that the lymphocyte B cells and T cells both develop from the stem cells in the red bone marrow:
•T cells form in the bone marrow (pre-T cells), and they migrate to the thymus where they complete their development and become immunocompetent T cells. Immunocompetent means they have the ability to carry out immune responses.
•B cells complete their development in the bone marrow.
Both B and T cells then circulate in the blood stream, but not before they have acquired surface proteins (antigen receptors), molecules that are capable of recognizing specific antigens.
The red bone marrow and thymus gland are primary lymphatic organs because they produce B and T cells. The thymus gland lies behind the sternum and extends upward to the base of the neck. At birth, it weighs about half an ounce (10-15 grams). It grows until puberty when it reaches about 1½ ounces (30-40 grams). After this, it begins to atrophy so that by age forty it is back to birth size and by age seventy it is less than ten grams. Most of the body’s T cells develop before puberty. Some develop throughout life, but to a lessened degree. As we advance in years, the effectiveness of our T cell response decreases making us more susceptible to disease.
The thymus liberates a hormone known as thymosin, which stimulates the growth of the thymus and the development of the lymphocytes. Once the lymphocytes have matured, they take up station in one of three places: some remain in the thymus and are the source of future T cell lymphocyte needs; some enter the blood circulation; and some lodge in other lymphoid tissue.
OTHER LYMPHOID TISSUE
These are secondary lymphatic organs: lymph nodes and spleen:
LYMPH NODES
Lymph vessels, lying in body tissues, course back toward the heart carrying body fluids known as lymph, a clear yellowish watery substance derived from body tissues that pass through lymph channels and oval structures known as lymph nodes. The lymph finally returns to the blood circulation of the body through the largest lymph vessel (thoracic duct) in the upper chest area.
The lymph nodes are from one to twenty-five millimeters in length (0.04 to 1 inch). Lymph nodes are scattered throughout the body, but predominantly concentrate in groups such as the axilla (under the arm), and groin area. The microscopic architecture of a lymph node includes an outer cortex and inner medulla.
The outer cortex contains follicles, the outer rim of which contains densely packed T lymphocytes plus macrophages. The inner region of the follicles, known as germinal centers, contain B lymphocytes. It is here that the B-lymphocytes evolve into plasma cells that make proteins (antibodies) that attack foreign or non-self cells. These plasma cells, along with some T cells, migrate to other parts of the body.
In the inner medulla, there are lymphocytes, macrophages, and plasma cells packed together in strands known as medullary cords.
The purpose of lymph nodes is to filter foreign substances from the lymph as it passes toward the blood stream. Here the macrophages attack (phagocytize) these foreign substances, and the lymphocytes kill by immune response.
SPLEEN
The spleen is an organ, about the size of a fist, located under the rib cage below the left diaphragm. The spleen has two different types of tissue:
•White pulp consisting mostly of B cell lymphocytes
•Red pulp consisting of red blood cells, lymphocytes, macrophages, other white blood cells and plasma cells.
Since no lymph vessels enter the spleen, it does not filter lymph. The spleen receives blood from the splenic artery that branches to it from the aorta. As the blood moves through the spleen the T cells monitor it for any non-self invaders. If they recognize any suspicious non-self or invading cell they will call up the appropriate memory B cell. This memory B cell will divide rapidly and produce antibodies (a protein formed in response to a specific antigen that reacts with or neutralizes that antigen). Then the antibodies will attack the invading antigen.
The macrophages in the spleen will destroy worn out red blood cells (they only live about three months), and destroy worn out platelets (they live a matter of days).
That concludes the discussion of the lymph nodes and the spleen, but there are other areas where lymphocytes congregate. Pressing the tongue against the interior of cheeks or lips will identify (for want of a better description) little round balls. These are lymphatic nodules and they are oval to rounded collections of lymphatic tissue. They are scattered throughout the connective tissue of our mucous membranes lining the gastrointestinal tract, the bronchial tubes, the reproductive tract, and the urinary tract. These nodules differ from lymph nodes: they do not have a capsule (outer cover). These lymphatic collections stand guard against any invaders that may penetrate the mucosal barrier. The name for this lymphoid tissue is mucosa associated lymphoid tissue (MALT). Included in this category of mucosal guards are patches of lymphoid tissue known as Peyer’s patches (located under the mucosa of the small intestine), and the five tonsils that form a ring in the back of our mouths (oropharynx).
MEMORY
Immunity is the body’s ability to defend against specific invaders. In addition to the property of specificity, the immune process also includes the ability of the body to prepare a future defense against the invader; in other words—memory.
When T and B cells are finally formed (T in the thymus…B in the bone marrow), they are said to be immunocompetent. This means that when the T cells leave the thymus and the B cells leave the bone marrow they have on their surface some distinctive proteins or antigen receptors labeled CD4 or CD8. These antigen receptors are programmed to attack specific antigens. Those T cells that have the CD4 protein are helper T cells, and those T cells that have the CD8 protein are killer T cells.
Before getting specific about killer T cells, it is important to know that there are two types of immunity:
1.The first is cell-mediated cell immune response (CMI) where CD8 cells become killer cells and attack an invading antigen. This also goes by the name of cellular immunity.
2.The second type of immunity is antibody-mediated humeral immune response (AMI) where B cells change into plasma cells that manufacture and secrete specific proteins called antibodies or immunoglobulins that bind to a specific antigen. This goes by the name of humeral immunity (humeral means body fluids).
CD4 cells become helper cells, and as the name implies they “help” the CD8 cell become killer cells, and help the B cell change into plasma cells, little factories that produce specific antibodies by the millions.
Cellular immunity is effective against pathogens that enter cells (intracellular pathogens) such as bacteria, fungi, viruses, cancer cells, and foreign transplants. An attack on an intracellular pathogen is an attack by cells on cells.
Humeral immunity (antibodies) attack antigens dissolved in body fluids, and extra cellular pathogens (mainly bacteria) that multiply outside of cells.
As you can see, each of these different forms of immunity has their preferred battlefield.
Here are some important points to remember:
•An immune system stockpiles an incredibly large arsenal of cells.
