GVHD occurs because the donor's T-lymphocytes (type of immune cell) contained in the transplanted organ or blood attack cells in the recipient's body.

Normally, the immune system uses markers called human leukocyte antigens (HLA) to recognize which cells are "self" cells and which are "foreign" cells. This allows the immune system to identify foreign invaders like bacteria, viruses, and fungi that may harm the body.

In GVHD, the donor's immune cells do not recognize the HLA antigens on the recipient's cells, so they attack them. To help avoid the development of GVHD, the donor's HLA antigens need to match recipient's as closely as possible. Selecting a closely matched donor or cord blood unit (blood from the umbilical cord that is rich in stem cells) may help reduce the risk of developing GVHD.

Acute: Acute GVHD develops within three months of the transplant and it is characterized by dermatitis (skin rash), hepatitis (liver inflammation), difficulty breathing (due to stiffening of lungs), diarrhea (sometimes containing blood), jaundice (yellowing of the skin and eyes), and inflammation of the intestine.

Chronic: Chronic GVHD typically develops longer than three months after the transplantation and it can last for months or years. Chronic GVHD can range from mild (skin rash) to life threatening (causing liver failure and severe infections). Symptoms may include dermatitis (skin rash), tightening or inflammation of the skin, lesions in the mouths, dry eyes and mouth, hair loss, liver damage, and indigestion.

Transfusion-associated: Transfusion-associated GVHD typically develops within four to 30 days after the blood transfusion. Symptoms are the same as acute GVHD.

General: In 1966, researcher R. Billingham described three criteria necessary to confirm a diagnosis of GVHD. These criteria are still used today: 1) the graft must contain healthy immune cells; 2) the host must have transplantation alloantigens (molecules that allow the body to recognize self cells) that are not present in the donor cells making the host appear foreign to the graft, which stimulates an immune response; 3) the host must not be able to launch an effective immune response against the graft.

Alloantigen assay: An alloantigen assay may be conducted to determine whether alloantigens are present in the patient's blood. Alloantigens are molecules on the body's cells that allow the body to identify them as self-cells. If the host contains alloantigens that are lacking in the donor, this indicates GVHD.

Infection: Because patients with GVHD have weakened immune systems, which are further weakened by immunosupressants, they are vulnerable to infections. Infection with one particular virus, called cytomegalovirus (CMV), is so common among GVHD patients that some experts recommend preventative medications.

Marrow aplasia: Marrow aplasia occurs when the bone marrow is unable to produce immune system cells. This condition is a frequent complication of transfusion-associated GVHD. However, this complication can be prevented by irradiating blood products with at least 2500 centigray (unit of radiation equal to one rad). During this process, X-rays are used to destroy the T-lymphocytes in the blood before an immunocompromised patient (like an HIV or chemotherapy patient) receives a blood transfusion. These T-lymphocytes are responsible for launching the immune response that causes GVHD.

Long-term effects: Some GVHD patients may experience long-term effects even after the disease is treated. Patients may experience photosensitivity (skin sensitivity to the sun or ultraviolet light). These patients are encouraged to wear sunscreen and avoid extensive sun exposure.

Other patients may develop cavities and gum disease, which is a result of dry mouth that is associated with GVHD. This is because saliva is necessary to prevent infection and tooth decay by controlling fungi and bacteria. These patients are encouraged to visit their dentists regularly. A fluoride rinse may be recommended. Other patients may experience dry or irritated eyes, which may be alleviated with over-the-counter eye drops. In some cases, plugged tear ducts may cause dry eyes.

Some patients may suffer from long-term diarrhea or difficulty digesting food. These patients should talk with the dietitians at their transplant centers to determine what foods to eat and what to avoid. In general, acidic and spicy foods are often difficult to digest in patients who suffer from gastrointestinal symptoms.