Intact skin and mucous membranes: First line of defense. 2. : Second line of defense. 3. : Third line of defense. 4. : Innate defense system. 5. : Adaptive defense system.

Skin and mucous membranes are the First line of defense of the body against disease.

First line of defense means that it is the first part which stops the disease infection and prevent it from infecting the animal body.

Skin also protects the body due to its rigid surface which does not allow pathogens inside the body.

Mucous contains white blood cells which attacks on the disease causing microorganisms and kill them before harming the body.