MHC class II

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MHC (major histocompatibility complex) Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs).

The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I); hence, the MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway.

Loading of class II molecules must still occur inside the cell; extracellular proteins are endocytosed, digested in lysosomes, and bound by the class II MHC molecule prior to the molecule's migration to the plasma membrane.

Contents 1 Structure 2 Reaction to bacteria 3 Synthesis 4 Genes 5 References 6 External links

[edit] Structure

Like MHC class I molecules, class II molecules are also heterodimers, but in this case consist of two homologous peptides, an α and β chain, both of which are encoded in the MHC.

Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24 amino acid residues long.

[edit] Reaction to bacteria

Because class II MHC is loaded with extracellular proteins, it is mainly concerned with presentation of extracellular pathogens (for example, bacteria that might be infecting a wound or the blood). Class II molecules interact exclusively with CD4⁺ ("helper") T cells (T_Hs). The helper T cells then help to trigger an appropriate immune response which may include localized inflammation and swelling due to recruitment of phagocytes or may lead to a full-force antibody immune response due to activation of B cells.

[edit] Synthesis

During synthesis, MHC class II is the result of dimerization of α and β chains, with the assistance of an invariant chain.^[1]

[edit] Genes

	Alpha	Beta
HLA-DM	HLA-DMA	HLA-DMB
HLA-DO	HLA-DOA	HLA-DOB
HLA-DP	HLA-DPA1	HLA-DPB1
HLA-DQ	HLA-DQA1, HLA-DQA2	HLA-DQB1, HLA-DQB2, HLA-DQB3
HLA-DR	HLA-DRA	HLA-DRB1, HLA-DRB2, HLA-DRB3, HLA-DRB4, HLA-DRB5

[edit] References

1. ^ School of Crystallography The Invariant chain

[edit] External links

• MeSH Histocompatibility+Antigens+Class+II
• MeSH MHC+Class+II+Genes

v • d • e The T cell receptor complex The antigen receptor TCR | TCRα | TCRβ | TCRγ | TCRδ Accessory molecules CD3 | CD3γ | CD3δ | CD3ε | ζ-chain (also called CD3ζ and TCRζ) Co-receptors CD8 (with two glycoprotein chains CD8α and CD8β) | CD4 Ligands MHC (MHC class I and MHC class II)

v • d • e Surface antigens Histocompatibility/Human leukocyte antigen MHC - MHC class I (HLA-A, HLA-B, HLA-C, HLA-E) MHC class II (HLA-DP, HLA-DQ, HLA-DR) - Minor histocompatibility antigen Other Arrestin - Calgranulin - Blood group antigens - Cell adhesion molecules - Differentiation antigens