Arthropods have the capacity of recognizing self from non-self in various defense phenomena including hemolymph clotting, phagocytosis, encapsulation, melanization and clearance of the foreign matter which must be initiated by a first step of molecular recognition. “Natural” and experimentally induced humoral factors have been described which act as hemagglutinins, bacterial agglutinins, precipitins, bactericidal factors, bacteriolysins, hemolysins, opsonins, clotting factors, and lysozymes. Their exact role in recognition functions has not been fully explored and their function remains unclear. Among these factors, the carbohydrate-binding molecules (lectins) are the best characterized in their specificity, physicochemical properties and molecular structure. Arthropod lectins are multimeric, high molecular weight protein (glycoprotein) molecules with a certain degree of heterogeneity in their specificity and structure. In particular, serum lectins from chelicerates (horseshoe crabs, scorpions and spiders) share a common property: the specificity for sialic acids. Arachnids and merostomes diverged in the earliest Cambrian. Since they occupy markedly different habitats, the sialic acid specific lectins most probably are a relict rather than an adaptative character. In addition to this common feature of specificity, lectins from chelicerates and other arthropods represent heterogeneous populations which can bind a wide variety of carbohydrates, many of them present on bacteria as D-galactose, 2-keto-3-deoxyoctonate, glucuronic acid, N-acetylmuramic acid, and colominic acid. Multiplicity in specificity suggests that serum lectins might contribute as a carbohydrate-based recognition system for the non-self. The requirement for avoiding self recognition would be that carbohydrate structures potentially recognized by the system would be absent, masked or out of reach of this humoral factor or cell associated recognition factors.