Explain why you can't become immune to a pathogen that has only encountered the first two lines of defence.

The immune system is composed of three lines of defence: the first line of defence (physical and chemical barriers), the second line of defence (innate immunity), and the third line of defence (adaptive immunity).

1. The first line of defence includes physical barriers like skin and mucous membranes, and chemical barriers like stomach acid and enzymes in tears and skin oils. These barriers aim to prevent pathogens from entering the body.

2. The second line of defence is the innate immune response, which is non-specific and reacts the same way to all pathogens. This includes the inflammatory response, fever, and the actions of phagocytic cells that engulf pathogens.

3. The third line of defence is the adaptive immune response, which is specific and tailored to each pathogen the body encounters. This response involves lymphocytes (B cells and T cells) and the production of antibodies.

The reason you can't become immune to a pathogen that has only encountered the first two lines of defence is because immunity is largely based on the third line of defence, the adaptive immune response. This response "remembers" pathogens it has encountered before, allowing it to respond more quickly and effectively when it encounters the same pathogen again. This is the basis of immunity.

If a pathogen only encounters the first two lines of defence, the adaptive immune response is not activated and the body does not "remember" the pathogen. Therefore, you cannot become immune to it.