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The Immune System & Lymphoid Organs SUMMARY OF KEY POINTS Basic Immunology

  • Innate immunity is present from birth and involves leukocytes (mainly granulocytes) and proteins such as defensins, complement, lysozyme, and interferons; adaptive immunity develops more slowly and is based on antigen presentation to lymphocytes.

  • Immune cells communicate with one another and regulate one another’s activities via polypeptide hormones called cytokines.

  • Antigens are the regions of macromolecules, usually proteins, that are recognized by lymphocytes to elicit a specific immune response against them.

  • Antibodies are immunoglobulins produced by plasma cells after a progenitor B cell is activated by a specific antigen and rearranges its immunoglobulin genes so that the antibody matches the antigen.

  • Surfaces of all nucleated cells bear fragments of their constituent proteins on MHC class I molecules.

  • Only antigen-presenting cells (APCs), mostly derived from monocytes, also present fragments of endocytosed foreign proteins (usually from microorganisms) on surface MHC class II molecules.

Lymphocyte Origins and Differentiation
  • Lymphocytes originate in the primary lymphoid organs: bone marrow for B lymphocytes and the thymus for T lymphocytes.

  • B cells produce antibodies for humoral immunity; T cells function in cell-mediated immunity.

  • T cells develop receptors (TCRs), usually containing α and β chains, that bind antigen along with another surface protein designated by a CD (“cluster of differentiation”) numbering system.

  • Important classes of T cells include CD4+ T helper cells; CD8+ cytotoxic T cells; CD4+CD25+ regulatory T cells; and γδ T cells, which have those TCR chains and are mainly in epithelia.

  • B-cell receptors (BCRs) are IgM or IgD antibodies on the surfaces of B lymphocytes that bind specific antigens when they contact them.

  • B and T cells are often activated, proliferate, and begin to function in the secondary lymphoid organs: the lymph nodes, all MALT, and the spleen.

  • In these organs, lymphocytes are distributed within a meshwork of reticulin produced by fibroblastic reticular cells, and most APCs are dendritic cells with many processes.

  • In secondary lymphoid tissues, BCRs bind antigens displayed in antibody-antigen complexes on the large surface of follicular dendritic cells (FDCs).

  • With cytokines from helper T cells, an FDC-activated B cell proliferates clonally to produce a large, transient lymphoid nodule (or follicle), which develops a pale germinal center.

  • From lymphoid nodules, cells produced there disperse as plasma cells, various T cells, and B and T memory cells that respond and proliferate quickly if their specific antigen reappears.

  • T lymphoblasts, or thymocytes, attach in the thymus to a cytoreticulum composed of interconnected thymic epithelial cells (TECs).

  • The TECs also secrete many cytokines, compartmentalize the thymus into a cortex and a medulla, and in the cortex surround blood vessels in the blood-thymus barrier.

  • Developing T cells with nonfunctional TCRs are detected and removed in the ...

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