Immunology: Animal Defense Systems

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Key Concepts

39.1 Animals Use Innate and Adaptive Mechanisms to Defend Themselves against Pathogens

39.2 Innate Defenses Are Nonspecific

39.3 The Adaptive Immune Response Is Specific

39.4 The Adaptive Humoral Immune Response Involves Specific Antibodies

39.5 The Adaptive Cellular Immune Response Involves T Cells and Their Receptors

The siege of Athens by Sparta and its Peloponnesian allies made the city crowded and vulnerable to the spread of an infectious disease.

The Athenian historian Thucydides wrote these words in 430 BCE. He was describing a rapidly spreading infectious disease, or plague, that was sweeping the city of Athens in ancient Greece, and would ultimately kill about one-third of its inhabitants.

As a rule, however, there was no ostensible cause; but people in good health were all of a sudden attacked by violent heats in the head, and redness and inflammation in the eyes, the inward parts, such as the throat or tongue, becoming bloody and emitting an unnatural and fetid breath.

One year earlier, in 431 BCE, the Spartans had surrounded Athens. Sparta was a rival state with a highly disciplined army, while Athens was near a port and had a powerful navy. The Athenian leader Pericles decided to take advantage of his naval supremacy. He instructed his soldiers to abandon the countryside to the Spartans, relying on the navy to keep the enemies out of the city and to keep the Athenians supplied with food. People living in the countryside poured into Athens.

As Athens became more crowded, deteriorating sanitation and close living conditions resulted in the city becoming an incubator for infectious diseases. In his famous History of the Peloponnesian War, Thucydides speculated that this disease came to Athens from Africa. His precise clinical description has provoked great debate among medical historians regarding its nature. In 1994 a burial ground was found near Athens that apparently contained the remains of several dozen people who died of the disease. Samples of dental pulp from the skeletons were examined using the polymerase chain reaction (PCR), and the results revealed the presence of DNA sequences from the bacterium that causes typhoid fever. Based on these results, it was proposed that the disease that killed so many Athenians was typhoid fever. Others disagree, using arguments based on a clinical description of typhoid fever today.

Whatever the cause, another passage from Thucydides (who was one of the few lucky survivors of the disease) is an interesting observation about immunity—the subject of this chapter:

Yet it was with those who had recovered from the disease that the sick and the dying found most compassion. These knew what it was from experience and now had no fear for themselves; for the same man was never attacked twice, never at least fatally.

Question 39.1

How can a person survive an infection and be resistant to further infection?

You will find the answer to this question on page 825.

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