The tonsils and mucosa associated lymphoid tissues are not structurally organs; however, they function as secondary lymphoid organs because they are sites of immune responses.
Tonsils
Tonsils (ton’-sils) are clusters of lymphoid tissue located just deep to the mucous membrane in the pharynx (fayr- inks), or throat, and oral cavity. Like all lymphoid tissue, they contain both lymphocytes and macrophages to fight pathogens. Their function is to intercept and destroy pathogens that enter through the nose and mouth before they can reach the blood.
There are three kinds of tonsils that are strategically located to carry out this function:
- The palatine tonsils are located bilaterally at the junction of the oral cavity and the pharynx.
- The pharyngeal (fah-rin’-je-al) tonsil is located posterior to the nasal cavity in the superior portion of the pharynx. This tonsil is commonly called the adenoid.
- The lingual (ling’-gwal) tonsils are located on the base of the tongue in the posterior oral cavity.
Tonsils are larger in young children and play an important role in their defense against pathogens. Sometimes the palatine tonsils and pharyngeal tonsil become so overloaded and swollen with pathogens, a condition called tonsillitis, that they must be surgically removed by a tonsillectomy.
Mucosa Associated Lymphoid Tissue
Individual lymphoid nodules, like those found within lymph nodes, are located throughout the body, especially in the areolar connective tissues of mucous membranes. These collections of numerous macrophages and lymphocytes trapped in reticular tissue provide additional barriers to invasion by pathogens. Large clusters of many lymphoid nodules located in the mucous membranes of the respiratory, digestive, urinary, and reproductive tracts are referred to collectively as MALT (mucosa associated lymphoid tissue). The appendix, an extension of the large intestine located in the right lower quadrant of the abdominopelvic cavity, is part of the MALT that helps control bacterial growth in the large intestine. Table below outlines the components of the lymphoid system.
Components of The Lymphoid System
Component Characteristics Function Lymphatic Capillaries Microscopic closed-ended tubes in interstitial spaces Collect interstitial fluid from interstitial spaces; collect and transport dietary lipids and lipid- soluble vitamins; once in a lymphatic capillary, fluid is called lymph Lymphatic Vessels Formed by merging of lymphatic capillaries; structure similar to veins; contain valves; merge to form lymphatic trunks that drain into either the right lymphatic duct or the thoracic duct Transport lymph and empty it into the subclavian veins Lymphoid Organs Sites of lymphocyte production or proliferation, and immune responses Primary Lymphoid Organs Red Bone Marrow Located mostly in spongy bone of the skeleton Site of origination of all lymphocytes Thymus Bilobed gland located superior to the heart; size decreases with age Site of T cells maturation; secretes hormones called thymosins, which stimulate maturation of T cells Secondary Lymphoid Organs Lymph nodes Small, bean-shaped organs arranged in groups along lymphatic vessels Sites of lymphocyte proliferation; house T cells and B cells that are responsible for immunity; macrophages phagocytose pathogens and cellular debris from lymph Spleen Large lymphoid organ containing venous sinuses RBC and platelet reservoir; macrophages phagocytose pathogens, cellular debris, and worn formed elements from the blood; house lymphocytes Lymphoid Tissues Tonsils Masses of lymphoid tissue within the mucosae of pharynx and oral cavity Protect against invasion of pathogens that are ingested or inhaled MALT (mucosa associated lymphoid tissue) Masses of lymphoid tissue within the mucosae of respiratory, digestive, urinary, and reproductive tracts Guards against pathogens that penetrate the epithelium of mucosa Nonspecific Resistance And Its Component
Nonspecific resistance provides protection against all pathogens and foreign substances, but it is not directed against a specific pathogen. Nonspecific defense mechanisms include mechanical barriers, chemical actions, phagocytosis, inflammation, and fever.
Mechanical Barriers
The most obvious mechanical barriers against pathogens are the skin and the mucous membranes. The closely packed epidermal cells of the skin make penetration by pathogens very difficult and the acidic pH of the skin discourages bacterial growth. Mucous membranes are less effective barriers than the skin. However, they secrete mucus that entraps pathogens and airborne particles and usually prevents their contact with the underlying membranes. The continuous flow of tears over the eyes, the production and swallowing of saliva, the movement of vaginal secretions, and the passage of urine through the urethra are examples of fluid mechanical barriers that help to flush away pathogens before they can attack body tissues.
Chemical Actions
Various body chemicals, including certain enzymes, provide a nonspecific defense against pathogens. A few examples will illustrate the effect of these chemicals.
Tears, saliva, nasal secretions, and perspiration contain the enzyme lysozyme, which destroys certain types of bacteria and helps to protect underlying tissues.
Mucus is continuously produced by epithelia lining the respiratory and digestive tracts. Pathogens entering the nose and mouth tend to be trapped in the mucus on the surface of the tonsils and are destroyed there. Pathogens the tonsils miss are swallowed at frequent intervals. Upon reaching the stomach, most pathogens are destroyed by gastric juice, either by its acidic pH or by the enzyme pepsin. Pepsin acts by digesting the proteins composing the pathogens.
Cells that are infected with a virus produce interferon, a substance that stimulates uninfected cells to synthesize special proteins that inhibit the replication of viruses within them. In this way, the rapid growth of viruses may be inhibited.
The blood contains a group of plasma proteins known as complement, which are named because their actions complement the actions of antibodies. Complement proteins can bind to certain pathogens initiating a chain of events that leads to the destruction of the pathogen. The binding of complement is known as complement fixation. The fixed complement punches holes in the pathogen’s plasma membrane, causing the cell to burst and the pathogen to be destroyed. Subsequently, the resulting debris is cleaned up by phagocytes (neutrophils and macrophages). Complement proteins also enhance phagocytosis and inflammation.
Phagocytosis
Phagocytosis is the engulfing and destruction (by digestion) of pathogens, damaged or cancerous cells, and cellular debris by neutrophils and macrophages. When an infection occurs, neutrophils and monocytes are quickly attracted to the infected tissues. Monocytes entering the tissues become transformed into macrophages, large cells that are especially active in phagocytosis.
Some macrophages wander among the tissues, searching out and phagocytizing pathogens and cellular debris. Others become fixed (stationary) in particular locations in the body, where they phagocytize pathogens that are passing by. Fixed macrophages are especially abundant along the internal walls of blood and lymphatic vessels and in the spleen, lymph nodes, liver, and red bone marrow. The wandering and fixed macrophages compose the tissue macrophage system, which plays a major role in the destruction of potential pathogens.
The most common pathogens affecting humans are bacteria and viruses. Bacteria are very small, single-celled organisms that lack a true nucleus and other complex cellular organelles. Their DNA is concentrated, but it is not enclosed in a nuclear envelope as in higher organisms. Bacteria are simple organisms but they have the necessary metabolic machinery required for life and reproduction. Bacterial pathogens may cause disease by releasing toxins (poisons), releasing enzymes that damage cells, or entering and destroying cells. Antibiotics are effective in treating most bacterial infections.
In contrast, a virus is composed of nucleic acids, either DNA or RNA, enveloped by a protein coat. Viruses are thousands of times smaller than bacteria. A virus attaches to a cell’s surface receptor and penetrates into the cell. Once inside, it takes over the cell’s DNA and metabolic machinery, causing the cell to replicate hundreds or thousands of viruses, which burst forth as the cell is destroyed. The released viruses then move on to attack other cells. Antibiotics are not effective in treating viral infections.
Inflammation
Inflammation is a localized response to infection or injury that promotes the destruction of pathogens and the healing process. It is characterized by redness, pain, heat, and swelling of the affected tissues.
When injury or infection occurs, several mechanisms produce chemicals, such as complement proteins and histamine, that cause dilation of the arterioles and increase the permeability of blood capillaries in the affected area. The increased blood flow to the local area produces redness and heat. The increased movement of fluids out of the blood capillaries produces swelling (edema) of the tissues. Pain results from irritation of nociceptors by pathogens, swelling, or chemicals released by infected cells.
Some of the chemicals of the inflammatory response attract WBCs to the affected area. In bacterial infections, neutrophils and macrophages actively phagocytize the pathogens and damaged cells. The accumulated mass of living and dead WBCs, tissue cells, and bacteria may form a thick, whitish fluid called pus.
Fluids from blood capillaries that enter the affected area contain both fibrinogen and fibroblasts. Fibrinogen may be converted into fibrin to form a clot that is subsequently penetrated and enveloped by fibers formed by the fibroblasts. This action tends to seal off the infected area and prevent the spread of pathogens to neighboring tissues.
The continued action of WBCs usually brings the infection under control. Then, the dead pathogens and cells are cleaned up by phagocytes, and new cells are produced by cell division to repair any damage to the tissues.
Fever
Fever is a high body temperature that accompanies infections and is a normal part of the immune response. It serves a useful purpose as long as the body temperature does not get too high. The increased body temperature inhibits growth of certain pathogens and increases the rate of body processes, including those that fight infection.
Summary of Major Components of Nonspecific Resistance
Component Function Mechanical Barriers Intact skin Closely packed cells and multiple cell layers prevent entrance of pathogens Intact mucous membranes Closely arranged cells retard entrance of pathogens; not as effective as intact skin Mucus Traps pathogens in digestive, respiratory, urinary, and reproductive tracts Saliva Washes pathogens from oral surfaces Tears Wash pathogens from surface of eye Urine Washes pathogens from urethra Vaginal secretions Chemical Actions
Wash pathogens from vaginal canal Acidic pH of skin Retards growth of many bacteria Gastric juice Kills pathogens that are swallowed Interferon Helps to prevent viral infections Lysozyme Antimicrobial enzyme in nasal secretions, perspiration, saliva, and tears that kills some pathogens Complement Group of plasma proteins that enhance inflammation and phagocytosis. Also cause direct death of pathogens by puncturing plasma membranes. Other Mechanisms Fever Speeds up body processes and inhibits growth of pathogens Inflammation Promotes nonspecific resistance; confines infection; attracts WBCs Phagocytosis Phagocytes engulf and destroy pathogens
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