Host responses against invading pathogens are basic physiological reactions of all living organisms. Since appearance of the first eukaryotic cells a series of defence mechanisms have evolved in order to secure cellular integrity, homeostasis and survival of the host. Invertebrates, ranging from protozoans to metazoans, possess cellular receptors which bind to foreign elements and differentiate self from non-self. This ability is in multicellular animals associated with presence of phagocytes, bearing different names (amoebocytes, haemocytes, coelomocytes) in various groups including animal sponges, worms, cnidarians, molluscs, crustaceans, chelicerates, insects and echinoderms (sea stars and urchins). Basically these cells have a macrophage like appearance and function and the repair and/or fight functions associated with these cells are prominent even at the earliest evolutionary stage. The cells possess Pathogen Recognition Receptors (PRRs) recognizing Pathogen Associated Molecular Patterns (PAMPS) which are well conserved molecular structures expressed by various pathogens (virus, bacteria, fungi, protozoans, helminths). Scavenger receptors, Toll-like receptors (TLRs) and Nod-like receptors (NLRs) are prominent representatives within this group of host receptors. Following receptor-ligand binding signal transduction initiates a complex cascade of cellular reactions which lead to production of one or more of a wide array of effector molecules. Cytokines take part in this orchestration of responses even in lower invertebrates which eventually may result in elimination or inactivation of the intruder. The evolutionary processes associated with these reactions are discussed.

Skin homeostasis is critical to preserve animal integrity. Although the skin of most vertebrates is known to contain a skin-associated lymphoid tissue (SALT), very little is known about skin B-cell responses as well as their evolutionary origins. Teleost fish represent the most ancient bony vertebrates containing a SALT. Due to its lack of keratinization, teleost skin possesses living epithelial cells in direct contact with the water medium. Interestingly, teleost SALT structurally resembles that of the gut-associated lymphoid tissue, and it possesses a diverse microbiota. Thus, we hypothesized that, because teleost SALT and gut-associated lymphoid tissue have probably been subjected to similar evolutionary selective forces, their B-cell responses would be analogous. Confirming this hypothesis, we show that IgT, a teleost immunoglobulin specialized in gut immunity, plays the prevailing role in skin mucosal immunity. We found that IgT ⁺ B cells represent the major B-cell subset in the skin epidermis and that IgT is mainly present in polymeric form in the skin mucus. Critically, we found that the majority of the skin microbiota are coated with IgT. Moreover, IgT responses against a skin parasite were mainly limited to the skin whereas IgM responses were almost exclusively detected in the serum. Strikingly, we found that the teleost skin mucosa showed key features of mammalian mucosal surfaces exhibiting a mucosa-associated lymphoid tissue. Thus, from an evolutionary viewpoint, our findings suggest that, regardless of their phylogenetic origin and tissue localization, the chief immunoglobulins of all mucosa-associated lymphoid tissue operate under the guidance of primordially conserved principles.