ABSTRACT
The immune system of fishes, as in other vertebrates, generally consists of two intercommunicating processes: innate and adaptive immunity. Cells and molecules involved in the non-specific recognition of foreign particles and organisms, largely pathogens, constitute the natural immune response, while specific mechanisms capable of discriminating between self and non-self via cell-mediated and humoral responses comprise the so-called adaptive immune system. The chapter reviews the appearance during fish ontogeny of both innate and adaptive immunity as well as of the cells and tissues involved in these reactions. Lymphocytes, the specialised cell type of adaptive immunity, and also natural killer cells, macrophages, granulocytes and other cell types that are involved in innate immune responses, are derived from multipotent haematopoietic stem cells.
