HUMAN BIOLOGY

ImmunIty and dIsease 161

complement system Set

of inactive proteins in blood

and tissue fluid; the proteins

are activated as part of

innate immunity.

fever Core body tempera-

ture above 37°C (98.6°F).

histamine Chemical

released by mast cells and

basophils that helps cause

inflammation; it dilates arte-

rioles and makes them leak.

inflammation General

response to tissue damage.

Symptoms are warmth, red-

ness, swelling, and pain.

membrane attack

complex Structure formed

by complement proteins

that punctures bacteria,

which then die.

Activated complement and cytokines from macrophages
both trigger the phenomenon we call inflammation. This
is a fast, general response to tissue damage or infection
(Figure 9.7A). In inflammation, mast cells (Figure 9.7B) and
basophils respond to an antigen or to the cascade of comple-
ment proteins by releasing histamine and other substances.
Histamine is a chemical messenger that makes arterioles
in the tissue dilate, so more blood flows through them.
As a result, the tissue
reddens and warms
with blood-borne meta-
bolic heat. Histamine
also makes capillaries
leak. The narrow gaps
between the cells of the
capillary wall become
a bit wider, so plasma
proteins and phago-
cytes slip out through them. Water flows out as well. Due
to these and other changes, the tissue balloons with fluid.
This swelling is called edema. The pain that comes with
inflammation is due to edema and the effects of inflamma-
tory chemicals.
The plasma proteins leaking into tissue fluid include
blood clotting factors. Clots can wall off inflamed areas
and delay the spread of microbes into nearby tissues.
A fever is a core body temperature above the normal
37°C (98.6°F). Fever develops when macrophages release
cytokines called interleukins, which stimulate the brain to

Figure 9.7 Animated! Inflammation is a general response to tissue damage. A This diagram illustrates
how invading bacteria might trigger inflammation. In addition to combating the attack, the process helps
prepare the damaged tissue for repair. B Mast cells are located close to blood vessels, nerves, and
mucous membranes near the body surface. (A: © Cengage Learning; B: Antonio Zamora, http://www.scientificpsychic.com))

What is innate immunity?

• Innate immunity is the body’s general defense that includes the
  complement system, inflammation, and fever.


• Complement proteins kill pathogens or chemically attract
  phagocytes that can destroy them.


• Mast cells and basophils release histamine and other
  substances that cause inflammation.

takE-homE mEssaGE

invading bacteria

complement (purple dot)

on bacterium

neutrophil

macrophage

basophil

eosinophil

capillary is becoming

more permeable

mast cell releasing

histamine

histamine

molecules

dilating anteriole

capillary

macrophage

entering

tissue

A

blood vessel

mast cell

release prostaglandins. These signal-

ing molecules in turn can raise the set

point on the hypothalamic thermo-

stat, which controls core temperature.

Fevers are not usually harmful. In

fact, a fever of about 39°C (100°F) is

actually helpful. Among other ben-

efits, it increases body temperature to

a level that is too hot for many patho-

gens to function normally. However,

a fever that rises above 42°C (107.6°F)

is a medical emergency because it can

result in organ damage or death.

Phagocytosis, inflammation, and

fever rid the body of most pathogens

before they do major harm. If an

infection does take hold, the adap-

tive immune system takes over. We

turn to this topic next.

Biology Pics/Science Source

B

Macrophage engulfing an invading cell

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