CHAPTER 43 THE BODY'S DEFENSES
The vertebrate body possesses two mechanisms which protect it from potentially dangerous viruses, bacteria,
1. One of these mechanisms is nonspecific, that is, it does not distinguish between nfective agents.
2. The second mechanism is specific in that it responds in a very specific manner (e.g., production of antibodies)
to the particular type of infective agent.
I. Nonspecific Defense Against Infection
Nonspecific defense mechanisms help prevent entry and spread of invading microbes in an animal's body.
- An invading microbe must cross the external barrier formed by the skin and mucous membranes.
- If the external barrier is penetrated, the microbe encounters a second line of defense:
interacting mechanisms of phagocytic white blood cells, antimicrobial proteins, and the inflammatory
A. The skin and mucous membranes provide first-line barriers to infection
The skin and mucous membranes act as physical barriers preventing entry of pathogens, and as chemical
- In humans, oil and sweat gland secretions acidify the skin (pH 3 to 5), which discourages microbial growth.
- Saliva, tears, and mucous secretions contain antimicrobial proteins and wash away potential invading
- An enzyme (lysozyme) in perspiration, tears, and saliva attacks the cell walls of many bacteria and
- In the respiratory tract, nostril hairs filter inhaled particles and mucus traps microorganisms that are then
swept out of the upper respiratory tract by cilia, thus preventing their entrance into the lungs.
- In the digestive tract, stomach acid kills many bacteria that enter with foods or those trapped in swallowed
mucus from the upper respiratory system.
B. Phagocytic cells, inflammation, and antimicrobial proteins function early in infection
Microbes that penetrate the skin or mucous membranes encounter amoeboid white blood cells capable of
phagocytosis or cell lysis.
1. Phagocytic and natural killer cells
- Attracted by chemical signals, they enter infected tissues by amoeboid movement
- Only live a few days as they destroy themselves when destroying pathogens
comprise only about 5% of the white blood cells, but they provide an even more effective
phagocytic defense. They mature, circulate for a few hours, then migrate to the tissues where they enlarge and
are large amoeboid cells that use pseudopodia to phagocytize microbe, which is then destroyed
by digestive enzymes and reactive forms of oxygen within the cell.
Macrophages are long lived.
- Most wander through interstitial fluid phagocytosing bacteria, viruses, and cell debris.