Antigen Processing

Antigen Processing
Johan K Sandberg, Karolinska Institute, Stockholm, Sweden
Rickard Glas, Harvard University, Boston, Massachusetts, USA

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Requirement for Antigen Processing
Infectious organisms such as bacteria and viruses produce proteins that are foreign (nonself) to the host immune system, and therefore serve as potential target structures for an immune response. The humoral immune response is effective against extracellular antigens, and is mediated by B lymphocyte-derived antibodies which bind the native conformation of the invading microorganism’s proteins. Thus, antibody-mediated recognition of native protein requires no antigen processing. In contrast, T lymphocytes recognize fragments of proteins that are processed into short peptides by proteases within the antigen-presenting cell (APC). These peptides are presented at the cell surface bound to major histocompatibility complex (MHC) molecules and represent a sampling of the intracellular and extracellular milieu. The MHC-presented peptides are scanned and recognized by circulating T lymphocytes via their antigen-specific cell surface T-cell receptors (TCRs). For these reasons, processing of antigens is crucial for activation of T lymphocytes during infection, and is therefore the key to a successful adaptive immune response.