4. An antibody is
a. a substance initiating an allergic response.
b. a marker on the cell surface of macrophages.
c. a protein produced by plasma cells.
d. a marker on the cell surface of macrophages.

5. Which of the following cell types secretes antibodies?
a. Macrophages
b. T cells
c. Plasma cells
d. Monocytes

6. Which of the following provide defense against viral infections?

a. Histamines
b. Antibiotics
c. Prostaglandins
d. Interferons