Human Physiology, 14th edition (2016)

504 Chapter 15

IgG subclass, whereas IgA is the most abundant subclass in

the secretions of mucosal membranes. For example, IgA anti-

bodies protect the mucosa of the intestine from both commen-

sal bacteria (chapter 18) and pathogens, and are present in the

external secretions of saliva and milk ( table 15.6 ). Antibodies

in the IgE subclass are involved in allergic (immediate hyper-

sensitivity) reactions, as discussed in section 15.6.

Antibody Structure

All antibody molecules consist of four interconnected polypep-

tide chains. Two long, heavy chains (the H chains ) are joined

to two shorter, lighter L chains. Research has shown that these

four chains are arranged in the form of a Y. The stalk of the Y

has been called the “crystallizable fragment” (abbreviated F (^) c ),
whereas the top of the Y is the “antigen-binding fragment”
( F (^) ab ). This structure is shown in figure 15.8.

Antibodies

Plasma proteins can be separated into five major classes by a

technique called electrophoresis, in which the proteins move

in an electric field. The five major classes of plasma proteins

are albumin, alpha-1 globulin, alpha-2 globulin, beta globulin,

and gamma globulin. The gamma globulin class of plasma pro-

teins contains the antibodies. Because antibodies are specific

in their actions, it follows that different types of antibodies

should have different structures. An antibody against smallpox,

for example, does not confer immunity to poliomyelitis and

therefore must have a slightly different structure than an anti-

body against polio. Despite these differences, antibodies are

structurally related and form only a few classes.

Antibody proteins are also known as immunoglobulins

(Ig). There are five Ig subclasses: IgG, IgA, IgM, IgD, and

IgE. Most of the antibodies circulating in the plasma are of the

Immunoglobulin Functions

IgG Main form of antibodies in circulation: production increased after immunization; secreted during secondary response

IgA Main antibody type in external secretions, such as saliva and mother’s milk

IgE Responsible for allergic symptoms in immediate hypersensitivity reactions

IgM Function as antigen receptors on lymphocyte surface prior to immunization; secreted during primary response

IgD Function as antigen receptors on lymphocyte surface prior to immunization; other functions unknown

Table 15.6 | The Immunoglobulins

Figure 15.8 Antibodies bind to antigens on bacteria. ( a ) A pathogen such as a bacterial cell has many antigens on its

surface, which can activate B cells that produce antibodies against those antigens. ( b ) Each antibody is composed of four polypeptide

chains—two are heavy (H), and two are light (L). Regions that have constant amino acid sequences are abbreviated C, and those with

variable amino acid sequences are abbreviated V. Antigens bind to the variable regions. Each antibody is divided into antigen-binding

fragments ( F (^) ab ) and a crystallizable fragment (F c ).
(a)
B cell
B-cell antibody
receptor
Surface antigen
Pathogen
Antigen molecule
Antibody
Fab
region
Fc region
(b)
VH
VL
VH
VL
CL CL
CH^1
CH^2 CH^2
CH^3 CH^3
CH^1