Which description matches the third line of defense in the immune system?

Which description matches the third line of defense in the immune system?

• A. Antibodies
• B. Hormones
• C. Red blood cells
• D. Enzymes

Answer: (A)

The third line of defense is the adaptive immune response, which uses antibodies to target specific pathogens. Antibodies are proteins made by B cells that bind tightly to particular antigens on invaders, neutralize them, block their ability to infect, and mark them for attack by other immune cells. This specificity and memory—producing targeted responses to familiar pathogens—define this part of immunity. Hormones regulate immune activity but aren’t defenses themselves; red blood cells carry oxygen; enzymes speed up chemical reactions and aren’t the targeted, pathogen-specific mechanism described by antibodies.

The third line of defense is the adaptive immune response, which uses antibodies to target specific pathogens. Antibodies are proteins made by B cells that bind tightly to particular antigens on invaders, neutralize them, block their ability to infect, and mark them for attack by other immune cells. This specificity and memory—producing targeted responses to familiar pathogens—define this part of immunity. Hormones regulate immune activity but aren’t defenses themselves; red blood cells carry oxygen; enzymes speed up chemical reactions and aren’t the targeted, pathogen-specific mechanism described by antibodies.