The Immunological System

7/30/2019 The Immunological System

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The Immunological SystemRecognition, Attack, and Memory

Microbes are constantly working to invade into other our bodies. Despite that, most of us arehealthy most of the time. Our usual state of good health is due to these three

Lines of Defense against Microbes :

- External barriers (keeps microbes out of the body)- Nonspecific Internal Defenses- Immune System (Specific Defenses)

External Barriers to Microbial Invasion:

- The Skin- Mucus Membranes (of the digestive and respiratory tract)

The Skin

Dry dead cells filled with protein

Prevents most microbes landing on the skin from obtaining needed water and nutrients

Natural antibiotics secreted by the sweat and sebaceous glands actively inhibit the growth of bacteria and fungi on the skin

Mucus Membranes

Mucus covers/protects the membranes of the digestive and respiratory tract.

Mucus contains antibacterial enzymes that destroy the cell walls of bacteria and physically trapsmicrobes that enter the body through the nose or mouth.

Cilia on the membranes sweep up the mucus, microbes included, until it is coughed up, sneezed, or swallowed . Swallowed microbes enter the stomach where they encounter a combination of extremeacidity and protein digesting enzymes that kills many enzymes. Further along the digestive tract,bacteria harmless to humans secrete substances deadly to other invading bacteria or fungi.

Conclusion: Mucus membranes are more vulnerable to invasion than the skin. Also, mucusmembranes and the skin cannot keep all microbes out of the body. When diseased organisms enterthe body through the skin or mucus membranes, they run into:

Nonspecific Internal Defenses

- Phagocytic and Natural Killer Cells- The Inflammatory Response- Fever

Phagocytic and Natural Killer Cells

- Monocytes and neutrophils


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Monocytes NeutrophilsPhagocytic white blood cells that travel though capillaries to wounds where bacteria have gainedentry, then ooze out through narrow openings in the capillary walls.After leaving the capillaries, monocytesdifferentiate into macrophages, amoeba like

cells that engulf and ingest bacteria and otherforeign particles.

The most numerous of white blood cells; likemacrophages also engulfs foreign invaders such

as bacteria and certain fungi.

Secrete molecules that attract other defensive cells to the invasion site.

- Natural Killer Cells

Another class of white blood cells

They identify and destroy body cells that have been invaded by viruses or that have becomecancerous.

Sensing molecular changes, natural killer cells bind to the compromised cell s cellular membrane andrelease proteins that pierce holes in it. Cytotoxic proteins then enter the abnormal cell and destroyits nucleus.

The Inflammatory Response

Breaches of the skin or mucus membrane elicit an inflammatory response.

1) Damaged cells release histamine into wounded area2) Histamine makes capillary walls leaky

3) Leaky capillaries cause a fluid to seep from the capillaries into the tissues around wound4) Wound becomes red, swollen and warm5) Meanwhile, other chemicals released by injured cells initiate blood clotting that both blocks

damaged blood vessels preventing microbes from escaping into the bloodstream and sealsoff wound from outside world and thus limiting entry of other microbes

6) Still other chemicals (released by wounded cells or microbial invaders) attract macrophages,neutrophils, and other phagocytic cells to wound where they engulf microbes, dirt and tissuedebris.

Pus (that collects around a wound) consists largely of:

- Microbes- Tissues debris- White blood cells (dead or alive)

Fever

IF enough microbes survive the inflammatory response and infect larger areas of the body, a fevermay be triggered.

The hypothalamus in the brain contains temperature sensing nerve cells that normally set the bodys

thermostat at about 37 C. However, in cases of infection, certain white blood cells release hormones


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called endogenous pyrogens that cause the hypothalamus to trigger behaviours that increase body temperature, shivering, increased fat metabolism, and a feeling of cold.

The release of pyrogens also results in reduced iron concentrations in the blood, which whencombined with a fever, slows the reproduction rate of bacteria.

Fever increases the activity of phagocytic white blood cells that attack bacteria.

Fever fights viral infections by increasing the production of protein interferon (synthesized by certainbody cells under invasion of viruses). Interferon travels to other cells and increases their resistanceto the viral attack.

Conclusion: While often effective, nonspecific internal defenses are not impregnable. When they failto ward off microbes, the body then mounts a highly specific immune response directed against theparticular organism that has successfully invaded the body.

Specific Internal Defenses (The Immune System)

Immunity to one disease confers no protection against other diseases. The immune system attacksone type of microbe, overcomes it, and provides future protection against that microbe but noothers, thus a specific defense against invasion.

The immune system consists of about two trillion specialized white blood cells called lymphocytes.Although many are distributed around the body (in blood and limbs), others are clustered in specificorgans, particularly the thymus, lymph nodes, and spleen .

The immune response results from the interactions among several types of lymphocytes and themolecules they produce.

Two types of lymphocytes:

- B-Cell- T-Cell

B-Cell T-CellArise from cells in the bone marrowDifferentiates in the bone marrow and migrateseither to the spleen or lymph nodes

Newly forming T-Cells migrate to the thymus anddifferentiate early in development

Immune responses consist of three steps:1) Recognizing invaders2) Launching an attack3) Retaining a memory of the invader for future infections

Recognizing Microbial Invaders Antibodies & T-Cell Receptors

Understanding Immune Response ( triggered by the binding of antigen to antibody)

1) How do immune cells recognize foreign molecules?2) How do immune cells produce specific responses to many different molecules?3) How do immune cells distinguish between foreign and self?


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Antibodies

Antibodies are either attached to the surface of B-Cells or dissolved in the blood plasma where theyare called immunoglobulins (IG).

They are Y-shaped molecules composed of four peptide chains, one large heavy chain and one smalllight chain on either side of the Y. Both heavy and light chains consist of a *constant region (similar not identical of all antibodies of the same class) and a variable region (differs per antibody).

The combination of light and heavy chains results in an antibody with two function parts: the armsand the stem of the Y. The variable regions on the arms form highly specific binding sites for largemolecules (generally proteins, polysaccharides and glycoproteins) that have particular shapes andelectrical charges. These molecules or antigens may either be attached to the surfaces of cells, thebodys own or invading microbes, or dissolved in the blood or extracellular fluid such as snakevenom.

The stem of the antibody molecules determines the activity of an antibody.

*e.g. the heavy chain constant regions forming the stem of one type of antibody may attach theantibody to the plasma membrane of a cell whereas in another type of antibody, the constantregions may bind to certain proteins in the blood

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The Immunological System