Why do you have 6 different genes to code for 6 different MHC II's? Why not just one?

Question

You have 6 genes that each code for an MHC II protein. . Truth be told, these cells actually have

SIX different looking MHC II. you have 6 genes that code for MHC II, each one

is a little different from the others.

With that being said I have another question.

We have 6 genes to code for 6 different MHC II's.

Some people however, have identical copies of some of these genes. In other

words, they have 6 MHC II genes, but 3 might be identical, and therefore only

make 4 different MHC II's PROTEINS!! They might have MHC II A, B, C, D, D, D.

6 MHC's, but only 4 DIFFERENT versions. Question? People that have fewer

"versions" of MHC II that get AIDS, have a much shorter life expectancy than

those people with a full 6 different MHC II versions. Why?

Answer 1

some of the most abundant proteins in our cells are expressed from more than one identical gene.  for example some ribosomal genes and some histone genes are duplicated.  Our cells need so much of their products that one gene wouldn't be enough

Answer 2

In short, gene duplication.

To quote wiki:

"MHC gene families are found in essentially all vertebrates, though the gene composition and genomic arrangement vary widely. Chickens, for instance, have one of the smallest known MHC regions (19 genes), though most mammals have an MHC structure and composition fairly similar to that of humans. Gene duplication is almost certainly responsible for much of the genetic diversity. In humans, the MHC is littered with many pseudogenes."