Final answer:
The body's immune system defends against pathogens through physical, chemical, and biological barriers, alongside inflammation and cellular responses of innate immunity. If pathogens bypass these defenses, the adaptive immune system's B and T cells provide a highly specific defense.
Step-by-step explanation:
The human body has developed a robust immune system that employs various defenses to protect against pathogens, which are disease-causing agents such as bacteria and viruses. The first line of defense includes physical and chemical barriers that prevent pathogens from entering the body. These barriers work non-specifically to protect against a wide range of pathogens, much like a castle's moat and high walls protect it from invaders.
Physical barriers, such as skin, mucous membranes, tears, and urine, essentially block pathogens mechanically. Additionally, chemical barriers like enzymes found in sweat and saliva work to destroy pathogens on the body's surfaces. Another vital part of this first line is biological barriers, which consist of beneficial bacteria that occupy spaces that could otherwise be infected by harmful bacteria.
Should these barriers be breached, the body's second line of defense kicks in, including inflammation and the innate immune system's cellular responses. This involves leukocytes, like macrophages and natural killer cells, which perform various functions such as phagocytizing pathogens and destroying cancerous or virus-infected cells. The complement system augments the ability of antibodies and phagocytic cells to clear microbes and damaged cells, marking pathogens for destruction and contributing to inflammation.
Lastly, the adaptive immune system, consisting of B and T cells, mounts a highly specific response against pathogens that have eluded the first line of defense. This specific immune response is tailored to the invading pathogen, ensuring a more effective and long-lasting protection.