Major histocompatibility complex facts for kids

A major histocompatibility complex, or MHC, is like a special tag found on the outside of your immune cells. Think of it as a tiny flag that helps your body tell the difference between its own cells and invaders. These tags are made from instructions found in a large group of genes. All animals with backbones (like humans, dogs, and fish) have them.

The main job of MHC molecules is to show your immune system what's inside your cells. They grab small pieces, called peptides, from things like pathogens (germs that make you sick). Then, they display these pieces on the cell's surface. This way, special immune cells called T-cells can recognize them. This process is super important for your adaptive immunity, which learns to fight off specific germs.

How MHC Helps Fight Germs

MHC molecules have two special features that make it hard for germs to hide from your immune system.

Many Different Genes (Polygenic)

First, MHC is polygenic. This means there isn't just one gene for MHC. Instead, there are several different genes. Because of this, each person or animal has a unique set of MHC molecules. These different molecules can grab and display slightly different kinds of peptides. This variety helps your body catch a wider range of invaders.

Many Variations in the Population (Polymorphic)

Second, the MHC genes are very polymorphic. This means there are many different versions, called alleles, of each MHC gene within a group of people. This variety is so high that, except for identical twins, no two people have exactly the same set of MHC genes and molecules.

Having so many different versions of MHC genes in a population is a big advantage. It means that if a new or changed germ appears, almost always someone in the group will have an MHC molecule that can recognize it. This helps protect the whole population from getting very sick from new diseases. The differences in MHC molecules are passed down through inheritance.

MHC and Organ Transplants

When someone needs a new organ, doctors have to find a good match. This is because the MHC system can react strongly to cells that aren't from your own body. Doctors do special tests to find the best tissue match, which means finding an organ with MHC molecules that are as similar as possible to the patient's. This helps prevent the patient's immune system from attacking the new organ.

MHC and Autoimmune Diseases

Sometimes, the immune system makes a mistake. An autoimmune disease happens when your immune cells don't recognize your own body's MHC molecules. Instead, they think your own cells are foreign invaders and start attacking them. This can cause many different health problems, depending on which parts of the body are attacked.

Images for kids

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  Schematic view of MHC class I and MHC class II molecules

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  Peptide binding for Class I and Class II MHC molecules, showing the binding of peptides between the alpha-helix walls, upon a beta-sheet base. The difference in binding positions is shown. Class I primarily makes contact with backbone residues at the Carboxy and amino terminal regions, while Class II primarily makes contacts along the length of the residue backbone. The precise location of binding residues is determined by the MHC allele.

See also

In Spanish: Complejo mayor de histocompatibilidad para niños