---

MHC class II

From MedBib.com - Medicine & Nature

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 See also 7 External links

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. ^[1]

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.

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_HC). 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.

Synthesis

During synthesis, MHC class II is the result of dimerization of α and β chains, with the assistance of an invariant chain.^[2] The invariant chain is a special polypeptide involved in the formation and deliverance of MHC class II protein.

The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway. The invariant chain also facilitates MHC class II's export from the ER in a vesicle. This fuses with a late endosome containing the endocytosed, degraded proteins. It is then broken down in stages, leaving only a small fragment called CLIP which still blocks the peptide binding cleft. An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome. The stable MHC class-II is then presented on the cell surface.

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-DR	HLA-DRA	HLA-DRB1, HLA-DRB3, HLA-DRB4, HLA-DRB5

References

1. ^ "Histocompatibility". http://www.cehs.siu.edu/fix/medmicro/mhc.htm. Retrieved 2009-01-21. 
2. ^ School of Crystallography The Invariant chain

See also

Cross-presentation

External links

• MeSH Histocompatibility+Antigens+Class+II
• MeSH MHC+Class+II+Genes
• http://www.merck.com/mmpe/sec13/ch163/ch163a.html

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 Major histocompatibility complex/ Human leukocyte antigen MHC class I HLA-A • HLA-B • HLA-C • HLA-E • HLA-F • HLA-G MHC class II HLA-DM (α, β) • HLA-DO (α, β) • HLA-DP (α1, β1) • HLA-DQ (α1, α2, β1, β2, β3) • HLA-DR (α, β1, β3, β4, β5) Minor histocompatibility antigen Other Arrestin - Calgranulin - Blood group antigens - Cell adhesion molecules - Differentiation antigens

---

This article is licensed under the GNU Free Documentation License.
All material adapted used from Wikipedia is available under the terms of the GNU Free Documentation License.
Seeking health information online: does Wikipedia matter?

---

---