Blood, Lymphatic, and Immune Systems
Chapters 5 and 6

Objectives

Functions of the Blood, Lymphatic, and Immune Systems

Homeostasis, or a "steady state," is a continual balancing act of the body systems to provide an internal environment that is comparable with life. The two liquid tissues of the body, the blood and lymph have separate but interrelated functions in maintaining this balance. They combine with a third system, the immune, to protect the body against pathogens that could threaten the organism's viability. The blood is responsible for the following:

The lymph system is responsible for the following:

The immune system is responsible for the following:

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The hematic and lymphatic systems flow through separate yet interconnected and interdependent channels. Both are systems composed of vessels and the liquids that flow through them. The immune system, a very complex set of levels of protection for the body, includes blood and lymph cells.

The above graphic shows the relationship of the lymphatic vessels to the circulatory system. Note the the close relationship between the distribution the distribution of the lymphatic vessels and the venous blood vessels. Tissue fluid is drained by the lymphatic capillaries and transported by a series of larger lymphatic vessels toward the heart.

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 Hematic System

The hematic system is composed of blood and the vessels that carry the blood throughout the body. Because blood can be an extremely important part of the diagnostic process, students need to understand its normal composition. Blood is composed of a solid portion called plasma. Blood cells make up 45% of the total blood volume, and plasma makes up the other 55%.

The solid portion of blood is composed of three different types of cells:

 

 

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 Components of Blood

Erythrocytes (Red Blood Cells)

The erythrocytes (which are normally present in the millions) have the important function of transport O2 and CO2 throughout the body. The vehicle for this transportation is a protein-iron pigment called hemoglobin.

The formation of RBCs in the bone marrow is stimulated by a hormone from the kidneys called erythropoietin. RBCs have a life span of approximately 120 days, after which they decompose into hemosiderin, an iron pigment resulting from hemolysis and bilirubin. The iron is stored in the liver to be recycled into new RBCs, and the bile pigments are excreted via the liver. 

Abnormal RBCs can be named by their morphology, the study of shape or form. RBCs normally have a biconcave, dislike shape. (Although the center is depressed, there is not an actual hole.) Those that are shaped differently often have difficulty in carrying out their function. For example, sickle cell anemia is a hereditary condition characterized by erythrocytes (RBCs) that are abnormally shaped. They resemble a crescent or sickle. An abnormal hemoglobin found inside these erythrocytes causes sickle-cell anemia in a number of Africans and African-Americans. Did You Know?

Leukocytes (White Blood Cells)

Although there are fewer leukocytes (thousands, not millions), there are different types with different functions. In general, WBCs protect the body from invasion by pathogens. The different types of cells provide this defense in a number of different ways. There are two main types of WBCs: granulocytes and agranulocytes.

Granulocytes

Named for their appearance, granulocytes also called polymorphonucleocytes have small grains within the cytoplasm and multilobed nuclei. Both names are used interchangeably.

These are three types of granulocytes, each with its own function. Each of them is named for the type of dye that it attracts.

  1. Eosinophils - are cells that absorb an acidic dye, causing them to appear reddish. An increase in eosinophils is a response to a need for their function in defending the body against allergens and parasites.
  2. Neutrophils are cells that do not absorb either an acidic or basic dye and consequently are a purplish color. They are also called phagocytes because they specialize in phagocytosis and generally combat bacteria in pyogenic infections. This means that these cells are drawn to the site of a pathogenic "invasion," where they consume the enemy and remove the debris resulting from the battle.
  3. Basophils are cells that absorb a basic (or alkaline) dye and stain a bluish color. Especially effective in combating parasites, they release histamine (a substance that initiates an inflammatory response) and heparin (an anticoagulant), both of which are instrumental in healing damaged tissue.

Be Careful!

Be Careful!

Agranulocytes

Agranulocytes are cells named for their lack of granules. The alternative names, mononuclear leucocytes, is so given because they have one nucleus. Both names are used interchangeably. Although these cells originate in the bone marrow, they mature after entering the lymphatic system. There are two types of these WBCs:

  1. Monocytes: These cells, named for their single, large nucleus, transform into macrophages, which eat pathogens and are effective against severe infections.
  2. Lymphocytes: these cells are key in what is called the immune response, which involves the "recognition" of dangerous, foreign (viral) substances, and the manufacture of their neutralizers. The foreign substances are called antigens, and the neutralizers are called antibodies

Thrombocytes (Platelets)

Platelets (also known as thrombocytes) have a round or oval shape and are so named because they look like small plates. Platelets aid in the process of coagulation, the process of changing a liquid to a solid. When blood cells escape their normal vessels, they agglutinate, or clump together, by the following process: First, they release factor X (formerly called thrombokinase), which, in the presence of calcium, reacts with the blood protein, prothrombin, to form thrombin. Thrombin then converts another blood protein, fibrinogen, to fibrin, which eventually forms a mesh like fibrin clot (blood clot), achieving hemostasis (control of blood flow; that is, stopping the bleeding).

Plasma

Plasma, the liquid portion of blood, is composed of the following:

  1. Water, or H2O (90%)
  2. Inorganic substances (calcium, potassium, sodium)
  3. Organic substances (glucose, amino acids, fats, cholesterol, hormones)
  4. Waste products (urea, uric acid, ammonia, creatinine)
  5. Plasma proteins (serum albumin, serum globulin, and two clotting proteins: fibrinogen and prothrombin)

Serum is plasma minus the clotting proteins. Serology is the branch of laboratory medicine that studies blood serum for evidence of infection by evaluating antigen-antibody reactions in vitro.

Did You Know

The clotting process

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Blood Groups

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Human blood is divided into four major different types: A, B, Ab, and O. The differences are due to antigens present on the surface of the blood cells. Antigens are substances that produce an immune reaction by their nature of being perceived as foreign to the body. In response, the body produces substances called antibodies that nullify or neutralize the antigens. In blood, these antigens are called agglutinogens because their presence can cause the blood to clot.

The antibody is termed an agglutinin. For example, type A blood has A antigen, type B has B antigen, type AB has both A and B antigens, and type O has neither A nor B antigens. If an individual with type A blood is transfused with type B blood, The A antigens will form anti-B antibodies because they perceive B blood as being foreign. Following the logic of each of these antigen-antibody reactions, an individual with type AB blood is a universal recipient, and an individual with type O blood is a universal donor.

Another antigen, the Rh factor, is important in pregnancy because a mismatch between the fetus and the mother can cause erythroblastosis fetalis, or hemolytic disease of the newborn. In this disorder, a mother with a negative Rh factor will develop antibodies to an RH + fetus during the first pregnancy. If another pregnancy occurs with an Rh + fetus, the antibodies will destroy the fetal blood cells.

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Lymphatic System

The lymphatic system is responsible for the following:

The lymphatic system is composed of lymph (or interstitial fluid), lymph vessels, lymph nodes, lymph organs (e.g. tonsils, adenoids, appendix, spleen,, thymus gland, and patches of tissue in the intestines called Peyer patches), and lymphoid tissue. Monocytes and lymphocytes pass from the bloodstream through the blood capillary walls into the spaces between the cells in the body. When they pass into this lymph or interstitial fluid that surrounds cells, they perform their protective functions. Monocytes change into macrophages, destroy pathogens, and collect debris from damaged cells. Lymphocytes are much more complicated and are essential to the immune response, so they are discussed in the next section. Once monocytes and lymphocytes pass into the lymphatic capillaries, the fluid is termed lymph or lymphatic fluid.

 

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Lymph moves in one directo to prevent pathogens from flowing through the entire body. The system filters out the microorganisms as the lymph passes through its various capillaries, vessels, and nodes. Lymph travels in the following sequence:

  1. From the interstitial spaces between the cells, then
  2. Toward the heart through lymphatic capillaries.
  3. To lymphatic vessels that carry lymph using a valvular system.
  4. To the lymphatic nodes, which are also called lymph glands, that filter the debris that has been collected through the use of macrophages. These nodes can become enlarged when pathogens are present. Note the major lymph nodes in the figure, including the cervical, axillary, inguinal , and mediastinal nodes.
  5. Then to either the right lymphatic duct or the thoracic duct, both of which empty into the large subclavian veins in the neck.
  6. Once in the venous blood, the lymph is then recycled through the body through the circulatory system.

The organs in the lymphatic system are the spleen, the thymus gland, the tonsils, the appendix, and Peyer's patches. the spleen is located in the upper left quadrant and serves to filter, store, and produce blood cells; remove RBCs; and activate B lymphocytes. The thymus gland is located is located in the mediastinum and is instrumental in the development of T lymphocytes (T cells). the tonsils are lymphatic tissue (lingual, pharyngeal, and palatine) that helps protect the entrance to the respiratory and digestive systems. The vermiform appendix and Peyer patches are lymphoid tissue in the intestines.

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Immune System

The immune system is composed of organs, tissues, cells, and chemical messengers that interact to protect the body from external invaders and its own internally altered cells. The chemical messengers are cytokines which are secreted by cells of the immune system that direct immune cellular interactions. Lymphocytes (leukocytes that are categorized as either B cells or T cells) secrete lymphokines. Monocytes and macrophages secrete monokines. Interleukins are a type of cytokine that send messages among leukocytes to direct protective action. The best way to understand this system is through the body's various levels of defense. The goal of pathogens is to breach these levels to enter the body, reproduce, and subsequently exploit healthy tissue, causing harm. The immune system's task is to stop them.

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The above graphic illustrates the levels of defense. The two outside circles represent nonspecific immunity and its two levels of defense. the inner circle represents the various mechanisms of specific immunity, which can be natural (genetic) or acquired in four different ways. Most pathogens can be contained by the first two lines of nonspecific defense. However, some pathogens deserve a "special" means of protection, which is discussed under "Specific Immunity."

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Nonspecific Immunity

This term refers to the various ways that the body protects itself from many types of pathogens, without having to "recognize" them. The first line of defense in nonspecific immunity (the outermost layer) consists of the methods of protection:

The second line of defense in nonspecific immunity comes into play if the pathogens make it past the first line. Defensive measures include certain processes, proteins, and specialized cells. Defensive processes include the following:

The protective proteins are part of the second line of defense. These include interferons, which get their name from their ability to "interfere" with viral replication and limit a virus's ability to damage the body. A second protein type, the complement proteins, exist as inactive forms in blood circulation that become activated in the presence of bacteria, enabling them to lyse (destroy) the organisms.

Finally the last of the "team" in the second line of defense are the natural killer (NK) cells. This special kind of lymphocyte acts nonspecifically to kill cells that have been infected by certain viruses and cancer cells.

 

Specific Immunity

Specific immunity may be either genetic - an inherited ability to resist certain diseases because of one's species, race, sex, or individual genetics - or acquired. Specific immunity is dependent on the body's ability to identify a pathogen and prepare a specific response (antibody) to only that invader (antigen). antibodies are also referred to as immunoglobulins (lg). The acquired form can be further divided into natural and artificial forms, which in turn can each be either active or passive. After a description of the specific immune process, each of the four types is discussed. Did You Know?

Specific immunity is dependent on the agraulocytes (lymphocytes and monocytes) for its function. The monocytes metamorphose into macrophages, which dispose of foreign substances. The lymphocytes differentiate into either T lymphocytes (they mature in the thymus) or B lymphocytes (they mature in the bone marrow or fetal liver). Although both types of lympocytes take part in specific immunity, they do it in different ways.

The T cells neutralize their enemies through a process of cell-mediated immunity. This means that they attack antigens directly. They are effective against fungi, cancer cells, protozoa, and unfortunately, organ transplants. B cells use a process of humoral immunity (also called antibody-mediated immunity). This means that they secrete antibodies to "poison" their enemies.

Types of Acquired Immunity

Acquired immunity is categorized as active or passive and then is further subcategorized as natural or artificial. All describe ways that the body has acquired antibodies to specific diseases.

Active acquired immunity can take either of the following two forms:

  1. Natural: Development of memory cells to protect the individual from a second exposure.
  2. Artificial: Vaccination (immunization) that uses a greatly weakened form of the antigen, thus enabling the body to develop antibodies in response to this intentional exposure. Examples are the DTP and MMR vaccines.

Passive acquired immunity can take either of the following two forms:

  1. Natural: Passage of antibodies through the placenta or breast milk.
  2. Artificial: Use of immunoglobulins harvested from a donor who developed resistance against specific antigens

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