The Immune System Essay, Research Paper

`Lymph organs include the bone marrow, lymph nodes, spleen,

and thymus. Bone marrow contains tissue that produces lymphocytes. B-lymphocytes (B-cells) mature in the bone marrow. T-lymphocytes

(T-cells) mature in the thymus gland. Other blood cells such as monocytes

and leukocytes

are produced in the bone marrow. Lymph nodes are areas of concentrated

lymphocytes and macrophages along the lymphatic veins. The spleen is similar to the lymph node

except that it is larger and filled with blood. The spleen serves as a reservoir

for blood, and filters or purifies the blood and lymph fluid that flows through

it. If the spleen is damaged or removed, the individual is more susceptible to

infections. The thymus secretes a hormone, thymosin, that causes pre-T-cells to

mature (in the thymus) into T-cells.Immunity Immunity is the body’s capability to repel foreign substances and

cells. The nonspecific responses are the first line of defense. Highly specific

responses are the second line of defense and are tailored to an individual

threat. The immune response includes both specific and nonspecific components.

Nonspecific responses block the entry and spread of disease-causing agents.

Antibody-mediated and cell-mediated responses are two types of specific

response. The immune system is associated with defense against disease-causing

agents, problems in transplants and blood transfusions, and diseases resulting

from over-reaction (autoimmune, allergies) and under-reaction (AIDS). General Defenses Barriers to entry are the skin and mucous membranes. The skin is a

passive barrier to infectious agents such as bacteria and viruses. The

organisms living on the skin surface are unable to penetrate the layers of dead

skin at the surface. Tears and saliva secrete enzymes that breakdown bacterial

cell walls. Skin glands secrete chemicals that retard the growth of bacteria.

Mucus membranes lining the respiratory, digestive, urinary, and reproductive

tracts secrete mucus that forms another barrier. Physical barriers are the

first line of defense.When microorganisms penetrate skin or epithelium lining respiratory,

digestive, or urinary tracts, inflammation results. Damaged cells release chemical signals such as histamine

that increase capillary blood flow into the affected area (causing the areas to

become heated and reddened). The heat makes the environment unfavorable for

microbes, promotes healing, raises mobility of white blood cells, and increases

the metabolic rate of nearby cells. Capillaries pass fluid into interstitial areas, causing the infected/injured area to swell. Clotting factors

trigger formation of many small blood clots. Finally, monocytes (a type of

white blood cell) clean up dead microbes, cells, and debris. The inflammatory response is often strong enough to stop the spread of

disease-causing agents such as viruses, bacteria, and fungi. The response

begins with the release of chemical signals and ends with cleanup by monocytes.

If this is not enough to stop the invaders, the complement system and immune response act.Protective proteins that are produced in the liver include the

complement system of proteins. The complement system proteins bind to a

bacterium and open pores in its membrane through which fluids and salt move,

swelling and bursting the cell.The complement system directly kills microbes, supplements inflammatory

response, and works with the immune response. It complements the actions of the

immune system. Complement proteins are made in the liver and become active in a

sequence (C1 activates C2, etc.). The final five proteins form a membrane-attack complex (MAC) that embeds itself into the plasma membrane

of the attacker. Salts enter the invader, facilitating water to cross the

membrane, swelling and bursting the microbe. Complement also functions in the

immune response by tagging the outer surface of invaders for attack by phagocytes.

The complement system of

proteins and their functioning. Interferon

is a species-specific chemical produced by cells that are viral attack. It

alerts nearby cells to prepare for a virus. The cells that have been contacted

by interferon resist all viral attacks.Specific Defenses The immune system also generates specific responses to specific

invaders. The immune system is more effective than the nonspecific methods, and

has a memory component that improves response time when an invader of the same

type (or species) is again encountered. Immunity results from the production of antibodies

specific to a given antigen

(antibody-generators, located on the surface of an invader). Antibodies bind to

the antigens on invaders and kill or inactivate them in several ways. Most

antibodies are themselves proteins or are a mix of protein and polysaccharides.

Antigens can be any molecule that causes antibody production. Lymphocytes White blood cells known as lymphocytes arise from by mitosis of stem cells

in the bone marrow. Some lymphocytes migrate to the thymus and become T cells

that circulate in the blood and are associated with the lymph nodes and spleen.

B cells remain in the bone marrow and develop before moving into the

circulatory and lymph systems. B cells produce antibodies. Macrophage Attacking E.coli (SEM

x8,800Antibody-mediated (humoral immunity) Antibody-mediated (humoral) immunity is regulated by B cells and the antibodies

they produce. Cell-mediated immunity is controlled by T cells. Antibody-mediated reactions defend against

invading viruses and bacteria. Cell-mediated immunity concerns cells in the

body that have been infected by viruses and bacteria, protect against

parasites, fungi, and protozoans, and also kill cancerous body cells. Human T-lymphocyte (SEM x12,080).Antibody-mediated Immunity Stages in this process are: Ø

antigen

detection Ø

activation of

helper T cells Ø

antibody

production by B cells Each stage is directed by a specific cell type.Macrophages Macrophages are white blood cells that continually search for foreign

(nonself) antigenic molecules, viruses, or microbes. When found, the

macrophages engulfs and destroys them. Small fragments of the antigen are

displayed on the outer surface of the macrophage plasma membrane.Helper T Cells Helper T cells are macrophages that become activated when they

encounter the antigens now displayed on the macrophage surface. Activated T

cells identify and activate B cells.B Cells B cells divide, forming plasma cells and B memory cells. Plasma cells make and release between 2000 and 20,000 antibody

molecules per second into the blood for the next four or five days. B memory

cells live for months or years, and are part of the immune memory system.Antibodies Antibodies bind to specific antigens in a lock-and-key fashion, forming

an antigen-antibody complex. Antibodies are a type of protein molecule known as

immunoglobulins. There are five classes of immunoglobulins: IgG, IgA, IgD, IgE, and

IgM. ?Antibodies are Y-shaped molecules composed of two identical long

polypeptide (Heavy or H chains) and two identical short polypeptides (Light or

L chains). Function of antibodies includes: Ø

Recognition and

binding to antigens Ø

Inactivation of

the antigen A unique antigenic determinant recognizes and binds to a site on the antigen, leading to the

destruction of the antigen in several ways. The ends of the Y are the

antigen-combining site that is different for each antigen. Helper T cells activate B cells that produce antibodies. Supressor T cells slow down

and stop the immune response of B and T cells, serving as an off switch for the

immune system. Cytotoxic (or killer) T cells destroy body cells infected with a virus or

bacteria. Memory T cells remain in the body awaiting the reintroduction of the

antigen. A cell infected with a virus will display viral antigens on its plasma

membrane. Killer T cells recognize the viral antigens and attach to that cell’s

plasma membrane. The T cells secrete proteins that punch holes in the infected

cell’s plasma membrane. The infected cell’s cytoplasm leaks out, the cell dies,

and is removed by phagocytes. Killer T cells may also bind to cells of

transplanted organs. The immune system is the major component of this defense. Lymphocytes,

monocytes, lymph organs, and lymph vessels make up the system. The immune

system is able to distinguish self from non-self. Antigens are chemicals on the

surface of a cell. All cells have these. The immune system checks cells and

identifies them as "self" or "non-self". Antibodies are

proteins produced by certain lymphocytes in response to a specific antigen.

B-lymphocytes and T-lymphocytes produce the antibodies. B-lymphocytes become

plasma cells which then generate antibodies. T-lymphocytes attack cells which

bear antigens they recognize. They also mediate the immune response.The immune system and memory of infections Secondary immunity, the resistance to certain diseases after having had

them once, results from production of Memory B and T cells during the first

exposure to the antigen. A second exposure to the same antigen produces a more

massive and faster response. The secondary response is the basis for

vaccination.Vaccination Vaccination is a term derived from the Latin vacca (cow, after the cowpox

material used by Jenner in the first vaccination). A vaccine

stimulates the antibody production and formation of memory cells without

causing of the disease. Vaccines are made from killed pathogens or weakened

strains that cause antibody production but not the disease. Recombinant DNA

techniques can now be used to develop even safer vaccines. The immune system can develop long-term immunity to some diseases. Man

can use this to develop vaccines, which produce induced immunity. Active

immunity develops after an illness or vaccine. Vaccines are weakened (or

killed) viruses or bacteria that prompt the development of antibodies.

Application of biotechnology allows development of vaccines that are the

protein (antigen) which in no way can cause the disease. Passive immunity is

the type of immunity when the individual is given antibodies to combat a

specific disease. Passive immunity is short-lived.