Antigen-presenting cells

Professional antigen-presenting cells (APC) take up allergen molecules and break them down into fragments, small polypeptides, which are antigenically active. MHC class II molecules attach the polypeptides and transport them to the cell surface. Only when prepared in this way can free antigens be actively presented by antigen-presenting cells (APCs) (so-called MHC restriction) and recognized by the cytotoxic T lymphocytes.

APCs act as:

• dendritic cells
• macrophages and
• B lymphocytes.

The type of APCs with which a helper cell comes into contact significantly determines its functional differentiation into TH1, TH0 or TH2 cells. The long-term survival of some of these antigen-specific lymphocytes results in the formation of an immunological memory. APCs are also crucial for the immune tolerance of the organism.