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The single-celled parasite known as Toxoplasma gondii infects more than half of the world's human population without creating any noticeable symptoms. Once inside the human body, Toxoplasma rapidly spreads to the heart and other organs. It can even penetrate the tight barrier that normally protects the brain from most pathogens. Yet, the blood of infected persons carries very few free-floating Toxoplasma cells. Scientists have long been puzzled by this ability of Toxoplasma to parasitize the human body without triggering an immune response and without an appreciable presence in the bloodstream. Recent research, however, has shed light on the ways in which Toxoplasma achieves its remarkable infiltration of the human body.
Though there are few individual Toxoplasma cells coursing freely in the blood of an infected person, scientists have discovered that the parasite is quite common in certain cells, known as dendritic cells, involved in the human immune system. Dendritic cells are found in the digestive tract and frequently come into contact with the various pathogens that enter the human body through food and water. When the dendritic cells encounter pathogens, they travel to lymph nodes and relay this information to other immune cells that then take action against the reported pathogen. Scientists have found, however, that Toxoplasma is capable of hijacking dendritic cells, forcing them from their usual activity and using them as a form of transportation to infect the human body quickly. Without this transport mechanism, Toxoplasma could not reach the better-protected areas of the body.
Toxoplasma invades the human body through consumption of the undercooked meat of infected animals, primarily pigs and chickens. Other animals, such as cats, can become infected as well. In fact, cats are a necessary component in the reproductive cycle of Toxoplasma, since the animal's intestines are the parasite's sole breeding ground. Toxoplasma creates egg-like cysts, known as oocysts, in the cats' intestines. These oocysts are shed in the cats' droppings and contaminate ground water and soil, eventually finding their way into the food chain. Because Toxoplasma must somehow find its way into a new host cat in order to reproduce, it cannot kill its current host. Instead, it waits for the host, usually a small rodent, to be eaten by a cat, thus providing Toxoplasma the opportunity to reproduce.
1. According to the passage, all of the following are true of Toxoplasma gondii EXCEPT
(A) it can contaminate ground water
(B) it enters the human body through the food chain
(C) it can alter the usual behavior of human cells
(D) the human body is incapable of detecting it
(E) it must find a host cat in order to reproduce
2. It can be inferred from the passage that which of the following statements is true of dendritic cells in the human body?
(A) They are produced by the lymphatic system.
(B) They are more numerous in the digestive tract than in any other part of the human body.
(C) Most dendritic cells of persons infected with Toxoplasma carry the parasite.
(D) They are the only cells capable of being infected by Toxoplasma.
(E) They are able to penetrate the membranes surrounding the brain.
3. Which of the following is the most likely outcome for Toxoplasma cells that invade the human body?
(A) They will be destroyed by the immune system.
(B) They will collect in the lymphatic system.
(C) They will not reproduce.
(D) They will be detected after several weeks.
(E) They will be destroyed by other pathogens in the bloodstream.
4. The second paragraph performs which of the following functions in the passage?
(A) It describes a paradox that scientists were unable to resolve for some time.
(B) It explains the effectiveness of a lethal biological pathogen.
(C) It describes the mechanism by which a biological event occurs in humans.
(D) It demonstrates how the infectious agent reproduces itself.
(E) It introduces information that is essential to understanding the role of Toxoplasma in human development.
5. The author mentions “pigs and chickens” in the final paragraph in order to
(A) provide specific examples of animals that can carry Toxoplasma
(B) provide specific examples of animals that are often eaten by cats
(C) provide specific examples of other animals whose dendritic cells are exploited by Toxoplasma
(D) provide specific examples of animals in which Toxoplasma can breed
(E) provide specific examples of animals that are immune to Toxoplasma
参考答案，做完才看 ：） DECCA
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发表于 2021-4-16 14:48:33
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