The first line of defence in the immune system is provided by the skin and mucous membranes.

The immune system is a complex network of cells, tissues, and organs that work together to protect the body from harmful pathogens such as bacteria, viruses, and parasites. The immune system has two main lines of defence: the innate immune system and the adaptive immune system. The first line of defence is the innate immune system, which provides immediate, non-specific protection against pathogens.

The skin and mucous membranes are the first line of defence in the immune system because they act as physical barriers that prevent pathogens from entering the body.

Role of the skin:
The skin is the largest organ in the body and serves as a physical barrier between the internal organs and the external environment. It consists of multiple layers of cells that are tightly packed together, making it difficult for pathogens to penetrate. The outermost layer of the skin, called the epidermis, is composed of dead skin cells that provide an additional layer of protection. The skin also produces sweat and sebum, which contain antimicrobial substances that can kill or inhibit the growth of pathogens.

Role of mucous membranes:
Mucous membranes are found in various parts of the body, including the respiratory tract, gastrointestinal tract, and genitourinary tract. They are made up of a layer of epithelial cells that secrete mucus, a sticky substance that traps pathogens and prevents them from entering the body. Mucous membranes also contain specialized immune cells, such as macrophages and neutrophils, which can detect and eliminate pathogens.

The importance of the first line of defence:
The skin and mucous membranes play a crucial role in protecting the body from pathogens. By preventing pathogens from entering the body, they reduce the risk of infection and disease. However, if the first line of defence is breached and pathogens manage to enter the body, the second line of defence, which includes the inflammatory response and the complement system, is activated to eliminate the pathogens.

In conclusion, the skin and mucous membranes serve as the first line of defence in the immune system by acting as physical barriers and preventing pathogens from entering the body. They play a crucial role in protecting the body from infection and disease.