**1. Introduction**

Parasitism is a complex relationship between two organisms and requires several adaptations at the molecular level to establish successful interactions throughout evolution. Intricate signaling systems are necessary to transduce each signal from host to pathogen and vice versa. These systems or networks are relevant because they capture various signals from the environment in which the parasite lives (host), release stimuli, and send signals between different organs and tissues to regulate complex biological processes.

Many host signals (molecules) modulate the development and growth of parasites and directly or indirectly interfere in the course of parasitic infection. It is as

#### **Figure 1.**

*Molecular cross-talk between parasite and host. Schematic representation of parasite and host cells, signaling pathways, and molecules secreted (potential ligands) by both organisms. The ellipses represent signaling elements as indicated in the figure and the different colors represent distinct signaling pathways; the hexagons represent second messengers, and the arrows indicate the sequential interaction between proteins of the signaling pathways that culminate in the activation of transcription of target genes that will trigger various cellular processes. Potential molecular targets for drugs and vaccines are highlighted in red circles.*

important to understand the mechanism of action of these molecules to improve the knowledge of the basic biology of parasitism as the description of the effects itself is necessary.

With the development of sequencing technologies, the difficulties and costs of sequencing a genome or transcriptome have been reduced significantly; consequently, the number of sequences available dramatically increased [1], including the sequences from platyhelminth genomes [2–6] and transcriptomes [7–12]. This data collection allowed the scientific community to perform evolutive studies and investigate the signaling elements such as receptor, kinase and phosphatase proteins, and transcription factors [13–16]. These advances reflect the understanding of the molecular crosstalk mechanism between host and parasites. Studying the signaling elements is essential to comprehend parasitism, parasite development and identify new targets for developing strategies against these diseases (**Figure 1**) [17, 18].

This chapter reviews the host molecules (cytokines and hormones) and their effects and signals transduction pathways in platyhelminths. The most studied model of platyhelminth is *Schistosoma mansoni* (*S. mansoni*) and most of the information available in this chapter refers to this parasite.
