HUMAN BIOLOGY

164 Chapter 9

This antibody can bind

to a type of virus

This antibody can bind

to a species of bacteria

immunoglobulin Ig; an
antibody.

antibodies: Defense against threats outside Cells

Because of the great diversity of antigen receptors,

there’s a good chance the lymph node also contains an

inactive B cell with receptors for the invader’s antigen. If

so, the B cell binds to the bacterial antigen and displays it

(again, in an antigen–MHC complex) at the B cell’s surface

(Figure 9.12B). This processing activates the B cell. It also

allows helper T cells in the node to recognize and bind to

the antigen–MHC complex. Now the T cell and active B cell

can interact. The T cell begins to release cytokines that spur

the B cell to divide. Its descendants become plasma cells or

memory B cells.

The plasma cells release huge numbers of antibodies

into the bloodstream—up to 2,000 each minute. When

any of these antibodies binds to an antigen, it marks the

invader for destruction by phagocytes and complement

proteins. The memory B cells will be available to respond

quickly to the antigen if it attacks the body another time.

n Antibodies flag invaders or toxins so that the pathogens

can be destroyed or eliminated.

antibody-mediated immune responses

produce a flood of antibodies

Antibodies don’t kill pathogens or directly destroy toxins

in body fluids. Instead, antibodies mark these invaders

so that other mechanisms prevent them from harming

body cells.

When a B cell forms, the genetic mechanisms involved

ensure that it has receptors for only one antigen—such as a

particular bacterial or viral protein. If a B cell is activated,

the antibodies that the resulting plasma cells make will

target the same antigen. Figure 9.11 shows the typical Y

shape of simple antibodies that bind bacteria or viruses.

The place where an antibody can bind an antigen usually

is near the tip of the two “arms.” As antibodies form, they

are embedded in a B cell’s plasma membrane so that the

two arms stick out.

Antibody-mediated immune responses get under way

in the lymphatic system, especially in lymph nodes and the

spleen. Dendritic cells, T cells, and B cells all participate.

Figure 9.12 takes you through the

basic steps of the response, starting

with the point when a dendritic cell

responds to the invasion.

Remember from Section 9.5 that dendritic cells serve

as antigen “presenters.” A dendritic cell that detects the

invader engulfs it, processes the antigen it bears, and then

displays it in an antigen–MHC complex (Figure 9.12A).

When receptors of a responding helper T cell bind to the

complex, the two cells trade signals. The T cell begins to

divide, giving rise to effector and memory helper T cells

that have the same antigen receptors. (When you have an

infection, the accumulation of these T cells is what makes

your lymph nodes swell.)

Figure 9.12 Animated! A An antibody-mediated immune

response occurs when B cells make antibodies to an antigen.

In this example, the invader is a bacterium. (© Cengage Learning)

Figure 9.11 Antibodies can bind to antigens. Each kind of

antibody can bind only one kind of antigen. The antigen fits

into grooves and bumps on the antibody molecules. (From Frances

Sienkiewicz Sizer; Eleanor Noss Whitney, Nutrition: Concepts and Controversies,

© 2002 Cengage Learning)

9. 6

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

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