The skin

Your skin needs to be strong and protected, since it has the most contact with the outside world, and can constantly be damaged. So, evolution has created an ingenious solution; dead cells. Strong, tough cells are born from stem cells about a millimeter deep. Then, they get into a layer with Desmosomes, spikes. After that, they die and make an acid layer to form the top of the skin. This way, when pathogens, or harmful things, come, they will be trapped. Even if they eat a layer, the skin will just put another layer in its place. Every second, you produce and shed about 40 thousand dead skin cells. How fast do the pathogens have to barrel through to beat that? 

Your immune system also reaches an agreement with bacterium on the skin- they get to somehow live there, but they don't go through the skin. If they do, they will disappear. 

Your skin has a PH of 4.7 to 5.75. Therefore, it is acidic.