Organization of the Immune System

The human immune system has sites of production, i.e., bone marrow and the thymus; and sites of differentiation and proliferation, i.e., lymph nodes and the spleen. Immune organization also occurs in the lung, gastrointestinal tract, skin, and adipose tissue. Cells can be tissue bound or move to sites during infection via blood or lymphatics. Pluripotent stem cells in bone marrow give rise to progenitors, which differentiate into lymphoid and myeloid cells as well as red blood cells and platelets. Myeloid cells include neutrophils, basophils, and eosinophils with functions in innate immune responses, as well as monocytes, macrophages, and dendritic cells, which are phagocytic and act as an interface with cells of the acquired immune system. Lymphocytes include antigen-specific cells with DNA-rearranged receptors, T cells and B cells, as well as cells termed innate lymphoid cells, which can be cytotoxic (natural killer cells) or noncytotoxic, with functions that mirror classic T-cell functions.