The first line of defense involves which structure (s) ?

T-cells

skin

blood

B-cells

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

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.