lymphatic and immune system

lymphoid nodules

Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens. The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with the mucous membrane epithelia. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes.Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer, found along the bifurcations of the bronchi and between bronchi and arteries.

Phagocytic cells of the innate immune system

A macrophage is an irregularly shaped phagocyte that is amoeboid in nature and is the most versatile of the phagocytes in the body. A neutrophil is a phagocytic cell that is attracted via chemotaxis from the bloodstream to infected tissues. A monocyte is a circulating precursor cell that differentiates into either a macrophage or dendritic cell, which can be rapidly attracted to areas of infection by signal molecules of inflammation.

Cells of the innate immune response

A phagocyte is a cell that is able to surround and engulf a particle or cell, a process called phagocytosis. The phagocytes of the immune system engulf other particles or cells, either to clean an area of debris or old cells, or to kill pathogenic organisms such as bacteria. The phagocytes are the body's fast-acting first line of immunological defense against organisms that have breached barrier defenses and entered the vulnerable tissues of the body.

B lymphocytes and antibodies

An antibody protein is essentially a secreted form of a B cell receptor. In fact, surface immunoglobulin is another name for the B cell receptor. Not surprisingly, the same genes encode both the secreted antibodies and the surface immunoglobulins. One minor difference in the way these proteins are synthesized distinguishes a naïve B cell with antibody on its surface from an antibody-secreting plasma cell with no antibodies on its surface. The antibodies of the plasma cell have exactly the same antigen-binding site and specificity as their B cell precursors. There are five different classes of antibody found in humans: IgM, IgD, IgG, IgA, and IgE.

recognition of pathogens

Cells of the innate immune response, the phagocytic cells, and the cytotoxic NK cells recognize patterns of pathogen-specific molecules, such as bacterial cell wall components or bacterial flagellar proteins, using pattern recognition receptors. A pattern recognition receptor (PRR) is a membrane-bound receptor that recognizes characteristic features of a pathogen and molecules released by stressed or damaged cells. Should the cells of the innate immune system come into contact with a species of pathogen they recognize, the cell will bind to the pathogen and initiate phagocytosis (or cellular apoptosis in the case of an intracellular pathogen) in an effort to destroy the offending microbe.

early induced proteins

Early induced proteins are those that are not constitutively present in the body, but are made as they are needed early during the innate immune response. Interferons are an example of early induced proteins. Cells infected with viruses secrete interferons that travel to adjacent cells and induce them to make antiviral proteins. Thus, even though the initial cell is sacrificed, the surrounding cells are protected. Other early induced proteins specific for bacterial cell wall components are mannose-binding protein and C-reactive protein, made in the liver, which bind specifically to polysaccharide components of the bacterial cell wall. Phagocytes such as macrophages have receptors for these proteins.

The spleen

In addition to the lymph nodes, the spleen is a major secondary lymphoid organ. The spleen is sometimes called the filter of the blood because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens. The red pulp of the spleen primarily functions as a filtration system of the blood, using cells of the relatively nonspecific immune response, and white pulp is where adaptive T and B cell responses are mounted.

The five classes of antibodies

In general, antibodies have two basic functions. They can act as the B cell antigen receptor or they can be secreted, circulate, and bind to a pathogen, often labeling it for identification by other forms of the immune response. Of the five antibody classes, notice that only two can function as the antigen receptor for naïve B cells: IgM and IgD.

Functions of the lymphatic system

In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary filtration. Once this filtrate is out of the bloodstream and in the tissue spaces, it is referred to as interstitial fluid. Of this, 17 liters is reabsorbed directly by the blood vessels. The remaining three liters is being drained by the lymphatic system and emptied back into the bloodstream via a series of vessels, trunks, and ducts. Lymph is the term used to describe interstitial fluid once it has entered the lymphatic system.An inappropriate accumulation of fluid referred to as lymphedema may lead to serious medical consequences. A lymph node is one of the small, bean-shaped organs located throughout the lymphatic system.

the adaptive immune system

Innate immune responses (and early induced responses) are in many cases ineffective at completely controlling pathogen growth. However, they slow pathogen growth and allow time for the adaptive immune response to strengthen and either control or eliminate the pathogen. The innate immune system also sends signals to the cells of the adaptive immune system, guiding them in how to attack the pathogen. Thus, these are the two important arms of the immune response.

lymph nodes

Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the filters of the lymph. Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system. The major routes into the lymph node are via afferent lymphatic vessels. Cells and lymph fluid that leave the lymph node may do so by another set of vessels known as the efferent lymphatic vessels. .

lymphatic capillaries

Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body. Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.

natural killer cells

NK cells are a type of lymphocyte that have the ability to induce apoptosis, or programmed cell death, in cells infected with intracellular pathogens, such as obligate intracellular bacteria and viruses. NK cells are able to respond to chemical signals and express the fas ligand. The fas ligand is a surface molecule that binds to the fas molecule on the surface of the infected cell, sending it apoptotic signals, thus killing the cell and the pathogen within it. The granules of the NK cells release perforins and granzymes. A perforin is a protein that forms pores in the membranes of infected cells. A granzyme is a protein-digesting enzyme that enters the cell via the perforin pores and triggers apoptosis intracellularly. Both mechanisms are especially effective against virally infected cells.

The cellular basis of immunological memory

One of the major features of an adaptive immune response is the development of immunological memory. During a primary adaptive immune response, both memory T cells and effector T cells are generated. Memory T cells are long-lived and can even persist for a lifetime. Memory cells are primed to act rapidly. Thus, any subsequent exposure to the pathogen will elicit a very rapid T cell response. This rapid, secondary adaptive response generates large numbers of effector T cells so fast that the pathogen is often overwhelmed before it can cause any symptoms of disease. This is what is meant by immunity to a disease. The same pattern of primary and secondary immune responses occurs in B cells and the antibody response.

4 parts of inflammatory response

•tissue Injury •vasodilation •increased vascular permeability •recruitment of phagocytes

The complement system

The complement system is a series of proteins found in the blood plasma. Made in the liver, they have a variety of functions in the innate immune response, using what is known as the alternate pathway of complement activation. Complement functions in the adaptive immune response as well, in what is called the classical pathway. Once activated, the series of reactions is irreversible, and releases fragments that have the following actions: •bind to the cell membrane of the pathogen that activates it, labeling it for phagocytosis (opsonization)•diffuse away from the pathogen and act as chemotactic agents to attract phagocytic cells to the site of inflammation •form damaging pores in the plasma membrane of the pathogen.

inflamatory response

The hallmark of the innate immune response is inflammation. Any activity that causes tissue damage will result in inflammation, characterized by: heat, redness, pain, and swelling. It is important to note that inflammation does not have to be initiated by an infection, but can also be caused by tissue injuries. The release of damaged cellular contents into the site of injury is enough to stimulate the response, even in the absence of breaks in physical barriers that would allow pathogens to enter (by hitting your thumb with a hammer, for example). The inflammatory reaction brings in phagocytic cells to the damaged area to clear cellular debris and to set the stage for wound repair.

Primary disease and immunology memory

The immune system's first exposure to a pathogen is called a primary adaptive response. Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective. Upon re-exposure to the same pathogen, a secondary adaptive immune response is generated, which is stronger and faster that the primary response. The secondary adaptive response often eliminates a pathogen before it can cause significant tissue damage or any symptoms. Without symptoms, there is no disease, and the individual is not even aware of the infection. This secondary response is the basis of immunological memory, which protects us from getting diseases repeatedly from the same pathogen.

larger lymphatic vessels, trunks, and ducts

The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. The superficial and deep lymphatics eventually merge to form larger lymphatic vessels known as lymphatic trunks. On the right side of the body lymph fluid is drained into the right subclavian vein via the right lymphatic duct. On the left side of the body, the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein.

structure of lymphatic system

The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream by a series of ducts. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body, the contraction of skeletal muscles during body movements, and breathing. One-way valves (semi-lunar valves) in lymphatic vessels keep the lymph moving toward the heart. Lymph is dumped into the circulatory system via the lymphatic ducts located at the junction of the jugular and subclavian veins in the neck.

organization of immune function

The modern model of immune function is organized into three phases based on the timing of their effects: barrier defenses, such as the skin and mucous membranes, which act instantaneously to prevent pathogenic invasion into the body tissues the rapid but nonspecific innate immune response, which consists of a variety of specialized cells and soluble factors the slower but more specific and effective adaptive immune response, which involves many cell types and soluble factors but is primarily controlled by white blood cells (leukocytes) known as lymphocytes, which help control immune responses.

Major trunks and ducts of the lymph system

The overall drainage system of the body is asymmetrical. The right lymphatic duct receives lymph from only the upper right side of the body. The lymph from the rest of the body enters the bloodstream through the thoracic duct via all the remaining lymphatic trunks. In general, lymphatic vessels of the subcutaneous tissues of the skin, that is, the superficial lymphatics, follow the same routes as veins, whereas the deep lymphatic vessels of the viscera generally follow the paths of arteries.

Soluble mediators of the innate immune response

The previous discussions have referred to chemical signals that can induce cells to change various physiological characteristics, such as the expression of a particular receptor. These soluble factors are secreted during innate or early induced responses, and later during adaptive immune responses. A cytokine is a signaling molecule that allows cells to communicate with each other over short distances. Cytokines are secreted into the intercellular space, and the action of the cytokine induces the receiving cell to change its physiology. A chemokine is a soluble chemical mediator similar to cytokines, except that its function is to attract cells (chemotaxis) from longer distances.

T cell mediated immune response

The primary cells that control the adaptive immune response are the lymphocytes, the T and B cells. T cells are particularly important, as they not only control a multitude of immune responses directly, but also control B cell immune responses in many cases as well. Thus, many of the decisions about how to attack a pathogen are made at the T cell level, and knowledge of their functional types is crucial to understanding the functioning and regulation of adaptive immune responses as a whole. T lymphocytes recognize antigens based on a two-chain protein receptor. The most common and important of these are the alpha-beta T cell receptors.

primary lymphoid organs

The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body. The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte, leaves the bone marrow and matures largely in the thymus gland. The thymus gland is a bilobed organ found in the space between the sternum and the aorta of the heart.

Secondary lymphoid organs

The secondary lymphoid organs include the lymph nodes, spleen, and lymphoid nodules. All of these tissues have many features in common, including the following: the presence of lymphoid follicles, the sites of the formation of lymphocytes, with specific B cell-rich and T cell-rich areasan internal structure of reticular fibers with associated fixed macrophagesgerminal centers, which are the sites of rapidly dividing B lymphocytes and plasma cells, with the exception of the spleenspecialized post-capillary vessels known as high endothelial venules

benefits of adaptive immune response

The specificity of the adaptive immune response—its ability to specifically recognize and respond to a wide variety of pathogens—is its great strength. Antigens, the small chemical groups often associated with pathogens, are recognized by receptors on the surface of B and T lymphocytes. The adaptive immune response to these antigens is so versatile that it can respond to nearly any pathogen. This increase in specificity comes because the adaptive immune response has a unique way to develop as many as 100 trillion different receptors to recognize nearly every conceivable pathogen.