HUMAN BIOLOGY

ImmunIty and dIsease 163

Two types of effector T cells form. Cytotoxic T cells

kill target cells. Hence the T cell response is called

cell-mediated immune response (Figure 9.9). Helper

T cells release cytokines that help boost adaptive immune

responses. Some of these chemicals are “interleukins”

(meaning between leukocytes) that promote B-cell activ-

ity. Others are growth factors that spur the activity of

cytotoxic T cells. Still others serve as a call to arms for

neutrophils and other white blood cells that operate in

innate responses.

Even if a T cell encounters an antigen that is a match for its

receptors, the antigen will be “invisible” until it is processed

by an antigen-presenting cell. Although macrophages

and B cells may provide this service, the presenter often is a

dendritic cell (Figure 9.10A). First the presenter engulfs a cell

or material that includes an antigen. Then enzymes (from the

presenter cell’s lysosomes) cut the antigen into pieces. Some

of the pieces then are joined with MHC markers to form an

antigen–MHC complex. This struc-

ture is part self (the MHC marker)

and part nonself (the antigen).

The presenter cell displays the

complex at its surface (Figure 9.10B).

A T cell that has the receptor for

the antigen part of the complex can

bind to it—and an adaptive immune

response can get under way.

The next two sections explain the

steps in the two types of adaptive

immune responses. We look first at

how an antibody-mediated immune

response develops, and then at how

a cell-mediated response occurs.

Figure 9.9 Animated! The two arms of adaptive immunity are the
antibody-mediated response and the cell-mediated response.

© Cengage Learning

Figure 9.10 Antigen-presenting cells process antigens. A A human T cell (pink)
interacting with a dendritic cell (blue). The interaction activates the T cell. B This
diagram shows how an antigen-presenting cell forms an antigen–MHC complex—
the chemical flag that can launch an immune response by lymphocytes.

lysosome

fuses with

endocytic

vesicle

cell engulfs an
antigen-bearing
particle endocytic
vesicle forms

endoplasmic

reticulum

nucleus

particle is

digested

into bits

MHC markers

bind fragments

of particle

antigen–MHC complexes

are displayed

on cell surface

Golgi bodies

B

Dr. Olivier Schwartz, Institut Pasteur/

Science Source

A

What are the body’s adaptive

immune defenses?

• An adaptive immune response is one that
  counteracts specific threats, such as a
  particular virus or species of bacteria.


• Adaptive responses are amazingly diverse.
  They also produce memory cells that can
  mount a faster, stronger response to an
  antigen that enters the body again.


• Antigen-presenting cells expose T cells
  to processed antigens, which they can
  recognize.


• Helper T cells produce cytokines, substances
  that stimulate immune responses of B cells,
  cytotoxic T cells, and other white blood cells.

takE-homE mEssaGE

Red blood cells

Platelets

Monocytes,

others

Bone marrow

Thymus

B cells T cells

Organs of lymphatic system

Foreign

invasion

Stem cells

B cells T cells

Antibody-mediated

immune response

Cell-mediated

immune response

© Cengage Learning

antigen-presenting cell A

cell that processes an anti-

gen in ways that allow a T

cell to detect it.

cell-mediated immune

response An adaptive

immune response mounted

by T cells.

cytotoxic T cell Effector T

cell that kills body cells that

have been infected by a

virus or altered by cancer.

helper T cell Effector T

cell that stimulates adaptive

immune responses.

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