The immune response is divided into two branches: innate immunity and adaptive immunity. Innate immunity involves barriers that keep harmful materials from entering your body. These barriers form the first line of defense in the immune response. Adaptive immunity (acquired or learned immunity) is initiated in response to specific molecular shapes and/or sequences termed antigens. Immune response plays a vital role in protecting against infectious agents. It is the main impediment against the occurrence of disseminated infections that are usually associated with a high death rate. It is a well-known fact that for virtually all infectious diseases, the number of individuals exposed to infection is much higher than those actually presenting with a disease. This indicates that most persons are able to destroy these microorganisms and thus prevent the progression of an infection. Bacteria are the microorganisms that most frequently cause infections in humans. The natural barriers against infectious agents as well as innate and adaptive immunity participate in the protection mechanism against bacteria.