Mast cell is a cell of $A$ of vertebrate animals. Mast cells mediate inflammatory responses such as $B$. Mast cells store a number of different chemical mediators including $C$ and $D$. Here $A$ to $D$ refers to:

• A
  $A-$immune system,$B-$allergic reactions,$C-$histamine,$D-$serotonin
• B
  $A-$bone marrow,$B-$allergic reactions,$C-$mucous,$D-$cerumen
• C
  $A-$thymus,$B-$allergic reactions,$C-$histamine,$D-$cerumen
• D
  $A-$immune system,$B-$allergic reactions,$C-$cerumen,$D-$serotonin