Lymphoid cells play a significant role in the adaptive immune response. During embryonic development, blood cell precursors (hematopoietic stem cells) originate in the fetal liver and other tissues; in postnatal life, the stem cells reside in the bone marrow. Stem cells may differentiate into cells of the myeloid or lymphoid series. The lymphoid progenitor cells develop into two main lymphocyte populations: B cells and T cells.

Stem cells destined to become B lymphocytes develop in the bone marrow. They rearrange their immunoglobulin genes and express a unique receptor for antigen on their cell surface. Following this step, they migrate to a secondary lymphoid organ (eg, the spleen) and may be activated by an encounter with antigen to become antibody-secreting plasma cells.

T cells are lymphocytes that are produced in the bone marrow but travel to the thymus to mature. Here, they undergo variable diverse joining (VDJ) recombination of their β chain T cell receptor (TCR) DNA and their α chain TCR DNA. Once TCR rearrangement has occurred and positive and negative selection has terminated, these cells form T cell subclasses with specific functions (eg, CD4 T cells, CD8 T cells). They are the source of cell-mediated immunity.

Figure 1 presents a summary of the specific immune processes that are reviewed in this section. The two arms of the immune response, cell-mediated and antibody-mediated, develop concurrently. In the antibody-mediated immune response, CD4 T lymphocytes recognize the pathogen’s anti gens bound to the class II MHC molecules on the surface of an antigen-presenting cell (APC) (eg, macrophage, B cell), and as a consequence of this interaction, cytokines are produced that stimulate B cells to express antibodies that display specificity for the antigen. The B cells undergo clonal proliferation and differentiate into plasma cells. In the cell-mediated immune response, the antigen–MHC class II complex is recognized by the CD4 T lymphocyte, whereas the antigen MHC class I complex is recognized by CD8 T lymphocytes. Both subsets of T cells produce cytokines, become activated, and expand by clonal proliferation. The CD4 T cells that develop stimulate B cells to produce antibodies and promote delayed hypersensitivity while the CD8 T cells direct their activity mainly at the destruction of cells in tissue grafts, tumor cells, or virus-infected cells.

Fig1. Schematic diagram of the cellular interactions in the immune response.

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مواضيع ذات صلة

The Major Histocompatibility Complex

Mechanisms of Innate Immunity

Overview of Acute-Phase Proteins

C-Reactive Protein

Hematopoietic Stimulators

The Complement System: Diagnostic Evaluation

Biological Functions of Complement Proteins

The Complement System: Alternative Pathway

The Complement System: Classic Pathway

The Complement System

Immune-Mediated Disease

Evaluation of Immunodeficiency Syndromes