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Structure of HLA molecules:
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Class I molecules consist of a transmembrane heavy a-chain (45 KDa), which is divided into three extracellular domains a1, a2 and a3. A small b2-microglobulin (12KDa) is non-covalently bound to the a-chain. This b2-microglobulin is not encoded by the MHC genes.
- Class II molecules are composed of a transmenbrane a-chain, which is divided into two domains a1 and a2 and a transmembrane b-chain consisting of two domains b1 and b2.
The class I HLA are found on all nucleated cells and are recognized by cytotoxic CD8+ T cells.
By contrast, class II is usually found only on antigen presenting cells (dentrictic cells macrophages) and B cells. Class II molecules are recognized by CD4+ T cells.
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