ImmunIty and dIsease 167What is a cell-mediated immune response?- A cell-mediated immune response occurs when certain types
of defensive cells attack infected or chemically altered body
cells. - Cytotoxic T cells target specific antigens.
- NK cells, macrophages, and various other white blood cells
make other, nonspecific responses.
takE-homE mEssaGEA cornea transplant is the only cure for advanced
cases of keratoconus, a condition in which the
cornea thins and becomes cone-shaped. As the
disease progresses, an affected person’s vision
becomes seriously distorted. Usually both eyes
are affected to some degree. Patients often are
diagnosed in their teens or early twenties. The
National Keratoconus Foundation (www.nkcf.org)
reports that about 1 in 2,000 people develop the
disorder, although its cause is unknown. What else
can you learn about keratoconus? What does a
cornea transplant entail?ThiNk OuTsiDE ThE bOOk
only when the donor and recipient share at least 75 percent
of their MHC markers, the best donor is a close relative of
the recipient, such as a parent or sibling, who is likely to
have a similar genetic makeup.
More commonly, however, the donated organ comes
from a fresh cadaver. In addition to having well-matched
MHC markers, the donor and recipient also must have
compatible blood types (Section 8.4).
After surgery, the organ recipient receives drugs that
suppress the immune system. The treatment also may
include other therapies designed to fend off an attack
by B and T cells. As with Kelly Perkins, the heart trans-
plant patient described in the chapter introduction, sup-
pression of the immune system means that the patient
must take large doses of antibiotics to control infec-
tions. In spite of the difficulties, many organ recipients
survive for years beyond the surgery and lead highly
active lives.
Interestingly, not all transplanted tissues provoke a
recipient’s immune defenses. Two examples are tissues
of the eye and the testicles. In simple terms, the plasma
membrane of cells of these organs is thought to bear recep-
tors that can detect activated lymphocytes. Before such
a defender can launch an attack, the protein signals the
soon-to-be-besieged cell to secrete a chemical that trig-
gers apoptosis in the approaching lymphocytes, so the
attack is usually averted. Our ability to readily transplant
the cornea—the outer layer of the eye that is vital to clear
vision—depends on this mechanism. The Think Outside the
Book feature on this page discusses a disorder for which a
cornea transplant is the only cure.
Figure 9.14 Cytotoxic T cells can kill cancer cells. A This color-
enhanced image captures two cytotoxic T cells (pink) killing a
tumor cell. B A cancer cell disintegrating during apoptosis.Steve Gschmeissner/Science SourceSteve Gschmeissner/Science SourceBACopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).