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

T-cells

skin

blood

B-cells

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.

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