What are the body's first lines of defence against pathogens?

The body's first lines of defence against pathogens are the skin, mucous membranes, and the immune system's innate response.

The skin is the body's largest organ and serves as a physical barrier against pathogens. It is composed of multiple layers of cells that are tightly packed together, making it difficult for pathogens to penetrate. Additionally, the skin is often dry and slightly acidic, conditions that are unfavourable for many types of bacteria and viruses. The skin also produces sebum, a substance that contains antimicrobial properties, further enhancing its protective role.

Mucous membranes line the body's internal surfaces that are exposed to the external environment, such as the respiratory, digestive, and urogenital tracts. These membranes produce mucus, a sticky substance that can trap pathogens. Some mucous membranes also have cilia, tiny hair-like structures that can move in coordinated waves to push trapped pathogens out of the body. For example, cilia in the respiratory tract can move mucus and trapped pathogens up to the throat, where they can be swallowed and destroyed by stomach acid.

The innate immune response is the body's non-specific defence mechanism against pathogens. It includes physical and chemical barriers, such as skin and mucous membranes, as well as cellular responses. For instance, if a pathogen manages to penetrate the skin or mucous membranes, it can be detected and destroyed by white blood cells called phagocytes. These cells can engulf and digest pathogens in a process called phagocytosis. The innate immune response also involves inflammation, which can help to isolate and destroy pathogens, and fever, which can inhibit the growth of some pathogens.

In addition to these first lines of defence, the body also has a more specific and sophisticated defence mechanism known as the adaptive immune response. This involves white blood cells called lymphocytes, which can recognise specific pathogens and mount a targeted attack against them. However, the adaptive immune response takes longer to develop than the innate response, so it is not considered part of the body's first lines of defence against pathogens.