activate intracellular signaling pathways when antigen
binds to the BCR; we discuss these invariant proteins later, when we consider how
B cells are activated with the assistance of helper T cells.
Each B cell clone produces a single species of BCR, with a unique antigenbinding
site. When an antigen and a helper T cell activate a naïve or a memory
B cell, the B cell proliferates and differentiates into an effector cell, which then
produces and secretes large amounts of soluble (rather than membrane-bound)
Ig. The secreted Ig is now called an antibody, and it has the same unique antigenbinding
site as the BCR (Figure 24–22).
A typical Ig molecule is bivalent, with two identical antigen-binding sites. It
consists of four polypeptide chains—two identical light chains and two identical
heavy chains. The N‐terminal parts of both light and heavy chains usually cooperate
to form the antigen-binding surface, while the more C‐terminal parts of the
heavy chains form the tail of the Y‐shaped protein (Figure 24–23). The tail mediates
many of the activities of antibodies, and antibodies with the same antigenbinding
sites can have any one of a number of different tail regions, each of which
gives the antibody different functional properties, such as the ability to activate
complement or to bind to receptor proteins on phagocytic cells that bind a specific
type of antibody tail.
Mammals Make Five Classes of Igs
In mammals, there are five major classes of Igs, each of which mediates a characteristic
biological response following antigen binding to an antibody: IgA, IgD,
IgE, IgG, and IgM, each with its own class of heavy chain—α, δ, ε, γ, and μ, respectively.
IgA molecules have α chains, IgG molecules have γ chains, and so on.
Moreover, there are four human IgG subclasses (IgG1, IgG2, IgG3, and IgG4), with
γ 1 , γ 2 , γ 3 , and γ 4 heavy chains, respectively. There are also two IgA subclasses in
humans. In addition to the various classes and subclasses of heavy chains, higher
vertebrates have two types of light chains, κ and λ, which seem to be functionally
indistinguishable. Either type of light chain may be associated with any of the
heavy chains, but an individual Ig molecule always contains identical light chains
and identical heavy chains: an IgG molecule, for instance, may have either κ or
λ light chains, but not one of each. As a result, an Ig’s antigen-binding sites are
always identical (see Figure 24–22).
The various heavy chains give a distinctive conformation to the tail region of
antibodies, so that each class (and subclass) has characteristic properties of its
own. IgM is always the first class of Ig that a developing B cell in the bone marrow
makes. It forms the BCRs on the surface of immature naïve B cells. After these cells
antigen
B cell
antigen
receptor B
(BCR)
PROLIFERATION
AND
DIFFERENTIATION
effector B cells
antigenic
determinant
resting
naïve or
memory
B cell
B B B B
secreted
antibodies
Figure 24–22 The B cell receptors
(BCRs) and secreted antibodies made
by a B cell clone. The binding of an
antigen to BCRs on either a naïve or
memory B cell (together with co-stimulatory
signals MBoC6 provided m25.17/24.23
by helper T cells—not
shown) activates the cell to proliferate
and differentiate into effector B cells.
The effector cells produce and secrete
antibodies with a unique antigen-binding
site, which is the same as that of the cellsurface
BCRs. Because antibodies have
two identical antigen-binding sites, they
can cross-link antigens, as shown for an
antigen with multiple identical antigenic
determinants.
antigenbinding
site
antigenbinding
site
H 2 N NH 2
NH 2
H 2 N
hinge
regions
S S
HOOC
tail region
HOOC
S
S
S
S
S S
COOH
light chain
heavy chain
COOH
Figure 24–23 A schematic drawing of
a bivalent antibody molecule. The two
heavy chains each have a hinge region,
which, because of its flexibility, improves
the efficiency with which the antibody can
cross-link antigens (see Figure 24–22). The
two heavy chains also form the tail of the
antibody, which determines its functional
properties. The heavy and light chains are
held together by both covalent S–S bonds
(red) and noncovalent bonds (not shown).