填空题Complete the summary below. Write NO MORE THAN TWO WORDS for each
answer. Every day the human body is fighting off {{U}}
{{U}} 1 {{/U}} {{/U}}by destructive pathogens. A person in good
health has natural protection in the form of an immune system which works best
against familiar microorganisms which may have been encountered during a
previous {{U}} {{U}} 2 {{/U}} {{/U}}or passed on by the mother
before or after birth. Vaccination is a way to cause {{U}}
{{U}} 3 {{/U}} {{/U}}immunisation by introducing a small amount of
pathogen into the body—just enough for the body's {{U}} {{U}} 4
{{/U}} {{/U}}to react by making antibodies. Passive immunization can be used
as a way of treating someone who is already sick. Proteins from animal {{U}}
{{U}} 5 {{/U}} {{/U}}are introduced into the patient to give him the
necessary antibodies to fight the disease. Dr. Edward Jennet
observed that people who had suffered and recovered from a serious disease
called smallpox did not get it again. He also noted that victims of a milder
disease, cowpox, which they caught from {{U}} {{U}} 6 {{/U}}
{{/U}}, were immune to smallpox. He carried out a successful {{U}} {{U}}
7 {{/U}} {{/U}}by deliberately giving a child cowpox in order to make
him immune to smallpox.