**2. Overview of the immune system**

#### **2.1 Adaptive immune system**

The vertebrate immune system is generally divided into two complementary arms. The adaptive immune system is highly specialized and recognizes specific protein motifs corresponding to individual pathogens. Adaptive immunity is primarily mediated by T and B lymphocytes, with T cells further divided into cytotoxic T cells (CD8+) and several classes of helper T cells (CD4+). Cytotoxic T cells recognize infected cells through interactions of the T cell receptor (TCR) with antigenbound class I major histocompatibility complex (MHC) molecules, and kill these cells via release of cytotoxins such as perforins and granzymes. Likewise, helper T cells recognize pathogens through interactions of their TCR with antigen-bound class II MHC molecules on antigen presenting cells, and serve to instruct activation

of B cells or CD8+ T cells through the release of various cytokines. Finally, B cells produce pathogen-specific antibodies, which bind extracellular pathogens to allow them to be neutralized by a variety of methods. Adaptive immune responses are highly important during SARS-CoV-2 infection and are key to successful clearance of the virus from the host. However, these aspects of immunity during COVID-19 are otherwise beyond the scope of this chapter, and we can point readers to a recent review article on this subject [22].
