Define and explain the functions of $B$-lymphocytes and $T$-lymphocytes.

• A
  $B$-lymphocytes produce antibodies; $T$-lymphocytes produce antigens.
• B
  $B$-lymphocytes produce antibodies; $T$-lymphocytes help $B$-lymphocytes produce antibodies.
• C
  $B$-lymphocytes kill pathogens directly; $T$-lymphocytes produce antibodies.
• D
  $B$-lymphocytes help $T$-lymphocytes; $T$-lymphocytes produce antibodies.