Immunology of Invertebrates: Humoral

Phagocytosis in unicellular animals represents a ubiquitous form of immune defence. Multicellular invertebrates possess phagocytotic cells and have evolved more complex functions attributed to immune defence, for example opsonisation, encapsulation, melanisation and specialised humoral immune response. Thus, all animals possess innate, natural, nonspecific, nonanticipatory, nonclonal, germline host defence functions. Pathogen-associated molecular pattern (PAMPs) are conserved structures or motifs of microbes and are recognised by different families of pattern recognition proteins (Toll pathway, cascade of serine proteases, agglutinins/lectins), thereby activating an immunological response of the host. This enhanced specificity conveys a special form of memory in invertebrates. At the humoral response antimicrobial proteins, iron-binding proteins, phenoloxidase, matrix metalloproteases, small effector molecules (reactive oxygen species and nitrogen intermediates) and complement-like molecules are synthesised by immuno-/haemocytes and fat body and released at infectious tissue and into the haemolymph. The communication between and regulation of the humoral immune response is orchestrated by cytokine-like molecules and protease inhibitors, which also bridge as regulator molecules the humoral with the cellular defence system. Key Concepts: The phylogenetically ancient innate response attacks infectious DNA/RNA carriers from the moment of first contact and is the fundamental defensive weapon of multicellular organisms. The process of phagocytosis in unicellular organisms represents the most ancient immune defence, followed by opsonisation, encapsulation and melanisation in metazoa. The pathogen-associated molecular patterns (PAMPs) are conserved structures and motifs of microbes, and, when recognised by pattern recognition receptors (PRRs), initiate an immune response of the host. The nonself-recognition of PAMPs by PRRs is already a special form of memory of invertebrate hosts. The immune surveillance concept based on self- and nonself-discrimination/recognition is extended and fine-tuned by the danger model. The important key genes and pathways of vertebrate immunity have much earlier origins and are expressed in some of the simplest of true animals. The invertebrate immunity is an example to understand a Darwinian principle of evolution that complex systems develop through small sequential steps. Keywords: phagocytosis; opsonisation; melanisation; pathogen-associated molecular patterns; pattern recognition proteins; Toll-like receptors; C-type lectins; antimicrobial peptides; phenoloxidase; iron-binding proteins