Lines of defence in vertebrates
The immune system is the body’s defence mechanism against harmful pathogens and foreign substances. It operates through a sophisticated network of barriers and responses designed to protect the body from infection and disease. This system is organised into three lines of defence: the first line includes physical and chemical barriers that prevent pathogens from entering the body; the second line consists of non-specific immune responses that target pathogens that breach the initial barriers, such as inflammation and phagocytosis; and the third line, the adaptive immune response, involves highly specific responses that provide long-lasting protection against previously encountered pathogens.
Innate defence mechanisms
Innate defences are the body’s first line of protection against pathogens and are an essential part of the immune system. Unlike the adaptive immune system, innate defences are non-specific, meaning they respond to a wide range of invaders without the need for prior exposure.
First line of defence
Skin and mucous membranes form the first line of defence against pathogens by serving as physical and chemical barriers that prevent harmful agents from entering the body.
Skin
The skin, composed of tightly packed cells in the outer layer (epidermis), acts as a barrier that is difficult for pathogens to penetrate. It also secretes antimicrobial substances, such as sweat and sebum, which contain enzymes and acids that inhibit the growth of bacteria and fungi, maintaining an acidic environment that is hostile to many pathogens.
Mucous membranes
Mucous membranes, found lining the respiratory, digestive, and urogenital tracts, are equally crucial in protecting internal tissues. They secrete mucus, a sticky fluid that traps pathogens, dust and other particles.
Cilia on the surface of some mucous membranes, like those in the respiratory tract, work together with mucus by sweeping trapped pathogens out of the body.
Additionally, enzymes such as lysozyme in mucus, saliva and tears can break down bacterial cell walls, providing a further layer of chemical defence.