**1. Introduction**

Exosomes were initially discovered in 1981 as exfoliating vesicles from different normal and neoplastic cell lines. Exosomes correspond to the family of extracellular vesicles (EVs), composed of microvesicles, apoptotic bodies, and exosomes [1–3]. Exosomes arise from the endo-lysosomal pathway and originate from the endosomal compartment known as the multivesicular bodies [4, 5]. The microvesicles are developed by sprouting from the plasma membrane. The microvesicles size is around 100–1000 nm and originates from sprout and fusion of plasma membrane into

extracellular space, and sharing out different models with the parental cells, involving membrane lipids, receptors, and different types of nucleic acids and proteins [6, 7]. Depending on their size and shape, exosomes can be divided into nine different subpopulations or categories, signifying that exosome that arises from a single cell line is different from morphologically functionally [8]. EVs play a potential role in many factors such as cellular homeostasis, physiology, and pathobiology [9, 10]. In the tumor milieu, EVs are isolated from cancer cells, immune cells, and non-immune host cells delivering as a critical component of the tumor microenvironment. Isolation of EV from various roots exhibits distinct roles in tumor immune cells, leading to tumor proliferation, metastasis, and drug resistance [11]. Examinations from the past years have shown a curious spike of exosomes initiating to fuse and communicate their vital functions. EV are linked with different cell types which also have required macromolecules comprising DNA, micro-RNA, messenger RNA, proteins, and lipids [12].

Immune checkpoint blockade therapy has modified an environment for the treatment of cancer [13]. Therefore, deeper knowledge on deciding the success and failure of this therapy is required. Tumor-derived EVs are linked in immunological crosstalk are likely to be an emerging biomarker for cancer immunotherapy [14, 15]. Tumorderived EVs are notably interesting targets as they are discharged from cancer cells as to normal cells and it has been segregated from different biospecimens such as blood, urine, cerebrospinal fluid, and saliva [16, 17]. Therefore, this chapter focuses on the current availability of biomarkers for cancer immunotherapy.
