ABSTRACT
Introduction 143 Cells of the immune system 143 Organs of the immune system 144 Innate immune responses 145 Generation of B and T cells 146 Generation of B cells 146 Generation of T cells 147 Cellular immune responses 148 T-independent immune responses 148 T-dependent immune responses 149 Extrafollicular reaction 149
Germinal center reaction 149 Memory B cells and plasma cells 150 Relationship between normal lymphocytes 151
and lymphomas Mechanisms of lymphoma pathogenesis 152 Genetic lesions 152 Microenvironmental factors 152 The role of viruses 152 Conclusions 153 Key points 153 References 153
The human body is constantly under threat to be colonized by infectious agents, in particular viruses, bacteria, and parasitic protozoans. While some of these are rather harmless, others can cause life-threatening diseases. During evolution, animals developed numerous specific defense mechanisms to be able to combat infectious agents. The simplest mechanisms involve physical barriers that prevent entry of pathogens into the body. These are the outer and inner epithelial layers of the body. A further layer of defense is provided by the production of substances that have toxic effects on infectious particles, such as the acidic condition in the stomach or bactericidal enzymes secreted into the saliva. However, these simple defense mechanisms are not sufficient to prevent life-threatening infections. A much more sophisticated defense system has evolved in vertebrates, the immune system. This system is most highly developed in mammals and involves numerous organs and cell types. Different arms and functions of the immune system can be distinguished for particular types of immune responses. Innate immune responses use a restricted set of receptors for pathogen recognition, whereas adaptive immune responses rely on antigen recognition by highly diverse antigen receptors on B and T lymphocytes. There are two types of adaptive immune responses. Cellular
cells by cytotoxic T cells, whereas humoral immune responses are mediated through antibodies produced by B lymphocytes.
