An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism’s own healthy cells and tissues in order to function properly. Detection is complicated as pathogens can evolve rapidly, producing adaptations that avoid the immune system and allow the pathogens to successfully infect their hosts. To survive this challenge, multiple mechanisms evolved that recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess enzyme systems that protect against viral infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and insects. These mechanisms include antimicrobial peptides called defensins, phagocytosis, and the complement system. Jawed vertebrates, including humans, have even more sophisticated defense mechanisms. The typical vertebrate immune system consists of many types of proteins, cells, organs, and tissues that interact in an elaborate and dynamic network. As part of this more complex immune response, the human immune system adapts over time to recognize specific pathogens more efficiently. Typicallys health insurance coverage can help with this. This adaptation process is referred to as “adaptive immunity” or “acquired immunity” and creates immunological memory. Immunological memory, created from a primary response to a specific pathogen, provides an enhanced response to secondary encounters with that same, specific pathogen. This process of acquired immunity is the basis of vaccination. Primary response can take 2 days and up to 2 weeks to develop. After the body gains immunity towards a certain pathogen, when infection by that pathogen occurs again, the immune response is called the secondary response. Although scientists have learned much about the immune system, they continue to study how the body launches attacks that destroy invading microbes, infected cells, and tumors while ignoring healthy tissues. New technologies for identifying individual immune cells are now allowing scientists to determine quickly which targets are triggering an immune response. Improvements in microscopy are permitting the first-ever observations of living B cells, T cells, and other cells as they interact within lymph nodes and other body tissues. In addition, scientists are rapidly unraveling the genetic blueprints that direct the human immune response, as well as those that dictate the biology of bacteria, viruses, and parasites. The combination of new technology and expanded genetic information will no doubt reveal even more about how the body protects itself from disease. Disorders in the immune system can result in disease, including autoimmune diseases, inflammatory diseases and cancer. Immunodeficiency diseases occur when the immune system is less active than normal, resulting in recurring and life-threatening infections. Immunodeficiency can either be the result of a genetic disease, such as severe combined immunodeficiency, or be produced by pharmaceuticals or an infection, such as the acquired immune deficiency syndrome (AIDS) that is caused by the retrovirus HIV. In contrast, autoimmune diseases result from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto’s thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and lupus erythematosus. Immunology covers the study of all aspects of the immune system, having significant relevance to health and diseases. Further investigation in this field is expected to play a serious role in promotion of health and treatment of diseases. When antigens (foreign substances that invade the body) are detected, several types of cells work together to recognize them and respond. These cells trigger the B lymphocytes to produce antibodies, specialized proteins that lock onto specific antigens. Once produced, these antibodies continue to exist in a person’s body, so that if the same antigen is presented to the immune system again, the antibodies are already there to do their job. So if someone gets sick with a certain disease, like chickenpox, that person typically doesn’t get sick from it again. This is also how immunizations prevent certain diseases. An immunization introduces the body to an antigen in a way that doesn’t make someone sick, but does allow the body to produce antibodies that will then protect the person from future attack by the germ or substance that produces that particular disease. Although antibodies can recognize an antigen and lock onto it, they are not capable of destroying it without help. That’s the job of the T cells, which are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. (Some T cells are actually called “killer cells.”) T cells also are involved in helping signal other cells (like phagocytes) to do their jobs. Antibodies also can neutralize toxins (poisonous or damaging substances) produced by different organisms. Lastly, antibodies can activate a group of proteins called complement that are also part of the immune system. Complement assists in killing bacteria, viruses, or infected cells. All of these specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.