Section description 

A healthy immune system functions through innate and adaptive immunity, the ability to defend against antigens, discriminate between self and nonself, remember previous infections, and cease to attack once the pathogen is removed from the system. The various types of immune cells, their intricate architecture and elaborate regulation, enable the body to maintain homeostasis and activate an adaptive immune response as needed. With flow cytometry, you are able to precisely and rapidly analyze a large number of immune cells, enumerate and characterize them, identify specific surface and intracellular markers of immune subtypes, and assess depletion of immune cells after procedures.

Innate immunity is the primary and immediate cellular response of the body to pathogenic attack through the recognition of conserved features of the pathogens. The recognition of molecular patterns associated with microbes is mediated by pattern-recognition receptors (PRRs), which include both surface receptors (Toll-like receptors (TLRs) and c-type lectin receptors (CLRs)) or cytoplasmic receptors (TLRs, nucleotide-binding oligomerization domain (Nod)-leucine-rich repeat-containing receptors (NLRs), RIG-I-like receptors (RLRs). PRRs recognize two types of ligands—conserved structures of microbial pathogens, known as PAMPs (pathogen associated molecular patterns), and danger signals from altered-self, known as DAMPs (danger associated molecular patterns). Activation of PRRs by their ligands gives rise to the mobilization of innate immune cells and the secretion of signaling and effector molecules (cytokines and antimicrobial peptides) leading to an inflammatory response to neutralize the pathogen or the danger.1 This first line of response is often a prerequisite to trigger initial responses from the adaptive immunity.

A hallmark of macrophages is their phagocytic capabilities supporting the clearance of intruders and priming immune responses. However, macrophages also enable other essential homeostatic functions such as metabolism.3 Macrophages are highly involved in tumor development and progression as tumor associated macrophages (TAMs) secrete factors inducing immune tolerance towards tumor cells. They are therefore of great interest in immuno-oncology and are central to investigations evaluating the impact of therapy on the tumor microenvironment (TME). Two main types of macrophages are described based on their stimuli induced polarization—M1 and M2. M1 macrophages are pro-inflammatory, promote an oxidative state with reactive oxygen species (ROS) production, and secrete inflammatory cytokines and chemokines, e.g., IL-1, IL-6, IL-12, TNF-α, CXCL9, CXCL10. M2 macrophages are considered immunosuppressive and secrete anti-inflammatory factors such as IL-10 and TGF-β.4