Antigen #2

I would like to resubmit my question about antigens. What is their lifecycle in the blood? and... How long does it take to clear them from the bloodstream considering their production source has been cut off.

Thank you, Dorf

[CCF-RN,MSN-rf]

Dear Dorf:  The following is from the Merck Manual of Medical Information:  

Cancer and the Immune System
The body's immune system attacks and eliminates not only bacteria and other foreign substances but also cancer cells. A cancer cell is not a foreign cell; rather, it is a cell whose biologic function has been altered in such a way that it doesn't respond to the body's normal mechanisms for controlling cell growth and reproduction. The abnormal cells can continue to grow, resulting in cancer.

Much of the body's protection against cancer is carried out directly by cells of the immune system rather than by antibodies circulating in the bloodstream. (see page 811 in Chapter 167, Biology of the Immune System) For example, the presence of tumor antigens on cancer cells can activate certain white blood cells (lymphocytes and, to a lesser degree, monocytes), which carry out an immunologic surveillance, looking for cancer cells and destroying them.

The immune system's critical role in controlling cancer cell development is exemplified by an astounding statistic: Cancer is 100 times more likely to occur in people who take drugs that suppress the immune system (for example, because of an organ transplant or a rheumatic disease) than in people with normal immune systems. In addition, sometimes a transplanted organ has a cancer that wasn't diagnosed before the transplantation. The cancer may have been growing slowly or not at all in the organ donor. However, the cancer starts growing and spreading rapidly in the recipient, whose immune system is suppressed by drugs to protect the transplant. Typically, when the immunosuppressive drugs are stopped, the transplanted organ is rejected and the transplanted cancer is destroyed as well.

Tumor Antigens
An antigen is a foreign substance recognized and targeted for destruction by the body's immune system. (see page 813 in Chapter 167, Biology of the Immune System) Antigens are found on the surface of all cells, but normally a person's immune system doesn't react to his own cells. When a cell becomes cancerous, new antigens--unfamiliar to the immune system--appear on the cell's surface. The immune system may regard these new antigens, called tumor antigens, as foreign and may be able to contain or destroy the cancer cells. However, even a fully functioning immune system can't always destroy all cancer cells.

Tumor antigens have been identified in several types of cancer, including malignant melanoma, bone cancer (osteosarcoma), and some gastrointestinal cancers. People with these cancers may have antibodies against the tumor antigens. The antigens generally don't elicit an immune response adequate to control the cancer. The antibodies seem unable to destroy the cancer and sometimes even seem to stimulate its growth.

Certain tumor antigens can be used to advantage, however. Antigens released into the bloodstream by some cancers can be detected with blood tests. These antigens are sometimes called tumor markers. Great interest has focused on whether tumor markers can be used as screening tests in people who have no symptoms of cancer. Because the tests are expensive and not very specific, for the most part their use in routine screening is currently inadvisable. Instead, tumor markers are much more valuable in both the diagnosis and treatment of cancer. For example, blood tests can help determine if a cancer treatment is effective. If the tumor marker no longer appears in the blood sample, the treatment has probably been successful. If the marker disappears and later reappears, the cancer has probably returned.

Carcinoembryonic antigen (CEA) is a tumor antigen found in the blood of people with cancer of the colon, breast, pancreas, bladder, ovary, or cervix. High levels of this antigen may also be found in people who are heavy cigarette smokers and in those who have cirrhosis of the liver or ulcerative colitis. Therefore, having a high carcinoembryonic antigen level doesn't always mean that a person has cancer. Measuring the carcinoembryonic antigen level in people who have been treated for cancer helps doctors detect a recurrence.

Alpha-fetoprotein (AFP), which is normally produced by fetal liver cells, is found in the blood of people with liver cancer (hepatoma). In addition, alpha-fetoprotein is often found in people with certain cancers of the ovary or testis and in children and young adults with pineal gland tumors.

Beta-human chorionic gonadotropin (-HCG), a hormone produced during pregnancy that serves as the basis for pregnancy tests, also occurs in women who have a cancer originating in the placenta and in men with various types of testicular cancer. Beta-human chorionic gonadotropin is a very sensitive tumor marker. Its use in monitoring the effects of treatment has helped improve the cure rates for these cancers to well over 95 percent.

Prostate-specific antigen (PSA) levels are high in men with noncancerous (benign) enlargement of the prostate and considerably higher in men with prostate cancer. What constitutes a meaningfully abnormal level is somewhat uncertain, but men with an elevated prostate-specific antigen level should be evaluated further for prostate cancer. (see page 1060 in Chapter 229, Disorders of the Penis, Prostate, and Testes) Monitoring the blood level of prostate-specific antigen after treatment for cancer can indicate whether the cancer has recurred.

Blood levels of CA-125, another antigen, are measurably increased in women with a variety of ovarian diseases, including cancer. Since ovarian cancer is often difficult to diagnose, some cancer experts advocate screening for CA-125 in women over 40 years old. However, its lack of sensitivity and specificity mean that it isn't yet a reliable screening test.

Elevated levels of CA 15-3 occur in breast cancer, CA 19-5 in pancreatic cancer, 2-microglobulin in multiple myeloma, and lactate dehydrogenase in testicular cancer, but none of them can be recommended for cancer screening. However, they are useful in monitoring the response to treatment of a person already diagnosed with cancer. (end of Merck Manual excerpt)

Regarding your question, the amount of time antigens live depends upon the antigen and the cells on which the antigens reside.  In speaking about tumor antigens, many of these exist on the surfaces of "normal" cells as well, so there may never be eradication of measurable antigen.  Also, tumor markers are rarely considered as a "one test, take action" test.  These markers are watched for trends and clinical correlation.  These tests have some inherent variability so the actual number is not the most important feature.  Rather, it is the rate of rise or fall and the trend up or down.  Only then, can reasonable interpretations be made.