How many heavy polypeptide chains are present in each antibody molecule?

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.

Interferons curb the infection of:

Interferons are:

Explain Acquired immunity.

$NK$ cells are which type of cells?