Invertebrates are pluricellular animals without vertebral column, articulated internal skeletum and they constitute the largest subphylum of the animal kingdom where there are included: arthropods (spiders, insects, millipedes, crustaceans), molluscs (clams, squid, octopus, snails), Porifera (sponges), Coelenterata (Jellyfish corals, polyps) echinoderms (starfish, sea urchins) Platyhelminthes (flat worms and parasites ), nematodes (roundworms), annelids (earthworms, leeches).

To explain the immunological complexity and variability of invertebrates we must take a look at their origin. The appearance of pluricellular animals is the consequence of the association of several cell types, then specialization appears and, among specializations, specialized cells develop in defensive functions of the organism, known as celomocytes if the invertebrate has celomic cavity, or haemocytes if it has not. So, both celomocytes and haemocytes are the phylogenetic novelty, as a contribution of invertebrates appearance to development of the immunitary system.

Some mechanisms present in protozoa are also present in invertebrates (calcification and melanization) and some new substances like pentraxines appear, which may be considered as a novelty in the invertebrates and phylogenetic predecessors of immunoglobulins in the vertebrates.

1) Physico-chemical barriers

a) the exoskeleton of arthropods is composed by chitin polysaccharide, a polymer with straight and simple chains ( non-ramified ) of N-acetyl-2-D-glucosamine, a monosaccharide that includes nitrogen in its composition. This layer, secreted by hypodermis, may contain calcium carbonate (crustaceans, crabs, lobsters) and sediments of excretion products sometimes with an antimicrobiane function just like cecropin and drosomicin, isolated on insects.

b) gelatinous excretions like the mucous secretions of annelids and molluscs, composed of mucopolysaccharides which may also contain antibacteriane peptides, enzymes (lysozyme) and glycoproteins.

2) Cellular response

a) phagocytosis based in the activity of celomocytes or haemocytes, recently mentionned. Those cells act by 3 processes: chemotaxis (approaching), recognition (adherence) and activation of hydrolases (digestion in lisosomes). This activity is transmitted during evolution to vertebrates macrophages.

b) capsulation of pathogens by forming a multicellular cover of celomocytes and sediments of melanin ( memories of the defensive mechanism of unicellular protozoa ).

c) formation of nodules as an evolution of previous process of capsulation.

d) recognition of own and strange compounds, made by a subpopulation of celomocytes or haemocytes which might be the predecessors of lymphocytes Tc and T in pluricellular vertebrated animals.

e) cytotoxicity derived from transformed cells lysis, done by celomocytes or haemocytes. This activity would be the predecessor of natural killer cells (NK) in pluricellular vertebrated animals.

3) Preserological response of soluble compounds.

a) opsonins which cover foreign agents and facilitate phagocytosis.

b) agglutinins to cause agglutination of foreign agents. For instance, tachilepsin present in blood serum of Limulus polyphemus.

c) lysins, and among them, lysozime to produce breaking of strange structures.

d) cyclic and lineal peptides.

e) pentraxins of proteinaceous kind which may be considered the ancestors of vertebrates immunoglobulins.