Figure 1. Phagocytic leucocyte from the earthworm that typifies this response in all multicellular invertebrates. Note that the cytoplasm is filled with numerous yeast particles to which the phagocyte has been exposed (×2000).

Figure 2. Hypothetical scheme of the putative innate immune response in earthworm involving cytotoxic factors. Septic injury provokes the release of coelomocyte cytolytic factor I (CCF) from coelomocytes. CCF possesses two distinct domains, i.e. the N-terminal domain interacts with lipopolysaccharide and 1,3-glucans and the C-terminal domain mediates its interactions with N,N-diacetylchitibiose and muramic acid. CCF can trigger the prophenoloxydase cascade (pro-PO) implicating serine proteases and fetidins (Fe). CCF also enhances phagocytosis by its opsonizing properties and potentiates lysis against various erythrocytes. CCF can be considered as a pattern-recognition molecule exerting a key role in the innate immune response of earthworm. Catalytic factors (fetidin, eiseniapore (EP), lysenins (Ly) are released from chloragocytes and coelomocytes. These proteins bind to and disturb the lipid bilayer only when particular sphingolipids are present and perform pore channels.

Figure 3. Transmission electron micrograph of the lymphocytic coelomocyte of the earthworm, involved in rejection of transplants and the killing of the tumour K562 in vitro. Note that the size of the cell is roughly that of a small lymphocyte in vertebrates. The cytoplasm is occupied almost entirely by the nucleus and the cytoplasm contains well-developed organelles: mitochondria, Golgi apparatus and endoplasmic reticulum. (×34 000). G, Golgi; N, nucleus, No, nucleolus; NC, nuclear cleft; M, mitochondrion; RER, rough endoplasmic reticulum. Reproduced from Salzet M, Tasiemski A and Cooper EL (2006) Innate immunity in lophotrochozoans: the annelids. Current Pharmaceutical Design 12: 3043–3050.

Figure 4. Agglutinin titre curve that represents the typical humoral response of the earthworm to an injection of antigen (rabbit red blood cell, RRBC). (a) Single injection of RRBC (blue circles) or saline (red circles); (b) Two injections of RRBC (green squares) 48 h apart.

Figure 5. The phylogenetic scheme of the animal kingdom. (a) Phylogeny of invertebrates; (b) Phylogeny of vertebrates (from Cooper, 1976).