Which macromolecules provide protection against infectious agents?

Which of the following pairs of amino acids contains non-polar '$R$' groups?

$A$ bond formed between the $C$-terminal of one amino acid and the $N$-terminal of another amino acid is known as:

Identify $X$ in the given chemical structure: $H_3N^+ - CH(R_1) - C(=O) - X - CH(R_2) - COO^-$.

Which group in the structure of an amino acid is basic?

Egg albumin is an example of . . . . . . .