Question

The first line of defense involves which structure(s)?
T-cells
skin
blood
B-cells

Answer 1

The first line of defense is skin, since it is the epidermis (top layer of skin) that protects all internal fluids and structures.

Answer 2

Skin.
The first being the outer barrier like skin, the second being non-specific immune cells like macrophages and dendritic cells, and the third line of defense being the specific immune system made of lymphocytes like B- and T- cells.