For most of the twentieth century, the practice of medicine treated the immune system primarily as a defense against pathogens. Beginning in the 1990s, however, immunologists began documenting significant immune activity directed at the body's own cells in the absence of disease. These observations forced a rethinking of what the immune system is actually for.
Which choice best describes the function of the second sentence in the text?
- Acheck_circle
It introduces a development that destabilizes the prior view and motivates the third sentence.
- B
It defines the term 'pathogen'.
- C
It supports the twentieth-century view introduced in the first sentence.
- D
It introduces an unrelated branch of medicine.
Explanation
Documented self-directed immune activity destabilizes the defense-only model and prompts the rethinking in sentence three. B captures this transitional function.