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Lymphocytes are formed from lymphoblasts. This process begins in the bone marrow, but goes on to mature in the lymphoid tissues of the spleen, lymph nodes, and thymus gland. Monocytes are formed from monoblasts. Collectively, lymphocytes and monocytes are known as agranulocytes.
Lymphocytes
Lymphocytes constitute 25-38% of the total white blood cells (WBCs). They are the most important cells in the immune function and the dominant cells of the lymphatic system.
The body contains roughly 1012 lymphocytes, with a combined weight of over a kilogram (over two pounds). Lymphocytes generally have a long lifespan. Roughly 80% will survive for 4 years, with some lasting 20 years or more.
Lymphocytes are constantly on the move throughout the body, but at differing rates. The fastest are the T-cells. Since they must have close contact with their targets, they move rather quickly, spending maybe 30 minutes in the blood, 5 or 6 hours in the spleen, and 15-20 hours in a lymph node.
B-cells are slower since they are responsible for antibody production. A typical B-cell might spend around 30 hours in a lymph node before moving to another location.
There are three classes of lymphocytes in the blood:
- T cells mature in the thymus gland under the effects of the hormone thymosin. Approximately 80% of the circulating lymphocytes are T cells responsible for cell-mediated immunity or cellular immunity. Cellular immunity is also concerned with a delayed type of response needed for such conditions as:
- transplant rejections
- the slow development of diseases
- delayed hypersensitivity reactions
- certain autoimmune diseases
- cells infected with viruses or parasites
- the formation of cancer cells
- many other functions
Types of T cells
- Killer T-cells(cytotoxic) attack antigens in a direct cell to cell combat using two methods:
- punching holes in the cell’s membrane through the release of a destructive protein called perforin
- secreting a group of hormones called lymphokines which enhance the activity of the phagocytes.
Killer T-cells are the most important lymphocytes in cellular immunity as they track down and attack bacteria, fungi, protozoa, parasites, and other foreign particles. They are also responsible for preventing the rejection of skin grafts or organ transplants from other donors.
- Helper T-cells also secrete lymphokines that stimulate both B-cells and other T-cells, thereby enhancing the immune response. Both killer and helper T-cells are called regulatory T-cells.
- Suppressor T-cells inhibit both T- and B-cell activity, stopping the immune response once the antigen has been destroyed. This is an important function; otherwise, antibodies would keep producing, ultimately causing an autoimmune response.
- Memory T-cells only function is to remember a particular antigen for future reference should it appear again. They do not participate in the destruction of an antigen. These memory cells have the ability to recognize at least ten million configurations of molecules viruses, parasites, fungi, bacteria and their toxins, pollens, strange blood cells, plus all the man-made molecules. The list is endless and is becoming harder to deal with as evidenced by the increase in disease and disorders.
- B cells interact with antigens indirectly through the secretion of antibodies called antibody-mediated immunity. Because the antibodies are carried by blood and tissue fluids (body humors), this type of immunity is also referred to as humoral immunity. In essence, it is a continuation of the cellular response by focusing on antibody/antigen/complement activities. Activated helper T-cells bind to inactive B-cells to secrete cytokines that accomplish four things:
- promote B-cell activation;
- stimulate B-cell division;
- accelerate plasma cell production;
- enhance antibody production.
B-cells also produce memory cells and perform the same role for antibody-mediated immunity that memory T-cells perform for cellular-mediated immunity. They remain in reserve until the reappearance of a particular antigen.
- Natural killer (NK) cells are a special type of lymphocyte. They act against the presence of antigens that have abnormal cell membranes, as those cells infected with viruses or cancer cells. When NK-cells encounter such antigens, they secrete special proteins that kill the abnormal cell by destroying its membrane. Unfortunately, some of these abnormal cells are able to avoid detection, a process called immunological escape. When this has occurred, the abnormal cell can multiply and spread without interference from the NK cells.
Monocytes
Monocytes are produced from monoblasts. They make up 3-8% of the total WBCs and are nearly twice the size of a typical erythrocyte. The nucleus is large and usually has a kidney-shape. Even though they are fewer than lymphocytes in number, monocytes are more efficient phagocytes.
Monocytes deposit themselves in various organs, producing macrophages which surround foreign particles to destroy them (phagocytosis). Macrophages are larger than microphages and can either be wanderers (free macrophages) or immobile (fixed macrophages).
- Wandering macrophages travel through the body, cleaning up pathogens and debris as they come across them. However, they are more numerous under the mucus membranes and the skin.
- Fixed macrophages remain in such connective tissue organs as the liver, spleen, lymph nodes, or red bone marrow. These organs are sometimes referred to as the tissue macrophage system. Macrophages release chemicals that attract and stimulate neutrophils, other monocytes and phagocytes to areas of infection or injury. They also secrete substances that attract fibroblasts that will begin producing scar tissue which walls off an injured area.
