HUMAN BIOLOGY

160 Chapter 9

What physical and chemical barriers help

prevent an infection?

• Intact skin and mucous membranes are physical barriers to
  infection.


• Chemicals such as lysozyme in mucus help destroy some
  microbes.

takE-homE mEssaGE

lysozyme Enzyme that
destroys many bacteria.

Barriers to infection

n Phagocytosis, inflammation, and fever are the body’s

“first responder” mechanisms that act at once to counter

threats in general and prevent infection.

n Links to Cells 3.1, blood cells 8.1, blood clotting 8.7

White blood cells and proteins carry out innate immune

responses. An extremely important part of these responses

is a set of proteins called the complement system, or

simply complement. These proteins circulate in the blood

and tissues. Contact with antigens or substances leaking

from damaged body cells activates them. Then a cascade of

reactions activate ever more complement proteins, until the

molecules flood a damaged area.

Complement proteins coat pathogens. Some also form

membrane attack complexes that create a pore—a

hole—in the cell wall or plasma membrane of invading bac-

teria (Figure 9.6A). The punctured cell then disintegrates.

A complement coating also attracts phagocytes like mac-

rophages, dendritic cells, and neutrophils to invaded tissues.

Macrophages are usually the first defenders on the scene.

They engulf virtually anything other than healthy body cells.

If a macrophage detects an antigen, it releases cytokines that

alert the adaptive immune system that a threat is present.

Neutrophils literally blow apart when they come in

contact with a particular combination of complement pro-

teins and other signaling molecules. The “remains” of each

neutrophil, including its granules, form a net that traps

nearby invaders (Figure 9.6B).

n Pathogens usually cannot get past the skin or the linings of

other body surfaces such as the digestive tract.

n Links to Tissue membranes 4.7, skin 4.9

At this moment, there are probably thousands of micro-

organisms on every square inch of your skin (recall the

microbiome mentioned in Section 3.5). They usually are

harmless as long as they stay outside the body. Some types

grow so densely that they help prevent more harmful spe-

cies from gaining a foothold (Figure 9.5).

Normally “friendly” bacteria in the mucosal lining of

the digestive tract also help protect you. In females, lactate

produced by Lactobacillus bacteria in the vaginal mucosa

helps maintain a low vaginal pH that most bacteria and

fungi cannot tolerate. Any change in the conditions in

which these organisms grow can cause an infection. For

example, some antibiotics can trigger a vaginal yeast infec-

tion because they also kill Lactobacillus. The fungus that

causes athlete’s foot may begin to grow between your toes if

the skin there is often moist and warm.

The inner walls of the respiratory airways leading to

your lungs are coated with sticky mucus. That mucus

contains protective substances such as lysozyme, an

enzyme that chemically attacks and

helps destroy many bacteria. Broom-

like cilia in the airways sweep out

the pathogens.

Lysozyme and some other chemicals in tears, saliva,

and gastric fluid offer more protection. Urine’s low pH and

flushing action help bar pathogens from the urinary tract.

In adults, mild diarrhea can rid the lower GI tract of patho-

gens. In children, diarrhea serves the same function but

must be controlled to prevent dangerous dehydration.

innate immunity

Figure 9.6 Animated! Complement proteins help launch innate

immune responses. A–B Membrane attack complexes can form

lethal holes in the plasma membrane of bacteria. C Two Klebsiella

bacteria (purple) snared in a neutrophil net. This species can cause

serious lung infections. (Top left: From Frances Sienkiewicz Sizer; Eleanor Noss

W hitney, Nutrition: Concepts and Controversies, © 2002 Cengage Learning; Top right:

Robert R. Dourmashkin, courtesy of Clinical Research Centre, Harrow, England; Bottom:

© 2010 Papayannopoulos et al. Originally published in J. Cell Biol. 191:6 7 7– 6 91,

doi: 10.1083/jcb.201006054. Image by Volker Brinkman and Abdul Hakkim)

9.3 9.4 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

Figure 9.5 Many types of bacteria live on body surfaces.

This image shows Staphylococcus epidermis, the most

common species of bacterium on human skin.

Scimat/Science Source

C

A B

membrane attack complex pore

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).