HUMAN BIOLOGY

ImmunIty and dIsease 157

What are the body’s three lines of defense?

• The three lines of defense are physical barriers, innate immunity,
  and adaptive immunity.


• Immune responses are executed by white blood cells and the
  chemicals they release.


• Innate immunity is inborn. Its responses occur quickly but do not
  provide long-term protection against specific pathogens.


• Adaptive immunity develops over a week or so, and it provides
  long-term protection against particular pathogens.

takE-homE mEssaGE

White blood cells and their chemicals are
the defenders in immune responses

As you’ve just read, white blood cells operate in both the
innate and adaptive immune responses. Many circulate
in blood, while some enter tissues. All release substances
that help muster or strengthen defense responses. These
chemicals include several types of cytokines, “cell mov-
ers” that promote and regulate many aspects of immunity.
Some white blood cells also secrete enzymes and toxins
that kill microbes.
Section 8.1 introduced the main white blood cells that
circulate in blood. Of the trillions of these cells in the body,
about two-thirds are neutrophils, which are phagocytes.
Eosinophils target parasites, such as worms, that are too
big for phagocytosis. They also play a part in allergies.
Basophils release substances from their granules that
cause inflammation.
Three other types of white blood cells perform their
functions while in tissues. Like basophils, mast cells have
granules containing chemicals that cause inflammation
(Figure 9.2). Macrophages are large phagocytes that arise
from circulating monocytes. Each one can engulf as many
as one hundred bacteria! Dendritic cells alert the adaptive
immune system when they detect antigens.
Central roles in adaptive immunity are filled by the cells
known as lymphocytes. Most of their activities occur in
tissues and organs of the lymphatic system (Section 9.2).

F i g u r e 9.1 Animated! An array of white blood cells carry out immune responses. These sketches show some of the major types.
(© Cengage Learning)

neutrophil eosinophil basophil macrophage

(from monocyte)

dendritic cell lymphocyte

Those called B cells and T cells are the white blood cells

that can recognize specific antigens. As you’ll read later in

this chapter, cells derived from B cells make the defensive

proteins called antibodies. Some T cells kill abnormal body

cells while others help activate B cells.

Many white blood cells circulate in lymph, watery

fluid carried in vessels of the lymphatic system. As the

next section describes, this system, which has major roles

in defense, also works with the cardiovascular system in

moving substances throughout the body.

Gender and stress influence

the immune system

Other things being equal, studies show that in females

immune responses to invading pathogens are stronger

than in males. This “turbocharged” immunity has a down

side, however. Females are statistically more likely to

develop common autoimmune disorders, such as multiple

sclerosis, lupus, and Hashimoto’s disease (which affects

the thyroid gland). As you will read in Section 9.9, in those

and other autoimmune disorders, an overvigilant immune

system attacks normal cells or proteins.

Prolonged fatigue, emotional upsets, and other forms

of ongoing stress can result in less effective immune

responses. This is because hormones that are produced in

response to stress suppress the chemical triggers that help

launch immune responses. The folk wisdom that you risk

getting sick when your “resistance is down” comes from

everyday observations of such biological effects of stress.

Figure 9.2 White blood cells include phagocytes and cells with
other roles. A Neutrophils are phagocytes. Here they have
surrounded bacteria (green) in lung tissue. B Here a fluffy-
looking dendritic cell (lavender) is engulfing cells of a fungus
(orange). (A: SPL/Science Source; B: Prof. Matthias Gunzer/Science Source)

A B

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).