White blood cells are like the guardians of your immune system, giving us clues about how healthy it is.

When we talk about someone having a strong or weak immune system, one way to check this out is by looking at their white blood cell count.

A normal count usually falls between 4,000 and 11,000 cells for every tiny drop of blood. †

For specific types of cells, like helper T-cells and suppressor T-cells, a healthy ratio is about 1.8 to 2.0. †

And for neutrophils, another type of white blood cell, the typical count is between 1,500 to 8,000 cells in that tiny drop.†

Well, it's all thanks to something called the Major Histocompatibility Complex (MHC), also known as the Human Leukocyte Antigen (HLA).

This system is like a badge that every cell in your body wears to say, "Hey, I belong here!"

If a cell doesn't have this badge, or if it's wearing the wrong one, your immune system sees it as an intruder.

The MHC has two main types of protein molecules that sit on the surface of almost every cell in your body.

In humans, these proteins come from specific genes on chromosome 6.

Because there are so many different versions of these genes, it's pretty rare for two people to have the exact same set of MHC molecules, which are also called a tissue type.

These MHC molecules are super important for your immune response. They help T lymphocytes, or T cells, spot cells that have been invaded by germs. They do this by showing off little pieces of the invader on the surface of the cell. When a T cell sees this, it either destroys the infected cell or helps it heal.

In healthy cells, MHC molecules show off normal pieces, and T cells usually ignore these. But if something goes wrong and T cells start attacking these normal parts, it can lead to autoimmune diseases.†

Pretty amazing, right? The immune system is a complex and fascinating part of our bodies!