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

1318 Chapter 24: The Innate and Adaptive Immune Systems
Table 24–2 Properties of the Major Classes of Antibodies in Humans
Class of antibody
Properties
IgM IgD IgG IgA IgE
Heavy chains μ δ γ α ε
Light chains κ or λ κ or λ κ or λ κ or λ κ or λ
Number of four-chain units 5 1 1 1 or 2 1
Percentage of total Ig in blood 10 <1 75 15 <1
Activates classical complement
pathway
+ – + (some subclasses) – –
Crosses from mother to fetus – – + (some subclasses) – –
Binds to macrophages and
neutrophils
+ (macrophages only) – + (some subclasses) + –
Binds to mast cells and basophils – – + (some subclasses) – +
Ig Light and Heavy Chains Consist of Constant and Variable
Regions
Both light and heavy chains have a variable amino acid sequence at their N‐terminal
ends but a constant sequence at their C‐terminal ends. Whereas the constant
region and variable region of a light chain are the same size, the constant region
of a heavy chain is about three or four times longer, depending on the class (Figure
24–25).
The variable regions of the light and heavy chains come together to form the
antigen-binding sites, and the variability of their amino acid sequences provides
the structural basis for the diversity of these binding sites. The greatest diversity
occurs in three small hypervariable regions in the variable regions of both light
and heavy chains. Only about 5–10 amino acids in each hypervariable region form
the actual antigen-binding site (Figure 24–26). As a result, the size of the antigenic
determinant that an Ig molecule recognizes is generally comparably small:
it can consist of fewer than 10 amino acids on the surface of a globular protein, for
example (see Figure 24–22).
Both light and heavy chains are made up of repeating segments—each about
110 amino acids long and each containing one intrachain disulfide bond. Each
repeating segment folds independently to form a compact functional unit called
an immunoglobulin (Ig) domain. As shown in Figure 24–27A, a light chain consists
of one variable (V L ) and one constant (C L ) domain, whereas a heavy chain
has one variable and three or four constant domains: the variable domains of the
light and heavy chains pair to form the antigen-binding region. Each Ig domain
has a very similar three-dimensional structure, consisting of a sandwich of two β
LIGHT CHAIN
variable
region
constant
region (κ type or λ type)
H 2 N
COOH
H 2 N
HEAVY CHAIN
COOH
variable region
constant region
(α, δ, ε, γ, or µ type)
Figure 24–25 Constant and variable regions of Ig chains. The variable regions of the light
and heavy chains form the antigen-binding sites, while the constant regions of the heavy chains
determine the other biological properties of an Ig protein. The different subclasses of IgG antibodies
have different γ‐chain constant regions.
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