Active immunity is obtained by

Where is the antigen-binding site located on an antibody?

Fill in the blanks: Our body, when it encounters a pathogen for the first time, produces a response called . . . . . . $a$ . . . . . . response, which is of . . . . . . $b$ . . . . . . intensity. Subsequent encounters with the same pathogen elicit a . . . . . . $c$ . . . . . . intensified . . . . . . $d$ . . . . . . response.

Which of the following types of antibodies activates $B$-cells?

For an organ transplant,it is an advantage to have an identical twin. Why?

In which lymphoid organ are all blood cells,including lymphocytes,produced?