The main barrier that prevents the entry of micro-organisms into our body is

The correct answer is Skin.

The human immune system is structured into different lines of defense to protect the body from infections. The first line of defense consists of physical and chemical barriers that physically block pathogens from entering body tissues.

The skin serves as the primary physical barrier. Its outer layer, the epidermis, is made of closely packed, keratinized epithelial cells that form a tough, continuous, and impermeable shield. Additionally, secretions from sweat and sebaceous glands create an acidic environment on the skin's surface, which further inhibits the growth of micro-organisms. As long as the skin remains intact, it successfully prevents most micro-organisms from gaining entry into the body.

Here is why the other options do not prevent the entry of micro-organisms:
• Antibodies: These are protective proteins produced by the immune system in response to antigens. They act inside the body to neutralize pathogens that have already entered.
• Macrophages and Monocytes: These are types of white blood cells involved in phagocytosis. They detect, engulf, and destroy foreign particles, but they operate within tissues and the bloodstream only after pathogens have bypassed the body's external barriers.