16. Following are the defenses of the colon except

A. sIgA
B. fast flow rate
C. dense resident microbiota
D. mucus

17. Natural killer cells are involved in

A. tumor rejection
B. allograft rejection
C. non-specific killing of virus-transformed target cells
D. all of these

18. Which of the following(s) is/are the determinants of antigenicity?

A. Chemical nature
B. Size of the antigen
C. Foreignness
D. All of these

19. Which of the following is correct?

A. All T Cells are CD4+
B. An epitope can have multiple antigens
C. T Cell receptors are generated by random assortment of gene cassettes
D. MHC Class II molecules are generated by random assortment of gene cassettes

20. Immunity is not long-lasting to

A. influenza
B. whooping cough
C. diphtheria
D. mumps