Pre-text assignment
Learn the key words and phrases:
tonsils, adenoids, lymphocytes, lymph fluid, complement proteins, antigen-specific recognition, gram-negative (gram-positive) organisms.
Organs of the Immune System
The immune system contains the following organs and cells: tonsils and adenoids, the thymus gland, lymph nodes, bone marrow, and white blood cells that leave blood vessels and migrate through tissues and lymphatic circulation. The spleen, appendix, and patches of lymphoid tissue in the intestinal tract are also parts of the immune system.
The essential job of this system is to distinguish self-cells from foreign substances and to recognize and take protective action against any materials that ought not to be in the body, including abnormal and damaged cells. The immune system can seek out and destroy disease germs, infected cells, and tumor cells. The immune system includes the following cells:
T lymphocytes (T cells);
B lymphocytes (B cells);
natural killer cells (NK cells);
dendritic cells;
phagocytic cells;
complement proteins.
These cells develop from «pluripotential hematopoietic stem cells» starting from a gestational age of about five weeks. They circulate through various organs in the lymphatic system as the fetus develops. T and B lymphocytes are the only units of the immune system that have antigen-specific recognition powers; they are responsible for adaptive immunity. In other words, the T and B cells are important in the immunity that vaccination promotes.
Lymphoid organs
Lymphoid organs are called lymphoid because they are home to lymphocytes – the white blood cells that are key operatives of the immune system. Within these organs, the lymphocytes grow, develop, and are deployed.
Bone marrow, the soft tissue in the hollow center of bones, is the ultimate source of all blood cells, including the immune cells.
The thymus is an organ that lies behind the breastbone; lymphocytes known as T lymphocytes, or just T cells, mature there.
The spleen is a flattened organ at the upper left of the abdomen. Like the lymph nodes, the spleen contains specialized compartments where immune cells gather and confront antigens.
In addition to these organs, clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract and the airways and lungs – gateways to the body. These tissues include the tonsils, adenoids, and appendix.
Small, bean-shaped lymph nodes sit along the lymphatic vessels, with clusters in the neck, armpits, abdomen, and groin. Each lymph node contains specialized compartments where immune cells congregate and encounter antigens.
Immune cells and foreign particles enter the lymph nodes via incoming lymphatic vessels or the lymph nodes' tiny blood vessels. All lymphocytes exit lymph nodes through outgoing lymphatic vessels. Once in the bloodstream, they are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic system to begin the cycle all over again.
How immunity works
The lymphatic system is a key participant in the body's immune actions. It is a network of vessels and nodes unified by the circulatory system. Lymph nodes occur along the course of the lymphatic vessels and filter lymph fluid before it returns to the bloodstream. The system removes tissue fluids from intercellular spaces and protects the body from bacterial invasions.
Types of immunity
Immunity is the ability of the body to resist the infecting agent. When an infectious agent enters the body, the immune system develops antibodies which can weaken or destroy the disease-producing agent or neutralize its toxins. If the body is re-introduced to the same agent at a later time, it is capable of developing antibodies at a much faster pace. As a result, the individual would likely not become sick, and immunity has developed.
Natural immunity is present when a person is immune to a disease despite not having either the disease itself or any vaccination against it. Acquired immunity may be either active or passive. Active immunity comes from having the disease or by inoculation with antigens, such as dead organisms, weakened organisms, or toxins of organisms. The antigens introduced during vaccination produce antibodies that protect the body against the infecting agent, despite the fact that the person does not become sick. Passive immunity is relatively short lived and is acquired by transferring antibodies from mother to child in the uterus or by inoculation with serum that contains antibodies from immune persons or animals. Passive immunization is used to help a person who has been exposed or is already infected to fight off disease. Although various types of serums may be used to produce passive immunization, gamma globulin is the most frequently used source of human antibodies.
Development of the immune response
Normal infants have the capability to develop responses to antigens at birth. Infants also start life with some immunoglobulin antibodies acquired from the mother. These antibodies cross the placental barrier, but not all types are transmitted equally. In particular, infants start with antibodies to viruses and gram-positive organisms, but not to gram-negative organisms. Gram is the name of a stain that distinguishes broad classes of bacteria. Gram-negative organisms are responsible for many diseases, including gonorrhea, pertussis (whooping cough), salmonella poisoning, and cholera.
Figure 1 – Organs of the Immune System
Escherichia coli (E. coli) is another common gram-negative organism.
Immunoglobulin antibodies are divided into five classes. The capacity of the body to produce each immunoglobulin varies with age. Newborn babies (premature and full-term) begin to synthesize antibodies at an increased rate soon after birth in response to antigenic stimulation of their new environment. At about six days after birth the serum concentration of specific antibodies rises sharply, and this rise continues until adult levels are achieved by approximately the end of the first year. Maternal immunity gradually disappears during the first six to eight months of life. A concentrated level of antibodies is reached and maintained by seven to eight years of age.