**2. Molecules in the Innate Immune System of** *A. fulica*

#### **2.1. Coagulation system in** *A. fulica*

Invertebrates are not able to synthesize immunoglobulins, rather they have developed a potential defense system against microbial surface antigens such as lipopolysaccharides (LPS)/endotoxins and glucans [13]. Among various kinds of innate immune mechanisms in invertebrates, two types of coagulation mechanisms are on record: (i) in crustaceans such as lobster, crayfish [14] and insects [15] clotting occurs through Ca‐dependent transglutamin‐ ase, (ii) serine protease zymogens dependent coagulation system is reported which is similar to mammalian system [13]. In *Limulus polyphemus*, commonly known as the horseshoe crab, endotoxins are sensed by amoebocytes. In invertebrates, amoebocytes are known to be associ‐ ated in both hemostasis and innate or nonadaptive immune responses against microbial infec‐ tions [16]. Amoebocytes behave like macrophages in mammals and can either bind pathogens directly or recognize and engulf pathogens that have been opsonized by serum proteins. This direct recognition plays a major role in host defense [17]. It has been proposed that activation of the innate immune system is initiated when pathogens bind to nonclonally distributed pat‐ tern recognition receptors on immune cells [18]. In *Limulus*, the ancient horse shoe crab, the components, termed as Factor C [19, 20], Factor B [21] and pro‐clotting enzyme [22], undergo endotoxin‐mediated sequential limiting proteolysis/activation followed by irreversible con‐ version of clottable protein (coagulogen) into insoluble gel (coagulin) [23]. A similar pattern of coagulation system is also reported in *A. fulica* akin to endotoxin‐mediated coagulation system in the circulating amoebocytes. An endotoxin‐sensitive factor (ESF) available in the *Achatina* amoebocyte lysate (AAL) has been purified and characterized to be a serine protease type. These factors undergo a series of events such as aggregation and rapid degranulation leading to coagulation [24]. The aggregation mechanism causes bacterial sequestration, while degranulation results in secretion of serine protease zymogens [16]. Although the molecular basis of coagulation in *A. fulica* and further characterization of AAL remains to be determined, amoebocytes are considered as one of the primary immune cells in innate immune system in *A. fulica*.
