Next are chapters that deal with the great variety of diseases involving immunological components. These vary from ineffective or absent immune responses (immunodeficiency) to those produced by aberrant immune responses (hypersensitivity) to responses to self‐antigens (autoimmunity). This is followed by chapters that describe the role of the immune response in transplantation and discuss antitumor reactions. Chapter 19 focuses on the spectrum of microorganisms that challenge the immune system and how immune responses are mounted in a vigilant, orchestrated fashion to protect the host from infectious diseases. Included is a discussion of immunoprophylaxis using vaccines that protect us from variety of pathogenic organisms. Without question, the successful use of vaccines helped revolutionize the field of medicine in the twentieth century. What lies ahead in the twenty‐first century are research efforts related to the development of crucial new vaccines to protect humankind from naturally occurring pathogenic microorganisms and viruses (most notably HIV and the novel coronavirus responsible for the recent COVID‐19 pandemic), as well as those that have been engineered as potential biological weapons, or have yet to be identified.
With the enormous scope of the subject and the extraordinary richness of detail available, we have made every effort to adhere to fundamental elements and basic concepts required to achieve an integrated, if not extensive, understanding of the immune response. If the reader’s interest has been aroused, many current books, articles, and reviews, and growing numbers of educational internet sites, including the one that supports this textbook (see the preface), are available to flesh out the details on the scaffolding provided by this book.
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2 CELLS AND TISSUES OF THE IMMUNE SYSTEM
INTRODUCTION
The immune system includes a wide variety of cells that function independently and in concert with one another. Chapter 1 introduced the major immune cells derived from lymphoid or myeloid progenitor cells in the bone marrow. In this chapter, we will further define the functional and phenotypic characteristics of these cells. In addition, the major lymphoid organs will be described which include the bone marrow, thymus, lymph nodes, and spleen. Given the coordination required for optimal immune responses to fully manifest, the anatomy of the immune system, including the strategic location of its major organs, is designed to maximize the ability of immune cells to efficiently detect and respond to antigens and to then disperse effector mechanisms throughout the body. A major route of immune cell and antigen transport is the lymphatic system.
THE LYMPHATIC SYSTEM
The lymphatic system is part of the vascular system comprising a network of lymphatic vessels that contain a clear fluid called lymph. Lymph is a fluid similar to plasma and contains lymphocytes, waste products and cellular debris, and, importantly, bacteria and proteins that stimulate immune responses. All interstitial spaces are drained by the lymphatic system, ensuring that wherever a foreign, potentially infectious pathogen enters the body, it will be swept away and deposited within a draining lymph node where antigen‐presenting cells and antigen‐specific T and B cells can initiate an immune response (Figure 2.1).
Primary and Secondary Lymphoid Organs
The organs in which lymphocyte maturation, differentiation, and proliferation take place are divided into two categories: primary and secondary organs. The primary or central lymphoid organs are those in which the maturation of B and T lymphocytes into antigen‐recognizing lymphocytes occurs. In other words, these are the organs where gene rearrangements occur to generate functional antigen‐specific BCRs and TCRs expressed by B and T cells, respectively. Mature B cells differentiate to fully mature cells within the bone marrow. Historically, the term “B cell” is derived from developmental studies in birds that demonstrated that antibody‐forming lymphocytes differentiate within an organ unique to birds called the bursa of Fabricius (hence, “B” for bursa). In contrast, T cells differentiate only partially within the bone marrow. Precursor cells destined to become mature T cells undergo final maturation within the thymus gland (hence, “T” for thymus). Histological characteristics of the thymus are discussed in the next section.
Mature B and T cells migrate through the bloodstream and lymphatic system to the peripheral lymphoid tissues, including the lymph nodes and spleen. Collectively, these are referred to as the secondary lymphoid organs and this is where antigen‐driven activation (proliferation and differentiation) of B and T cells takes place (Figure 2.2). Histological properties of the secondary lymphoid organs are also presented below.
Figure 2.1. Lymphoid fluid draining interstitial spaces.
Source: © John Wiley & Sons, Inc.
