What is your body's first line of defense against pathogens?
best guess: white blood cells

The body's first line of defense is actually the non-specific immune system which is also called innate immunity.

Innate immunity employs  both physical and chemical barriers that form the first line of defense. These include:

1. The intact skin. The skin is made up of keratin  in its outermost  layer. Keratin makes skin tough and resistant and pathogens usually cannot penetrate unless the skin is broken.

2. The mucus membrane. This is a thin membrane that forms a protective lining inside many organs and other parts of the body  such as the trachea. Mucus produced by the membrane traps bacteria and other harmful particle which are then expelled from the body.

3. Tears. Tears from our eyes contain lysozyme, a chemical that kills bacteria.

If the pathogen gets through or evades these types of defenses, then it is dealt with by the second line of defense which is called specific immunity or acquired immunity. It is here that  white blood cells namely the B lymphocytes and T lymphocytes  attack, kill or neutralize pathogens.