Adapted by Thyroid Guide, Mary J. Shomon
Return to Index for "Understanding the Immune System"
Lymphocytes are small white blood cells that bear the major responsibility for carrying out the activities of the immune system; they number about one trillion. The two major classes of lymphocytes are: B cells, which grow to maturity independent of the thymus, and T cells, which are processed in the thymus. Both B cells and T cells recognize specific antigen targets.
B cells work chiefly by secreting soluble substances called antibodies into the body's fluids, or humors. (This is known as humoral immunity.) Antibodies typically interact with circulating antigens such as bacteria and toxic molecules, but are unable to penetrate living cells. T cells, in contrast, interact directly with their targets, attacking body cells that have been commandeered by viruses or warped malignancy. (This is cellular immunity.)
Although small lymphocytes look identical, even under the microscope, they can be told apart by means of distinctive molecules they carry on their cell surface. Not only do such markers distinguish between B cells and T cells, they distinguish among various subsets of cells that behave differently. Every mature T cell, for instance, carries a marker known as T3 (or CD3); in addition, most helper T cells carry a T4 (CD4) marker, a molecule that recognizes class II MHC antigens. A molecule known as T8 (CD8), which recognizes class I MHC antigens, is found on many suppressor/cytotoxic T cells. In addition, different T cells have different kinds of antigen receptors-either alpha/beta or gamma/delta.
Thyroid Information, News and Support To Help You Live Well
- About F.Y.I. -- About Thyroid Disease Weekly Newsletter --
Stay up on new developments in autoimmune disease and thyroid problems. Subscribe to
Mary Shomon's free weekly thyroid newsletter.Thyroid F.Y.I. Signup Page or Sign Up
Now Right Here:
- About Thyroid Bookstore -- Be sure to read the books that will help you live well with and triumph over your thyroid condition. Visit the About Thyroid Bookstore.
- FIND SUPPORT at the
Here are some of the latest discussions at the main Thyroid Forum, one of many popular Thyroid Forums, which include Thyroid Diet/Weight Loss, Top Doctors, and Alternative Medicine.Jump into the conversations now!
Updated 2002: This information has been made available from the National Cancer Institute and the National Institute of Allergy and Infectious Diseases.