2.How does the immune system recognize and destroy the pathogenic bacteria?

Once bacteria enters into our body, the cells within our body recognize the bacteria. The cells have several pattern recognition receptors, which recognize components that are conserved among broad groups of bacteria. For example, these receptors recognize bacteria specific DNA, RNA and bacterial cell walls, respectively, but do not recognize product from ourselves. There are more than 30 receptors at the surface and inside of the cells.
However, our immune system must destroy only pathogenic bacteria because our body originally has many non-pathogenic and useful bacteria. It is still unclear how our immune systems identify only pathogenic bacteria but not non-pathogenic bacteria, though both groups of bacteria have the same components recognized by pattern recognition receptors.

After the recognition of bacteria, many molecules trigger the release of several cytokines. These cytokines are responsible for communication between white blood cells. These cytokines recruit one of the white blood cells, neutrophils and macrophages which eat and clear the bacteria. This step is called the innate immune response and it is early but non-specific.

Additionally, the macrophages present a part of bacterial components at the surface of the cells as a sign that bacteria is coming. Then, special types of white blood cells, T cells, contact the macrophages and recognize what kind of bacteria is coming. T cells induce cytokines and activate more macrophage. On the other hand, T cells contact with B cells, which are the other white blood cells, and these B cells produce antibodies which bind to a specific bacteria and eliminate them. This step, which is called the adaptive response, is slow but specific. The B cells memorize each specific pathogen and induce an earlier and stronger response if the pathogen is detected again.