HUMAN BIOLOGY

ImmunIty and dIsease 165

What is an antibody-mediated immune

response?

• In an antibody-mediated response, antibodies form and bind
  to antigens of pathogens and toxins that are outside cells.


• The steps that result in an antibody-mediated response produce
  plasma cells, which make antibodies, and memory B cells.


• Antibodies do not directly kill pathogens. They flag them for
  destruction by other defenders.


• Plasma cells secrete five classes of antibodies
  (immunoglobulins) that help protect the body against
  diverse threats.

takE-homE mEssaGE

secreted antibodies

IgG Main antibody in blood; activates complement,

neutralizes toxins; protects fetus and is

secreted in early milk.

IgA Abundant in exocrine gland secretions

(e.g., tears, saliva, milk, mucus), where it

occurs in the form shown here. Interferes

with binding of pathogens to body cells.

Membrane-bound antibodies

IgE Anchored to surface of basophils, mast cells,

eosinophils, and some dendritic cells. IgE

binding to antigen induces anchoring cell to

release histamines and cytokines. Factor in

allergies and asthma.

IgD B cell receptor.

IgM B cell receptor, as a monomer.

Also is secreted as pentamer

(group of five), as shown here.

Table 9.2 Antibodies in the human body

there are five classes of antibodies

Plasma cells make five classes of antibodies. Collectively

they are called immunoglobulins, or Igs. We abbreviate

them as IgM, IgD, IgG, IgA, and IgE. Each type has antigen-

binding sites and other sites with special roles. As you can

see in Table 9.2, some antibodies are secreted into body fluids

while others are anchored to the surface of defensive cells.

IgM and IgD serve as the B cell receptors. IgM also

is the first antibody secreted during immune responses

and the first one produced by newborns. IgM molecules

cluster into a structure with ten antigen- binding sites. This

makes it more efficient at binding clumped targets, such as

agglutinating red blood cells (see Section 8.4) and clumps

of virus particles.

IgG makes up about 80 percent of the anti bodies

in your blood. It’s the most efficient one at turning on

complement proteins, and it neutralizes many toxins. This

long-lasting antibody easily crosses the placenta. It helps

protect the developing fetus with the mother’s acquired

immunities. IgG secreted into early milk is also absorbed

into a suckling newborn’s bloodstream.

IgA is the main immunoglobulin in substances secreted

by exocrine glands, such as tears, saliva, and breast milk.

It also is in mucus that coats the respiratory, digestive,

and reproductive tracts—areas microbes can easily access.

Bacteria and viruses can’t attach to the cells of mucous

membranes when IgA is bound to them. In this way, IgA

is effective in fighting the pathogens that cause salmonella,

cholera, gonorrhea, and influenza.

IgE is involved in allergic reactions, including asthma,

hay fever, and hives. IgE also triggers inflam mation after

attacks by parasitic worms and other pathogens. When it

binds to an antigen, basophils and mast cells release hista-

mine that causes the inflammation response.

Other

cytokines

Antigen–MHC complex

Helper T cells

A

Signals from

cytokines

Bacterial cell with

antigens on its surface

Dendritic cell

that engulfs the

invader and

processes it

B cell

receptor

B cell

Helper T cell

Cytokines

B cells that can make

antibodies to the invading

bacteria

Plasma cells

Antibodies

Memory

B cells

B

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