Understanding the vertebrate immune system: insights from the reptilian perspective

Abstract

Reptiles are ectothermic amniotes, providing the key link between ectothermic anamniotic fishes and amphibians, and endothermic amniotic birds and mammals. A greater understanding of reptilian immunity will provide important insights into the evolutionary history of vertebrate immunity as well as the growing field of eco-immunology. Like mammals, reptile immunity is complex and involves innate, cell-mediated and humoral compartments but, overall, there is considerably less known about immune function in reptiles. We review the current literature on each branch of the reptilian immune system, placing this information in context to other vertebrates. Further, we identify key areas that are prime for research as well as areas that are lagging because of lack of reagents in non-model systems. * Adaptive immunity : Antigen-specific immune responses, which includes the development of memory. Requires prior exposure to mount a full response. Allograft : Tissue transplant between individuals from the same species but with different genetic makeup. Antigen : Any substance that will elicit a specific immune response. Basophil : Leukocyte which, when triggered by an antigen, degranulates and releases histamine. Release in reptiles is dependent upon antigen concentration and temperature. Chemokine : Small cytokines involved in directing the migration of phagocytic cells and lymphocytes. Complement system : A series of proteins that exist in the plasma and kill invading bacteria through opsonization or by lysis. Three pathways exist: classical, alternative and lectin. Concanavalin A (Con A) : A lectin from the jack bean ( Canavalia ensiformis ) that stimulates T cells to proliferate. Cytokine : Low molecular weight substances that mediate cellular interactions. Cytotoxic T cell : A T cell involved in killing other cells. Expresses CD8 marker. Defensin : Antimicrobial proteins that have a characteristic β-sheet-rich fold and six disulfide-linked cysteines. They have been found in mammals, birds and reptiles. Eosinophil : Leukocyte that plays key role in defense against parasites. Germinal centers : Sites in secondary lymphoid tissue where intense selection, proliferation and maturation of B cells occurs. Granuloma : A mass of cells at the site of persistent inflammation. Chronic granulomas in reptiles can be heterophilic, which consist of degranulated and necrotic heterophils, or can be histiocytic, which consist of necrotic macrophages. Helper T cell (TH cell) : A T cell that regulates other immune cells. Expresses CD4 marker. Two types of TH cells exist: TH1, which regulates cell-mediated immunity, and TH2, which regulates humoral immunity. Heterophil : Leukocyte that is functionally equivalent to mammalian neutrophils. Helps to suppress microbial invasion while also involved in the inflammatory response. Histamine : An amine that triggers the inflammatory response while causing dilation of local blood vessels and smooth muscle contraction. Immunoglobulin (Ig) : A general term for an antibody. Each Ig consists of two heavy chains and two light chains held together by disulfide bonds, and has two sites that can bind antigen. Innate immunity : A rapid, non-specific defense that does not require previous exposure to mount a full response. Involution : Reduction in the size of an organ. Leukocyte : White blood cell. Lymphocyte : A T or B cell. Express antigen-specific receptors. Macrophage : A monocyte that has entered the surrounding tissue, a macrophage is phagocytic and processes and presents antigens while also releasing cytokines. MHC : Major histocompatibility complex. A cluster of genes that encodes for molecules displayed on the cell surface that are involved with interactions with T cells. Mitogen : A substance that triggers the proliferation of lymphocytes. Monocyte : Phagocytic leukocyte found in blood that is the precursor to tissue macrophage. Neutrophil : A granular leukocyte that helps suppress microbial invasion and is also involved in the inflammatory response. Opsonization : The binding of an antibody or a component of complement to a particle to mark it for ingestion which leads to enhanced phagocytosis by phagocytic cells. Phagocytic cell : A cell that can engulf particles or microorganisms. Phytohemagglutinin (PHA) : Mitogen from red kidney beans that polyclonaly activates T cells. Red pulp : Area of the spleen that contains a high number of red blood cells and macrophages. Spleen : Secondary lymphoid organ that filters blood and traps foreign substances. Contains red and white pulp. T cell : Lymphocyte that differentiates in the thymus. Thymus : Primary lymphoid organ involved in T cell differentiation. White pulp : Follicles within the spleen that are rich with lymphocytes.