Antigen-presenting cells (APCs) are a group of function ally defined cells capable of taking up antigens and presenting them to lymphocytes in a form that they can recognize.

APCs take up antigens (e.g., dendritic cells, macrophages, B cells, even tissue cells) in various ways. Some are collected in the periphery and transported to the secondary lymphoid tis sues; other APCs normally reside in lymphoid tissues and intercept antigen as it arrives. B cells recognize antigen in a native form.

There are two major pathways of antigen processing for the APC and target cell, endogenous and exogenous. The endogenous pathway processes proteins that have been internalized, processed into fragments, and reexpressed at the cell surface membrane in association with MHC molecules. In this path way, proteins in the cytoplasm are cleaved into peptide fragments about 20 amino acids in length. These fragments are then transported into the lumen of the endoplasmic reticulum by the transporter associated with the antigen-processing com plex, where the fragments encounter newly formed, heavy chain molecules of MHC class I and their associated beta2-microglobulin (β2m) light chains. The heavy chain, light chain, and peptide form a trimeric complex, which is then transported to and expressed on the cell surface.

T cells that express the CD8+ cell surface marker recognize antigens presented by MHC class I molecules. CD8+ functions as a coreceptor in this process, binding to an invariant region of the MHC class I molecule. Pathogen clearance requires that CD8+ effector cells produce inflammatory cytokines and develop cytolytic activity against infected target cells, after which a small number of memory cells survive that rap idly regain effector function in the event of rechallenge. During this process, a relatively homogeneous pool of naïve CD8+ T cells differentiates into heterogeneous pools of effector and memory CD8+ T cells.

In the exogenous pathway, soluble proteins are taken up from the extracellular environment, generally by specialized or so-called professional APCs. The antigens are then processed in a series of intracellular acidic vesicles called endosomes. During this process, the endosomes intersect with vesicles that are transporting MHC class II molecules to the cell surface. CD4+ T cells recognize antigens that are presented by MHC class II molecules. As with CD8, the CD4 molecule functions as a coreceptor, increasing the strength of the interaction between the T cell and APC.

For both systems of antigen presentation, recognition of the antigen by the T cells is described as being MHC-restricted, a process whereby T cells recognize only antigen presented by self MHC molecules.

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مواضيع ذات صلة

Antigen Recognition by T Cells

Cytotoxic T Lymphocytes

T Regulatory Lymphocytes

Helper T Lymphocytes

Sites of Lymphocyte Development

MONOCYTE-MACROPHAGE DISORDERS

Clinical Features of HIV Infection and AIDS

Pathogenesis of AIDS

Congenital Immunodeficiencies : Defects in Lymphocyte Activation and Function

Congenital Immunodeficiencies: Defects in Lymphocyte Maturation

Congenital Immunodeficiencies: Defects in Innate Immunity

Congenital (Primary) Immunodeficiencies