If you’re looking at blood test results and see “MHC,” you’re almost certainly looking at MCH, which stands for mean corpuscular hemoglobin. It’s one of the standard measurements on a complete blood count (CBC) and tells you the average amount of hemoglobin packed into each red blood cell. There is, however, a separate concept called MHC (major histocompatibility complex) that relates to immune system testing. This article covers both so you can figure out which one applies to you.
MCH: The CBC Value You Probably Saw
MCH appears on nearly every routine blood panel. It measures the weight of hemoglobin, the oxygen-carrying protein, inside an average red blood cell. The normal range is roughly 27 to 33 picograms per cell. Your doctor uses MCH alongside two related values, MCV (the size of your red blood cells) and MCHC (how concentrated the hemoglobin is within each cell), to evaluate and classify anemia.
A low MCH means your red blood cells carry less hemoglobin than normal. This often points to iron deficiency or chronic disease that limits iron availability. You might feel fatigued, short of breath, or pale. A high MCH, on the other hand, typically shows up when red blood cells are larger than usual. This pattern is common in vitamin B12 or folate deficiency. In either case, the MCH number itself doesn’t diagnose anything on its own. It’s a clue that guides further testing.
MHC: The Immune System Meaning
MHC stands for major histocompatibility complex, a set of proteins that sit on the surface of your cells and help your immune system distinguish your own tissue from foreign invaders. These proteins grab tiny fragments of whatever is inside or around a cell and display them on the surface, essentially showing passing immune cells a sample of what’s going on. If the fragment looks like it belongs to a virus, bacterium, or other threat, immune cells launch an attack.
In humans, MHC proteins are called HLA (human leukocyte antigens). When a doctor orders “MHC testing,” what the lab actually runs is an HLA typing test. Your body has 12 unique HLA markers inherited from your parents, and the possible combinations number in the millions. This extreme diversity is why finding a matching donor for a transplant can be so difficult.
Why HLA Typing Is Ordered
HLA typing has two main clinical purposes: matching donors for transplants and investigating autoimmune or inflammatory conditions.
Transplant Matching
Before a bone marrow or organ transplant, both the recipient and potential donors have their HLA markers mapped. The goal is to find the closest possible match, which improves the odds that donated cells will engraft successfully and reduces the risk of graft-versus-host disease (GVHD), a serious complication where the donor’s immune cells attack the recipient’s body.
Matches fall into three broad categories. A full match means every HLA marker lines up between donor and recipient. A partial match covers most but not all markers. A haploidentical match, common when a parent or child serves as the donor, means half of the markers are shared. Full matches produce the best outcomes, but advances in transplant medicine have made partial and haploidentical transplants increasingly viable.
Autoimmune Disease Screening
Certain HLA markers are strongly associated with specific diseases. The most well-known example is HLA-B27, a marker linked to ankylosing spondylitis, a type of inflammatory arthritis that primarily affects the spine. That association was discovered over 50 years ago and remains one of the strongest known links between a single genetic marker and a disease. HLA-B27 is also associated with reactive arthritis and a form of eye inflammation called anterior uveitis.
Other HLA variants connect to other conditions. HLA-B51 is the major risk marker for Behçet’s syndrome, an inflammatory disorder affecting blood vessels. HLA-C*06 is linked to psoriasis. Having one of these markers doesn’t guarantee you’ll develop the associated disease. It raises the probability and, combined with symptoms and other tests, helps confirm a diagnosis.
How HLA Testing Works
The test requires a simple blood draw or, in some cases, a cheek swab. No fasting or special preparation is needed. From there, the lab identifies which HLA variants you carry using one of several techniques.
Older methods relied on serology, where lab technicians exposed living white blood cells to antibodies targeting specific HLA types. If the antibodies matched, the cells would react in a visible way. This approach has largely been replaced by DNA-based methods. The most common use PCR (polymerase chain reaction) to amplify the relevant stretches of your DNA, then identify which HLA alleles are present using either probe matching, specific primers, or direct sequencing. Newer labs use next-generation sequencing, which reads longer stretches of DNA in a single pass and provides higher-resolution results. This precision matters most in transplant settings, where even small differences between donor and recipient can affect outcomes.
How Results Are Reported
HLA results look nothing like a typical blood test. There’s no single number with a “normal range.” Instead, you receive a list of your specific HLA alleles, usually identified by standardized names like HLA-A*02:01 or HLA-B*27:05. Each person inherits one set of markers from each parent, so you’ll see two alleles at each HLA gene location.
For transplant purposes, your results are compared directly against a potential donor’s profile. The report focuses on how many markers match. For autoimmune screening, the presence or absence of a particular marker (such as HLA-B27) is the key finding. A positive result means you carry the marker, which supports a suspected diagnosis when combined with your symptoms and imaging. A negative result makes the associated condition less likely but doesn’t completely rule it out.
MHC Class I vs. Class II
If you encounter the terms “Class I” and “Class II” on your results or in reading about your condition, here’s the distinction. Class I MHC proteins appear on nearly every cell in your body. They display fragments of proteins made inside the cell, which is how your immune system detects cells that have been infected by a virus or have become cancerous. Class II MHC proteins appear only on specialized immune cells. These proteins display fragments of material the immune cell has engulfed from outside, like pieces of bacteria. Together, the two classes give your immune system a way to monitor threats both inside and outside your cells.
In clinical testing, both classes matter for transplant matching. For disease-association testing, the specific marker your doctor orders depends on the condition being investigated. HLA-B27, for instance, is a Class I marker.