The defence system of gastropods lack any anamnestic reactions, i.e. any type or equivalent of memory cells, a phenomenon that is true of all invertebrates. They can distinguish, at the species level, between "own" and "foreign" but not, at the individual level, between "self" and "not-self". For the latter reaction, the interaction of particular cells are necessary under MHC restriction. However, invertebrates do not possess any equivalent to the major histocompatibility complex. These facts are relevant for the effects of the defence reaction at the epidemiological level. In short, a double infection by the same pro- or eukaryotic pathogen is statistically improbable in r-strategists because of their relatively limited life expectancy. However, because of their mostly seasonal emergence, invertebrate populations need an immediate (i.e. without latency) defensive reactivity against microbial and viral pathogens transmitted contaminatively or by contact. At the beginning of the relevant season, the snail population builds up. Its population density increases rapidly and a directly transmitted microbial infection would spread quickly throughout it, because a high population density favorites transmission. Therefore, an immediately acting defence mechanism is necessary. As mentioned above, the trematodesí cercariae are not transmitted from snail to snail, and thus the increased population density is rrelevant.
The IDS comprises humoral- and cell-mediated components that cooperate in the recognition and killing of invaders. At least four types of cells are involved, three of which are sessile. (1) Endothelial cells that fix foreign substances delivered by the epithelial lining of the body cavities. These cells should not be named "antigen-fixing" because antigens are substances that induce acquired immune reactions. (2) Reticulum cells fixed to tissues by extra-cellular fibrils. Both types phagocytoze many foreign particles. (3) Pore cells associated with the reticulum cells. (4) Mobile cells, generally named haemocytes, are the most important cells. They float in the haemolymph and migrate in gastropods as amoebocytes through the tissues because of the open circulatory system. They are heterogeneous with respect to their morphology and biochemical reactions. Some are named "round cells" or hyalinocytes because of their small nucleus. They harbour few lysosomes, disperse on glass only to a limited extent, form few pseudopodia and phagocytoze hardly any particles. Other cells, named granulocytes, exhibit these functions fully. Oxygen radicals have often been found in their phagosomes. Phagocytozing haemocytes show luminol-dependent chemi-luminescence; therefore, metabolites of oxygen including H2O2 are created.
Invertebrates recognize foreign particles by Toll-like receptors (TLRs) and lectins. The latter are secreted by haemocytes and fix carbohydrates specifically and agglutinate particles. In some cases, they function as opsonins supporting phagocytosis. Intensive contact with foreign material mostly induces a state of increased reactivity, sometimes lasting for two months. This is caused by a specifically altered reactivity of the haemocytes. However, after a return to the normal state, this increased reactivity cannot be transferred passively by haemocytes from a donor snail to other recipient snails. They react equally against this as to other foreign specifities. No alterated secondary reaction occurs with the same foreign specifity; in short, there is no immune memory.
Genetically resistant and susceptible strains of Biomphalaria glabrata have been found. Their plasma can be distinguished with respect to its influence on floating haemocytes. In experiments in vitro, the plasma of resistant snails increases the phagocytozing activity of red blood cells, whereas that of susceptible snails reduces and delays this reactivity. Therefore, in vivo, a parasite could escape the defence reaction by masking itself or by other processes.
A phenotypically increased resistance that is genetically based but with low or no specifity can be found and selected from wild populations. The polymorphism with respect to the susceptibility guarantees that, even at hyperendemic conditions, not all individuals of an IH population will be infected. The different genetically based multiple resistance factors are present at limited rates and in variable combinations. They can be accumulated experimentally by breeding but do not accumulate under natural conditions, irrespective of whether the parasite is present. In the field, only a small proportion of snails harbour the parasite; the others may be resistant or, more often, have not had contact with the parasite. Resistant and susceptible snails persist permanently, a feature that is a prerequisite of the permanent coexistence of a parasite and its host.