The immune system provides protection against foreign invaders, such as viruses, bacteria, fungi and pollen. There are two distinct forms of defenses against foreign invaders: innate (non-specific) immune system and adaptive (specific) immune system. While only vertebrates possess specific immune responses, nearly all organisms have a non-specific immune response, including plants and animals. Innate immune response provides an immediate, although non-specific response, whereas the adaptive immune response is more specific.
In the immune system, there are two major mechanisms of defense which allow us to resist most infections. The innate immune system is the first line of defense against foreign organisms. If invading agents are able to evade the first line of defense, they´re detected by a second line of defense known as the adaptive immune system. The innate immune system exerts an immediate response upon infection to a variety of organisms. The adaptive immune system reacts only to specific antigens, and retains immunological memory to react more rapidly on subsequent encounters with a similar organism. Both systems depend on the ability to recognize self from non-self molecules.
Protection barriers of increasing specificity prevent pathogens, including viruses and bacteria from entering the body. An organism defends itself from invaders by implementing mechanical, chemical, and biological barriers. The skin is the first mechanical line of defense against infection. When invaders enter the body, other systems, including the lungs, urinary tract and intestines, react to protect the body. Coughing and sneezing mechanically eject pathogens from the respiratory system. Urine mechanically expels pathogens from the system. Mucus, when secreted, carries pathogens found in the gastrointestinal and respiratory tracts, out from the body.
Chemical fluids, including tears and saliva which contain an enzyme known as lysozyme, which functions as antibacterial, prevent foreign organisms from inhabiting the body. Breast milk contains antibacterial properties, as well. Vaginal secretions become slightly acidic to protect from infection, especially following menarche. Semen contains defensins which act actively against bacteria, fungi and many viruses. Gastric acid and proteases, in the stomach, function as effective chemical barriers against ingested microbes. Beneficial microorganisms in the stomach create conditions that prevent the survivability of harmful bacteria.
Inflammation is a nonspecific response to infection that occurs when the tissues of the body are damaged. The signs of inflammatory infection include redness, heat, swelling, and pain. Once infecting agents have entered the body, damaged tissues release chemical messengers, including histamines, leukotrienes, and prostaglandins, increasing blood flow, and causing vasodilation. Phagocytes and clotting factors are released by capillaries into the affected area. Phagocytes engulf pathogens and cellular debris in a process known as phagocytosis. Inflammation is the immediate reaction of damaged tissue, and it is the first stage of wound healing.
Fever is an abnormal increase in body temperature in response to pathogen invasion. Fever is stimulated by the immune response in an attempt to gain advantage over pathogenic agents, including viruses and bacteria, which react to the sense of temperature. Fever makes the body less favorable for the replication of viruses and bacteria. Biochemical substances called pyrogens induce the hypothalamus, which sits at the brain, to generate or retain more heat. This increases body temperature, producing a fever. Interferons (IFNs) are proteins that are released by host cells in the presence of pathogens. Interferons inhibit the replication of bacteria and increase the ability of uninfected cells to resists infection.
Fever can help fight infection; however, sometimes a fever may be so high that it can threaten the function of proteins adapted to the body´s typical temperature variations, leading to infarctions, cellular stress, seizures, and delirium. Whether the immune response mechanism is as complex as a fever and inflammation, or as uncomplicated as the physical barriers to infection and phagocytosis, both specific and non-specific defensive responses protect the body against foreign agents. According to a study, exposure to bacteria early in life, is essential for developing a healthy immune system.