The immune system is the body's resistance system against attacks by foreign substances that are exposed to our bodies. These foreign substances can come from outside or within the body itself. Examples of foreign substances that come from outside the body (exogenous) are bacteria, viruses, parasites, fungi, dust, and pollen. Meanwhile, foreign substances from inside the body can be in the form of dead cells or cells that change their shape and function. These foreign substances are called immunogens or antigens.

If an immunogen is exposed to our body, our body will respond by forming an immune response from the immune system. The immune system is literally a beneficial self-defense system, but under certain conditions it can create adverse circumstances.

The immune response is divided into 2 phases, namely the innate immune response phase and the adaptive immune response phase. A natural immune response will occur at the beginning of exposure to immunogens in our body. If this natural immune system can defend the body from immunogen attacks, then we will not suffer from illness (first phase). Conversely, if the natural immune system cannot defend against immunogen attacks, then we will get sick/infected (second phase).

The body's cells that work in the immune system (immune system cells) are a group of white blood cells (leukocytes). In carrying out their duties, leukocyte cells are divided into 2 groups. The first group plays a role in the natural immune system, including macrophage cells, neutrophil cells, eosinophil cells, and dendritic cells; called APC cells (antigen presenting cells). APC cells are cells that are tasked with recognizing and processing immunogens, which will later be handed over to cells that play a role in adaptive immune responses. In addition to APC cells, there are NK (natural killer) cells that play a role in the natural immune response. The second group of cells are cells that play a role in the adaptive immune response, namely B lymphocyte cells (which produce antibodies) and T lymphocyte cells which play a role in producing cytokines. These cytokines will activate cells that play a role in the immune system to be more active in defending the body against microbial attacks that are highly infective, such as gram-negative bacteria, gram-positive bacteria, and viruses.