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Source: Illustration created by Jake Nikota for this Surgeon General's report.

Note: To illustrate the various aspects of the immune system, the figure is divided into physical barriers, innate, and adaptive immunity. While this separation is convenient, there is an intimate interaction between innate and adaptive immunity, and individual components never respond in isolation. The ciliated respiratory epithelium forms a physical barrier through tight junctions between individual cells and protects by sweeping particles away in the overlying mucus gel layer. A number of innate defense molecules, including defensins, are found in the epithelial lining fluid. In addition to their barrier function, epithelial cells also have potent innate defense capabilities. The main cell types associated with innate immunity are mononuclear lineage cells (monocyte and macrophage subpopulations) and granulocytes (neutrophils and eosinophils). Macrophages are considered the most important phagocytes, playing a critical role in the destruction of pathogens and the removal of dying cells. Other innate immune cells are natural killer cells, natural killer T cells, mast cells, dendritic cells, and nuocytes. Innate immune responses are activated rapidly but do not hold a molecular memory. T and B lymphocytes (T and B cells) are part of the adaptive immune system. Adaptive immunity retains memory, providing protection against subsequent insult by the same pathogen. T cells respond to short peptide fragments of foreign proteins (antigens) presented by specialized antigen-presenting cells, of which dendritic cells are the most important. Dendritic cells reside within the tissue where they capture antigen. Following activation, dendritic cells migrate to secondary lymphoid organs and present antigen to T cells. Cluster of differentiation (CD)4 T cells exert their effects by differentiating into effector cells that are capable of secreting cytokines and chemokines that regulate inflammation. CD8 T lymphocytes differentiate into cytotoxic cells that kill cellular targets. B cells exert their effect largely by producing antibodies. This process requires antigen presentation and help from CD4+ T cells.