Figure 2.

The main invertebrate haemocytes involved in immune response. Pro-haemocyte. Immature cell identified as pro-haemocyte or lymphocyte-like cell. These cells, present in ascidians, crustaceans, insects and probably in the haemopoietic tissues of other invertebrates, are able to differentiate in mature haemocytes. The undifferentiated pro-haemocyte is small, with a big nucleus containing a large amount of heterochromatin and a prominent nucleolus. Amoeboid phagocytes are motile vacuolated cells present in annelids, insects, echinoderms and ascidians. Depending on the species, amoeboid phagocytes are involved in phagocytosis, migration, wound repair, non-self-recognition, transplant reaction, cytotoxicity, encapsulation, endocytosis, and enzymatic digestion of engulfed material. Granular cells are mature cells found in ascidians, crustaceans, insects and bivalves. They are able to synthesize a number of cytotoxic and defense factors and store them in granules. Degranulation occurs upon challenge with stressors. Hyaline cells are vacuolated or non-vacuolated cells, abundant mostly in ascidians, crustaceans, and bivalve molluscs. They are mainly involved in phagocytosis. In ascidians, hyaline cells rapidly clump together in vitro. Oenocytoid cell. These cells are widely present in insect species. They are large cells with a low nuclear-cytoplasmic ratio, which show phenoloxidase activity in the cytoplasm. This suggests that oenocytoid cells could play a role in the melanization process. Spherule or morula cell. These haemocytes, present in some cnidarians, annelids, insects, echinoderms, and ascidians, are berry-shaped cells, sometimes pigmented, with highly refractive cytoplasmic inclusions. They are actively involved in encapsulation and synthesize, transport and release various defensive factors during infections, including antimicrobial proteins, cytotoxic factors, and opsonins. Lamellocyte. These flat cells with adhesive properties are present in insects, in particular in Diptera. Lamellocytes appear in the lymph glands and haemolymph during larval development and differentiate in response to parasite infection. They are active in neutralizing and encapsulating materials recognized as “non-self,” too large to be phagocytosed.