The Immune System

 

 

Nonspecific Defense Mechanisms

 

A). Skin and Mucus membranes

 

If not broken, the skin is an impenetrable barrier to viruses and bacteria.

It is reinforced by chemical defenses. Oil and sweat give the skin a pH of 3-5.

Bacteria that live on the skin help by releasing acids and wastes to kill off unwanted microorganisms. Lysozyme is found in tears and saliva which can destroy the cell walls of most bacteria. Gastric juices kill off bacteria in food. Mucus captures bacteria in the nose and throat cavities of the body.

 

B). Phagocytes and Natural Killer Cells

 

Phagocyte are cells that eat other cells or objects found in the body.

Macrophages a type of phagocyte that eats bacteria, viruses, and the debris of damaged cells. They develop from a class of white blood cells called MONOCYTES. Neutrophils are another type of white blood cell capable of becoming phagocytic.

 

Natural Killer Cells destroy the bodies own infected cells, especially those containing viruses. They may attack cells that might form tumors. They do not eat the cell but lyse it

 

C). Inflammatory Response

 

Damage to tissue by physical injury will trigger an inflammatory response.

Small blood vessels in the area will dilate and become leakier. This causes the redness and heat of the infected area. This enhances the migration of phagocytes into the area. Neutrophils arrive first, followed by monocytes that develop into macrophages. Clotting proteins seal off the infected area.

Injured cells release HISTAMINE which induces the dilation of the capillaries.

Sometimes white blood cells release chemicals called pyrogens. They act on the bodies thermostat and raise the temperature in an effort to kill the microorganism.

 

Anti microbial Proteins

 

Interferons: Acts in warning other cells of a viral invasion. It is produced by infected cells.

 

Complement: Circulate in the body in an inactive form. They are activated by chemical markers, or the onset of the immune response.

 

SPECIFIC DEFENSE MECHANISMS:

• Immune Response = a recognition system that distinguishes "self" form "non-self".
• Antigen = Foreign substance.
• Antibody = specific defensive proteins which help to counter antigens is various ways.
• Immunity = protection afforded to an organism by previous exposure to an antigen.
• Active Immunity = Immunity conferred by an encounter with the actual virus or bacterium. The body actually produces antibodies in its own defense.
• Passive Immunity = Temporary acquired antibodies passed on to the organism. No antigen is passes on.

 

The Immune response is governed by 2 different parts of the immune system.

• Humoral Immunity: results in the production of antibodies that circulate around the body in blood and lymph (humors). Defends against: free bacteria and viruses.
• Cell-mediated Immunity: defense activities are carried out by specialized cells circulating around the body. Defends against: host cells infected with bacteria and viruses, fungi and protozoans, tissue transplants, own cancer cells.

Cells of the Immune System:

• Lymphocytes: White blood cells that are divided into 2 types:
• B lymphocytes or B cells = secrete antibodies act in humoral immunity.
• T lymphocytes or T cells = function in cell-mediated immunity.

 

Both originate from the bone marrow. They become B or T depending on their point of maturation. The T develop in the THYMUS gland found in the chest. The B develop in the bone. After this maturation process is complete they attain IMMUNOCOMPETENCE. This is their ability to do their appointed jobs.

These cells eventually migrate to the lymph nodes and spleen waiting for their appropriate antigens to destroy.

 

Immunological Memory:

 

The Humoral Immune Response: is provoked by the binding of antigens to specific receptors sticking out of the plasma membrane of B cells. This occurs most commonly in the lymph nodes.

Activation of B cells:

1. Capping: The serial (many repeated forms of antigens) antigenic determinants bind to several antibodies on the surface of the B cell. Pulling the antibodies together. The B cell is activated when this whole cluster is ingested by the cell.

2. Help from specialized T cells: Helper T cells release a chemical called interleukin II. This double action ( antigen and interleukin II) results in a clone of plasma cells, which secrete antibodies against the free antigens.

Each B cell is coated with only one type of receptor. They are considered selective in nature. Once an activated B cell begins to grow and multiply, this clone includes memory cells and effectors cells called plasma cells. B cells secrete very few antibodies but the plasma calls give off as many as 2000 antibodies per sec. for their 4-5 day life span.

Antibody Structure: Antibodies constitute a class of plasma proteins called Immunoglobulins, abbreviated Ig. These antibodies have the ability to recognize and help destroy specific antigens in the body. Each molecule consists of 2 distinct regions. a). the variable region (V) and b). the constant region(C).

A typical antibody consists of 2 pairs of polypeptide chains- 2 short identical light(L) chains and 2 longer identical heavy(H) chains. The chains are joined by disulfide bridges and non covalent associations to form a Y shaped molecule. The ends of the Y contain the antigen binding sites.

Examples :

• IgA: found in body secretions, help attach antigen to body wall where they can be swept out.
• IgM: largest of the antigens, produced early in infection, helps produce complement.
• IgG: most abundant of circulating antibodies, confers passive immunity in babies, occurs when infection is well established, also activates complement
• IgD: always bound to the plasma membrane of B cells,
• IgE: associated with the bodies allergic responses, binds to mast cells which release histamine.

Humoral Effector mechanisms: Antibodies do not posses the power to destroy the antigens directly, they tag them and mark them for destruction by a variety of effector mechanisms.

• Neutralization: surround the antigen cutting it off from the organism, phagocytes then dispose of the complex.
• Agglutination: bind together many antigens into clumps for phagocytes to eat.
• Precipitation: binding together of antigen molecules (not cells) for removal by phagocytes.
• Activation of the Complement system: These chemicals will lyse the invader and make it easier for phagocytes to do their work.

Cell Mediated Immunity: Once pathogens have invaded the body's cells, antigens cannot harm them. This type of immunity destroys the invaded cells directly. This type of immune response cannot be activated by direct antigen contact. T cells only react to the antigenic determinants displayed on the cell surface of the body's own cells. These are called T-Cell receptors.

T-cell Receptors and Histocompatibility Restriction:

The T-cell receptor actually recognizes a combination of the antigen along with the body's "self" markers. "Self" is signaled by the major histocompatibility complex (MHC), a group of proteins unique to the individual that is present on the surface of the cell. There are 2 forms of MHC: MHCI and MHCII. I is found on nucleated cells

of the body, II is found on macrophages, B cells and some T cells. The T-cell recognized both types of proteins at once. This complex as as a red flag to the T cells.

They will divide and produce memory cells and effectors called cytotoxic T cell.

There are 2 types of T cells with regulatory functions within the body. helper-T cells and suppressor T cells.

Helper T cells: They eat the antigen, digest it and display parts of it on their cell surface. They bind with macrophages causing the macrophages to release a chemical called interleukin I, which stimulates the reproduction of the T cells. The activated T cells then produce a chemical called interleukin II, this increases the division and growth rate of the T cells. This is an example of positive feed back.

Cytotoxic T Cells: These are the only T cells that can kill off other cells. They recognize the cells by the MHC-antigen complex. They release a chemical called perforin.

Suppressor T cells: They release cytokines that inhibit the activity of other T cells.