Organization And Components Of The Immune System

The immune system is organized into discrete compartments to provide the milieu for the development and maintenance of effective immunity. Those two overlapping compartments: the lymphoid and reticuloendothelial systems (RES) house the principal immunologic cells, the leukocytes. Leukocytes derived from pluripotent stem cells in the bone marrow during postnatal life include neutrophils, eosinophils, basophils, monocytes and macrophages, natural killer (NK) cells, and T and B lymphocytes. Cells of the immune system intercommunicate by ligand-receptor interactions between cells and/or via secreted molecules called cytokines. Cytokines produced by lymphocytes are termed lymphokines (interleukins and interferon-γ) and those produced by monocytes and macrophages are termed monokines.

Lymphoid System

Cells of the lymphoid system provide highly specific protection against foreign agents and also orchestrate the functions of other parts of the immune system by producing immunoregulatory cytokines. The lymphoid system is divided into 1) central lymphoid organs, the thymus and bone marrow, and 2) peripheral lymphoid organs, lymph nodes, the spleen, and mucosal and submucosal tissues of the alimentary and respiratory tracts. The thymus instructs certain lymphocytes to differentiate into thymus-dependent (T) lymphocytes and selects most of them to die in the thymus (negative selection) and others to exit into the circulation (positive selection). T lymphocytes circulate through the blood, regulate antibody and cellular immunity and help defend against many types of infections. The other classes of lymphocytes, B cells (antibody-forming cells) and natural killer (NK) cells, are thymic-independent and remain principally in peripheral lymphoid organs.

Reticuloendothelial System

Cells of the RES provide natural immunity against microorganisms by 1) a coupled process of phagocytosis and intracellular killing, 2) recruiting other inflammatory cells through the production of cytokines, and 3) presenting peptide antigens to lymphocytes for the production of antigen-specific immunity. The RES consists of 1) circulating monocytes; 2) resident macrophages in the liver, spleen, lymph nodes, thymus, submucosal tissues of the respiratory and alimentary tracts, bone marrow, and connective tissues; and 3) macrophage-like cells including dendritic cells in lymph nodes, Langerhans cells in skin, and glial cells in the central nervous system.


Leukocyte magnified 7766x
Credit: CDC/ Janice Carr

Leukocytes, the main cells in the immune system, provide either innate or specific adaptive immunity. These cells are derived from myeloid or lymphoid lineage. myeloid cells include highly phagocytic, motile neutrophils, monocytes, and macrophages that provide a first line of defense against most pathogens. The other myeloid cells, including eosinophils, basophils, and their tissue counterparts, mast cells, are involved in defense against parasites and in the genesis of allergic reactions. In contrast, lymphocytes regulate the action of other leukocytes and generate specific immune responses that prevent chronic or recurrent infections.


  • Medical Microbiology. 4th edition. Armond S. Goldman and Bellur S. Prabhakar