Our immune system is an intricate network of specific and non-specific checks and balances that constantly interact. Its major functions are to differentiate self from non-self and to maintain host defenses against foreign substances and pathogens.

The first lines of defense are the body’s own natural barriers such as the skin and mucous membranes preventing potentially harmful bacteria from entering the body. The lining of the mouth, nose, and entire respiratory tract, the gastrointestinal tract, as well as the genitourinary tract, are composed of mucous membranes. These areas are well equipped with defense mechanisms forming effective barriers to infection. Tears, saliva, sweat and gastric acid juices have natural antibacterial potential. Even the natural microbial flora is regarded as a protective barrier.

The innate immunity is relatively non-specific and reacts similarly to all unknown invaders. Its major cellular components are phagocytes, including granulocytes, monocytes (white blood cells), and specialized tissue cells (macrophages). After migration into the tissue, monocytes become macrophages, which means "large eaters". Phagocytes are attracted to a site of infection or inflammation by biochemical messengers. Activated phagocytes develop arms to enclose bacteria before ingesting them into a vacuole (bubble) inside the cell. The killing process is usually confined to the intracellular vacuole which encloses the pathogen. One of these processes works by way of fusion with another vacuole which is filled with special enzymes and toxic agents that will kill and digest the bacteria. Macrophages and neutrophil granulocytes also have the capacity to take up oxygen and generate reactive oxygen containing molecules to kill bacterial pathogens inside the vacuole.

Phagocytosis is facilitated if the bacteria are being coated with complement proteins or antibodies. Antibodies are small molecules which are able to recognize previously encountered foreign substances (antigens) and thus are able to amplify the body’s immune response. Antibodies are part of the specific immunity. The specific immune components are more sophisticated, having the ability to learn, to adapt, and to memorize. Lymphocytes are the cellular component of the specific immunity and can be divided in to two major groups: the T-cells and B-cells. T-cells play a primary role in regulating and modulating immunological responses. Activated B cells (plasma cells) produce antibodies in order to facilitate recognition, phagocytosis and destruction of pathogens. Therefore, people overcoming a specific infection can develop acquired immunity, which means that from then on they are protected against that particular disease, e.g. German measles or small pox. These skills of our immune system are used for vaccination techniques.