1. Top tier answers:
invading pathogens
2. Mid-tier answers:
a. may encounter surface barriers of the body that form the: first line of defence
b. may encounter internal, fast-responding, and nonspecific cells and chemicals that form the: second line of defence
c. may encounter antigen-specific and adaptive cells and chemicals that form the: third line of defence
3. Last tier answers:
a. is/are composed of: mucous membranes
b. is/are composed of: phagocytes and natural killer cells
c. may produce an immune response that induces: inflammation of fever
d. is/are composed of: B and T lymphocytes
e. may produce an immune response that induces: antibodies
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