Your body contains, by some estimates, more immune cells than any other cell type except red blood cells — trillions of white cells distributed across blood, lymph, bone marrow, lymph nodes, spleen, thymus, mucous membranes, and tissues. Their job is to recognize what is you and what is not, and to kill the not-you while sparing the you. The task is staggeringly hard — novel pathogens evolve constantly, the body has hundreds of cell types that must all be tolerated, and getting either side wrong is catastrophic (too aggressive: multiple sclerosis, type 1 diabetes, lupus; too lax: pathogens kill you) — yet the immune system gets it right most of the time.
The vertebrate immune system has two interlocking arms. The innate immune system — older, faster, less specific — provides immediate response through physical barriers (skin, mucous membranes), the complement cascade, innate immune cells (macrophages, neutrophils, dendritic cells, natural killer cells, mast cells), pattern-recognition receptors (Toll-like receptors — Hoffmann, Beutler, Akira, Nobel 2011 — recognize PAMPs like bacterial flagellin and viral RNA), and cytokines (interleukins, interferons, TNF-α) that coordinate the response. The adaptive immune system — younger, slower, exquisitely specific — provides targeted, memory-equipped response through T cells (matured in the thymus: helper CD4+, cytotoxic CD8+, regulatory T cells) and B cells (matured in bone marrow) that make antibodies (IgG, IgA, IgM, IgE, IgD). The adaptive system's specificity is generated by V(D)J recombination (Tonegawa, Nobel 1987): immune-receptor genes are cut and pasted during lymphocyte development to produce ~10¹¹ distinct receptors per individual. Self-tolerance is enforced by negative selection in the thymus and bone marrow. Antigen presentation signals through MHC: infected cells display fragments on MHC class I, professional antigen-presenting cells display fragments on MHC class II. Immunological memory — memory B and T cells persisting for years after first pathogen encounter — produces a faster, stronger response on subsequent encounters, and vaccines exploit this (the mRNA vaccine revolution — Karikó and Weissman, Nobel 2023 — delivers mRNA encoding the SARS-CoV-2 spike).
Immunotherapy has been one of the largest medical advances of the past fifteen years: checkpoint inhibitors (anti-PD-1, anti-CTLA-4; Allison and Honjo, Nobel 2018) release the brakes on T cells and have produced durable remissions in melanoma, lung cancer, and other cancers previously considered terminal. CAR-T cell therapy — genetically engineering a patient's T cells to recognize tumor antigens — is FDA-approved for some leukemias and lymphomas (Kymriah, Yescarta) with cure rates previously inconceivable in advanced disease. Autoimmune disease treatment has improved with biologics targeting specific cytokines (TNF inhibitors for rheumatoid arthritis, IL-17 inhibitors for psoriasis). The COVID-19 pandemic exposed the immune system to public discourse at unprecedented scale, and immunology of aging (inflammaging as chronic low-grade inflammation contributing to age-related disease) is a major research front.