One of the major differences between the first and second line of defense is that a. the first line defenses are anatomical barriers whereas the second line defenses are mechanisms.
b. the first line defenses involve complement proteins whereas the second line defenses involve antibodies.
c. the first line defenses involve the respiratory tract whereas the second line defenses involve the gastrointestinal tract.
d. the first line defenses are mechanisms whereas the second line defenses are physical barriers to infection.