Final answer:
The second line of defense against pathogens, once they've entered the body, involves phagocytic cells of the innate immune system, like macrophages and neutrophils, which engage in phagocytosis. Various mechanisms of resistance exist that allow certain pathogens to evade destruction. The immune system also deploys soluble mediators such as cytokines to support in combatting the invasion.
Step-by-step explanation:
If you were a pathogen that breached the external barriers, the second line of defense you would face are the cells of the innate immune system, which include a variety of phagocytic cells. These phagocytes, such as neutrophils, macrophages, and dendritic cells, would recognize and attack you, either as a virus or a bacterium, through a process called phagocytosis. During phagocytosis, the pathogen is engulfed and enclosed in a phagosome, which then fuses with a lysosome containing digestive enzymes, effectively killing many pathogens. However, some bacteria have evolved mechanisms to resist this process.
For viruses, while phagocytes can engulf and attempt to destroy them, specialized cells such as natural killer (NK) cells and certain types of white blood cells play a critical role. They can recognize and destroy virus-infected cells before the virus has an opportunity to replicate further. The innate immune system also uses soluble mediators like cytokines, complement, and interferon to combat pathogens. Some pathogens, such as Streptococcus pneumoniae and Staphylococcus aureus, have developed evasive techniques like capsules and toxins to hinder phagocytosis and adaptive immunity, posing unique challenges to the immune system's ability to eradicate them.