HUMAN BIOLOGY

166 Chapter 9

apoptosis Programmed cell
death, which is governed by
genes.

NK cell Natural killer cell; a
lymphocyte that kills virus-
infected cells and some
types of cancer cells.

programmed cell death is called apoptosis (a-poh-toe-

sis). The term comes from a Greek word meaning “to fall

apart,” and that’s what happens to the cell (Figure 9.14B). As

it disintegrates, its cytoplasm dribbles out, and its DNA and

organelles are broken up. After a cytotoxic T cell has done

its defensive work, it disengages from the doomed cell and

moves on.

Other kinds of cells make more general responses.

These cells include macrophages as well as lymphocytes

called NK cells (natural killers). NK cells are present in tis-

sues and organs of the lymphatic system. They can detect

and kill virus-infected body cells and some cancer cells.

Helper T cell cytokines stimulate NK cells, but NK cells

don’t need to have an antigen presented to them. Instead,

they simply attack any body cell that has too few or altered

MHC markers, or that antibodies have tagged for destruc-

tion. They also kill body cells flagged with chemical

“stress markers” that develop when a cell is infected or has

become cancerous.

Cytotoxic t cells cause the body

to reject transplanted tissue

Cytotoxic T cells cause the rejection of tissue and organ

transplants. This is partly because features of the MHC

markers on donor cells differ enough from the recipient’s

to be recognized as antigens.

To help prevent rejection, before an organ is trans-

planted the MHC markers of a potential donor are

analyzed to determine how closely they match those of

the patient. Because such tissue grafts generally succeed

n Antibodies can’t reach threats inside cells. Accordingly,

when cells become infected or altered in harmful ways,

other “warrior” cells must come to the defense.

n Link to blood types 8.4

Many pathogens evade antibodies. They hide in body

cells, kill them, and often reproduce inside them. They

are exposed only briefly after they slip out of one cell and

before they infect others. Viruses, bacteria, and some fungi

and protozoans all can enter cells. Cell-mediated immune

responses are the body’s weapons against these dangers as

well as against abnormal body cells such as cancer cells.

Figure 9.13 shows a simplified version of a cell-mediated

immune response to infection by a virus. Initially, antigens

from decaying virus particles in the cell are displayed at

the infected cell’s surface in an antigen–MHC complex.

Next, the two cells are linked when a T cell having recep-

tors for the antigen binds the complex. Together with cyto-

kines, this linking activates the T cell. The cell now begins

dividing, producing active cytotoxic

T cells that can kill virus-infected

cells. The response also produces

memory cytotoxic T cells.

Cytotoxic T cells release various

“killer” substances. Perforins are pro-

teins that literally perforate the tar-

get cell, making holes in its plasma

membrane. This “direct hit” kills the target cell in much the

same way that complement membrane attack complexes

do (Figure 9.14). Cytotoxic T cells also secrete chemicals

that cause the target cell to self-destruct. This genetically

F i g u r e 9.13 Animated! T cells are the warriors in cell-mediated immune responses. (© Cengage Learning)

9.7 Cell-mediated responses: fighting threats inside Cells

Antigen

Perforins

Virus-infected

cell

Virus-infected

cell

Infected cell

destroyed

T cell linked to

infected cell

Cytotoxic

T cell

Cytotoxic

T cells

T cell

receptor

Copyright 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).