The correct answer is option 3, skin.

Skin is considered the first line of defence for the body because it acts as a physical barrier between the external environment and the internal organs. It covers the entire body surface and helps to prevent the entry of harmful microorganisms, chemicals, and other harmful substances into the body.

The outermost layer of the skin, called the epidermis, is composed of tightly packed cells that provide a waterproof barrier. This barrier helps to prevent the loss of water from the body and keeps the internal organs protected.

In addition to acting as a physical barrier, the skin also contains sebaceous glands that produce sebum, an oily substance that helps to keep the skin moisturized and prevents the growth of certain bacteria.

Although options 1 and 2, white blood cells (WBC) and antibodies, are important components of the immune system and play a role in defending the body against infections, they are not considered the first line of defence.

Option 4, nails, do provide some protection to the fingertips, but they are not considered the first line of defence for the entire body.

Therefore, the correct answer is skin.