Xing-Hua Gao and Hong-Duo Chen - Practical Immunodermatology

1.1 An Overview to the Fundamental Concepts in Immunology

Immunology refers to the study of the structure and function of human immune system and the mechanism under which the body fights against infection. We live in an environment surrounded by invading pathogens, but we are rarely infected by these pathogens because of our immune system. Immunology describes the underlying process and battling between pathogens and the immune system, leading to either the clearance of pathogens and thus the cure of pathogen-induced diseases or the persistent invasion of microorganisms with a possible over-reactive immune injury to the body. Immunology shares a close connection with cellular biology, molecular biology, genetics, etc., turning this branch of science into one of the most cutting-edge and fundamental discipline in both life sciences and medical sciences.

Immunology is a relatively complex and newly growing science. Over 2000 years ago, it was discovered that people who survived infection during pandemic may exhibit resistibility against such infection during their second challenge, which may be referred to as immunity. Edward Jenner is always considered as the founder of modern immunology. He introduced the term vaccination by demonstrating that smallpox may be prevented by inoculating with cowpox in the eighteenth century. After almost two centuries of this elegant work, smallpox vaccination became universal, and in 1979 the World Health Organization announced its complete eradication. Disease is mainly caused by infectious agents that were defined as microorganisms by Robert Koch in the nineteenth century. We now classify infectious microorganism into four categories: viruses, bacteria, fungi, and parasites. In the 1880s, Louis Pasteur devised a vaccine against cholera in chickens and a rabies vaccine against bitten by a rabid dog. In the early 1890s, Emil von Behring and Shibasaburo Kitasato led to the first discovery of antibodies in the serum of animals immunized with diphtheria or tetanus that could confer short-lived protection against the effects of diphtheria or tetanus toxins in people.

A complete functional immune system includes immune organs, immune cells, and immune molecules (Table ). The immune system not only can recognize and clear pathogens but also may recognize mutated cells, senescent cells, and other toxic components. The three main functions of the immune system are immune defense, immune surveillance, and immune homeostasis. Immune defense prevents foreign pathogens from entering the body and clears pathogens and other toxic substances that are already inside of the body. Immune surveillance discovers and clears nonself components such as tumor cells, senescent cells, and apoptotic cells from the body. Immune homeostasis maintains the balance of the microenvironment by immunotolerance and immunoregulation.

Immune response is the process of the immune system to recognize and clear nonself substances from the system which may be divided into innate immunity and adaptive immunity (Table ). Innate immunity, or the innate immune response, is the first barrier against microorganisms. It involves nonspecific cells such as monocytes/macrophages, dendritic cells, granulocytes, etc., that recognize pathogen-associated molecule patterns (PAMPs) through their pattern recognition receptors (PRRs). Innate immune response is an immediate response that combats a wide range of pathogens without lasting for a long time. Adaptive immunity, or the adaptive immune response, is a specific immune response of T and B cells to induce a series of functional effects against a particular pathogen after self-activation, proliferation, and differentiation upon encountering this nonself antigen. In many cases, an adaptive immune response also results in the phenomenon known as immunological memory, which confers lifelong protective immunity to reinfection with the same pathogen. Innate immunity and adaptive immunity always coordinate with each other, and innate immune response is often the prerequisite to initiate an adaptive immune response.

On one side, the immune system provides defense against foreign pathogens; however, on the other side, when the equilibrium of the immune system is broken, it often results in either over-activation or over-inhibition of the immune responses that leads to serious immunopathology. For example, immune cells may attack self tissues and organs, which results in autoimmune diseases; severe inflammatory process induced by immune responses often results in hypersensitivity; immune responses against harmless antigens often result in allergy; defective immune responses often result in persistent infections; and tolerized immune responses often result in the progression of tumors.

Springer Science+Business Media Dordrecht 2017

Haoyu Sun 1, Rui Sun 1 and Zhigang Tian 1

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Immune system is mainly composed of immune organs, tissues, cells, and molecules, all of which interact with each other to perform immune functions. Immune tissues may also be referred to as the lymphoid tissues or lymphoid organs, which are divided broadly into central lymphoid organs and peripheral lymphoid organs.