Human body's immune response.

The primary function of the immune system is to defend the body frompathogens, which are disease-causing organisms such as vi- ruses and bacteria. Tissues, cells, and proteins in the immune sys- tem work together to achieve this function.

How Immunity Works To fight infections, the immune system must be able to identify pathogens. Pathogens have molecules calledantigenson their sur- face. Antigens provide a unique signature for the pathogen that en- ables immune system cells to recognize different pathogens and dis- tinguish pathogens from the body’s own cells and tissues. When a pathogen gets into the body, the immune system reacts in 2 ways.

•Theinnate immune responseis a rapid reaction. Innate immune cells recognize certain molecules found on many pathogens. These cells also react to signaling molecules released by the body in re- sponse to infection. Through these actions, innate immune cells quickly begin fighting an infection. This response results in inflam- mation. The cells involved in this reaction can kill pathogens and can also help activate cells involved in adaptive immunity.

•Theadaptive immune responseis slower than the innate re- sponse but is better able to target specific pathogens. There are 2 main cell types involved in this response: T cells and B cells. Some T cells kill pathogens and infected cells. Other T cells help control the adaptive immune response. The main function of B cells is to makeantibodiesagainst specific antigens. Antibodies, also known as immunoglobulins, are proteins that attach themselves to patho- gens. This signals immune cells to destroy the pathogen.

It takes time for T and B cells to respond to the new antigens when a pathogen causes an infection. Once exposed to the pathogen, these cells develop a memory for the pathogen so that they are ready for the next infection. As part of the adaptive immune response, some T and B cells change into memory cells. Memory cells mostly stay in the lymph nodes and the spleen and “remember” particular antigens. If a person becomes infected with the same pathogen again, these cells are able to quickly and vigorously begin fighting the infection.

Disorders of the Immune System Immunodeficiencyresults when the body does not have enough of certain kinds of immune cells or the cells do not function prop- erly. When that happens, a person is more vulnerable to infections.

Immunodeficiency can be primary (genetic) or secondary (due to other conditions). Secondary immunodeficiency can be caused by •Medications: steroids, chemotherapy drugs, other drugs that sup- press the immune system•Medical conditions: diabetes, kidney disease, liver disease •Infection: HIV, which can lead to AIDS •Other conditions: malnutrition, surgery, trauma, extremes of age (newborn and elderly) Autoimmune diseaseoccurs when the immune system over- reacts against the body’s own cells and tissues. Lupus, multiple scle- rosis, rheumatoid arthritis, and celiac disease are all types of auto- immune disease.

Author:Amy E. Thompson, MD Sources:Chinen J, Shearer WT. Secondary immunodeficiencies, including HIV infection.J Allergy Clin Immunol. 2010;125(2)(suppl 2):S195-S203.

Haynes BF, Soderberg KA, Fauci AS. Introduction to the immune system. In: Longo DL, Fauci AS, Kasper DL, et al, eds.Harrison’s Principles of Internal Medicine. 18th ed. New York, NY: McGraw-Hill; 2012:2650-2685.The JAMA Patient Page is a public service ofJAMA. The information and recommendations appearing on this page are appropriate in most instances, but they are not a substitute for medical diagnosis. For specific information concerning your personal medical condition,JAMAsuggests that you consult your physician. This page may be photocopied noncommercially by physicians and other health care professionals to share with patients. To purchase bulk reprints, call 312/464-0776. Pathogens INFECTED CELL ED Some T and B cells become memory cells that quickly fight future infections by the same pathogen Innate immune cells engulf and kill pathogens and release molecules to enhance the immune response DD ADAPTIVE IMMUNE RESPONSE T cells kill pathogens and attack infected cells B cells make antibodies that target specific pathogens IMMEDIATE RESPONSE DELAYED RESPONSE RER INNATE IMMUNE RESPONSE INNATE IMMUNE CELLS T CELL age s B CELL Signaling molecules The Immune Response Time FOR MORE INFORMATION •National Institute of Allergy and Infectious Diseases www.niaid.nih.gov/topics/immunesystem/pages/features.aspx •National Library of Medicine www.nlm.nih.gov/medlineplus/ency/article/000818.htm To find this and previous JAMA Patient Pages, go to the Patient Page link onJAMA’s website atjama.com.Many are available in English and Spanish.

1686 JAMAApril 28, 2015 Volume 313, Number 16(Reprinted)jama.com Copyright 2015 American Medical Association. All rights reserved.Downloaded From: http://jamanetwork.com/pdfaccess.ashx?url=/data/journals/jama/933829/ by a UNIV OF MINN LIBRARIES User on 03/26/2017