Dizygotic twins are connected to a single placenta during their embryonic development. These twins

Which one of the following amino acids is catalyzed by activated macrophages to produce reactive nitrogen species?

A. Arginine

B. Asparagine

C. Cysteine

D. Histidine

Recombinant live attenuated vaccine against hepatitis B was prepared from

A. Plasma of chronically infected individual

B. Recombinant yeast expressing hepatitis B surface antigen

C. Recombinant vaccinia virus expressing hepatitis B surface antigen

D. Transgenic plants expressing hepatitis B surface antigen

Which of the following statements about immune response are correct?
P. T cells are activated by antigen-presenting cells.
Q. Foreign peptides are not presented to helper T cells by class II MHC proteins.
R. Dendritic cells are referred to as professional antigenpresenting cells.

A. P and R only

B. P and Q only

C. Q and R only

D. P, Q and R

Which one of the following complement proteins is the initiator of the membrane attack complex?

A. C3a

B. C3b

C. C5a

D. C5b