Overview of Fish Immune System

Fish immune system can be categorized into innate and adaptive immune systems. The innate immune system comprises physical barriers such as mucus, skin, gill, gut and nostril epithelium, effector molecules such as antimicrobial peptides, interferons, complement proteins, natural antibodies, growth inhibitors, protease inhibitors and cells such as macrophages, monocytes, non-specific cytotoxic cells (NCC) and neutrophils. The innate immune molecules are generally heat labile and are more active at lower environmental temperature ranges. The onset of innate immune response occurs early in the life of teleosts than the adaptive immune response. The adaptive immune system comprises lymphoid organs such as head kidney, thymus, spleen and mucosa-associated lymphoid tissue and, the effector molecules comprise IgM, IgD and IgT, while the cells involved in the adaptive immunity are B and T lymphocytes. The age at maturation of the adaptive immune system of fish varies from species to species. Although the innate immunity is more diverse in fish, protection offered by adaptive immune system is also significant, especially when stimulated by vaccination. The adaptive immune response is modulated by various factors such as temperature, age, stress and route of vaccine administration and, hence, a better understanding of the immune system will help to formulate prophylactic measures such as vaccination to prevent disease outbreaks in aquaculture farms.