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The daunting role of the immune system is to afford protection. It serves as a host defense system against infectious diseases and foreign (nonself) antigens. To accomplish this goal, the immune system is equipped with a rapid response mechanism, exquisite specificity, adaptability, an intricate regulatory network, and memory.

Over the past several decades, dramatic progress has taken place in the field of immunology. As a consequence, significant advances have been realized not only in the research realm but also in the diagnostic and clinical arena. These advances have allowed us to better understand how the immune system works and have provided insight into a variety of immune disorders, such as infectious diseases, allergy, autoimmunity, immunodeficiency, cancer, and transplantation. This information has led to better diagnosis, new treatment strategies, and improved management for patients with these disorders.

This chapter focuses on the basic principles of immunology, particularly as they relate to response to infection. More detailed discussions on the various aspects of the immune system are available in the reference section.

The Immune Response

As the immune system defends the host against pathogens, it uses different recognition systems to effectively eliminate the invading pathogen or its products. A response generated against a potential pathogen is called an immune response. The first line of defense, which is nonspecific to the invading pathogen, is rapidly mobilized at the initial site of infection but lacks immunologic memory and is called innate immunity. The second defense system is called adaptive immunity. It is specific for the pathogen and confers protective immunity to reinfection with that pathogen. Adaptive immunity can specifically recognize and destroy the pathogen because lymphocytes carry specialized cellular receptors and produce specific antibodies. A protein that is produced in response to a particular pathogen is called the antibody, and the substance that induces the production of antibodies is called the antigen. In summary, the innate immune response is effective and critical in eliminating most pathogens. However, if this initial mechanism fails, the adaptive immune response is induced that specifically confronts the pathogen and establishes immunity to that invading pathogen. Hence, both systems interact and collaborate to achieve the final goal of destroying the pathogen.


Innate immunity is an immediate response to a pathogen that does not confer long-lasting protective immunity. It is a nonspecific defense system and includes barriers to infectious agents, such as the skin (epithelium) and mucous membranes. It also includes many immune components important in the adaptive immune response, including phagocytic cells, natural killer (NK) cells, toll-like receptors (TLRs), cytokines, and complement.

Barrier Functions of Innate Immunity

Few microorganisms can penetrate body surfaces. These surfaces have an epithelial cell layer as their barrier, which is present in the skin, airways, gastrointestinal (GI) tract, ...

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