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SUMMARY OF KEY POINTS
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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.
Thymus 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 ...