The Immunology of Teleosts

The Latin ‘immunis’ means ‘exempt from’, and the term immunology is used to mean the study of defence mechanisms against infectious disease. Defence mechanisms fall into two broad categories: nonspecific and specific. The former are found in all living organisms and are termed nonspecific because the response can be elicited by a variety of very different stimuli ranging from infectious agents to inorganic irritants such as talcum powder. Specific mechanisms are found only in vertebrates and are specific in that the response is elicited by and directed towards the molecular structure of the stimulus and involves an adaptive change in the lymphoid system resulting in specific immune memory. Molecules capable of eliciting a specific immune response (e.g. antibody production) are termed antigens. Usually, whole protein molecules or even whole microorganisms are referred to as antigens , but it must be remembered that large molecules have many different sites which are capable of eliciting and binding to antibodies with different specificities. These sites are termed antigenic determinant sites , or epitopes. While the defence mechanisms can be categorised into specific and nonspecific groupings, it is important to realise that in vivo they act in concert with each other, the two systems being interdependent in many ways. For example, macrophages are capable of phagocytosing many bacteria but this capacity is greatly enhanced if the bacteria are coated with antibody. On the other hand, macrophages are required to process antigens before the lymphoid system can respond in a specific manner. The sum total of defence factors responsible for immunity to a particular disease is a highly complex interrelationship of specific and nonspecific mechanisms. This chapter will describe the mechanisms of defence against infectious agents, and it is convenient to organise this description with regard to the chronology of events which occur during the course of infection. The first-line defence mechanisms include all the nonspecific factors since they are constitutive (i.e. a part of the normal body constituents), although the levels or activities of these factors may change during infection. The specific immune mechanisms, requiring adaptive changes in the lymphoid system, form the second line of defence.