The immune systems line of defense against infectious diseases includes the skin

First line of defense.

Explanation: The immune system's way of protection initially begins using the skin to protect against any physical and chemical harm. This is called the first line of defense. Amongst other first line defense mechanism includes; tears to protect from harmful substance in dust and air, cillia inside the nose, the internal mucosal linings to protect against harsh chemicals as well as freindly bacteria called our normal microflora.

nucleic acid molecules are incredibly complex, containing the code that guarantees the four different nucleotide units comprise dna, the nucleic acid of interest to the genealogist. and one of four nitrogen-containing molecules called nucleotides