Breaking Biology

[06032014] Immune System Quiz ≡

Posted on Tuesday, March 11, 2014 at 8:15 AM

Explain how the immune system achieves all of the following.

Provides an immediate nonspecific immune response
Activates T and B cells in response to an infection
Responds to a later exposure to the same infectious agent
Distinguishes self from nonself

1) The skin and mucous membranes provide a nonspecific first line of defense against invaders entering through the skin or through opening into the body (like your nasal cavity and oral cavity). Nonspecific defense is not specialized for a particular invader. Instead, it is a general defense against all king of pathogens.

Nonspecific first line of defense features the following characteristics:

Skin is covered with oily acidic (pH from 3 to 5) secretions from sweat glands, creating a physical and hostile barrier.
Antimicrobial proteins like lysozyme, which breaks down the cell walls of bacteria, are contained in saliva, tears and other secretions found on mucous membranes.
Cilia line the lungs, and serve to sweep invaders out of them.
Gastric juice of the stomach kills most microbes.
Symbiotic bacteria found in the digestive tract and the vagina outcompete many other organisms that could cause damage.

The second line of defense,too, involves several nonspecific mechanisms including:

Phagocytes are white blood cells (leukocytes) that engulf pathogens by phagocytosis. Some examples include neutrophils and monocytes. Monocytes enlarge into large phagocytic cells called macrophages. Other white blood cells called natural killer cells (NK cells) attack abnormal body cells, such as tumors, or pathogen-infected body cells
Complement is a group of about 20 proteins that "complement" defense reactions. These proteins help attract phagocytes to foreign cells, and assist in the destruction of foreign cells by promoting cell lysis (breaking open the cell).
Interferons are substances secreted by virus-invaded cells that stimulate neighboring cells to produce proteins that help them defend against the viruses.
Inflammatory response is a series of nonspecific events that occur in response to pathogens. For example, when skin is damaged and bacteria or other organisms enter the body, the following events occur:

Histamine is secreted by the basophils, white blood cells found in connective tissue.
Vasodilation (dilation of blood vessels) stimulated by histamine, increases blood supply to the damaged area and allows for easier movement of white blood cells, as well as other body fluids, through the blood vessel walls. This also causes redness, swelling and an increase in temperature. An increase in temperature, like a fever, may stimulate white blood cells, and may also make an inhospitable environment to pathogens.
Phagocytes, attracted to the injury by chemical gradients of complement, arrive and engulf pathogens and damaged cells.
Complement helps phagocytes engulf foreign cells, stimulate basophils to release histamine, and help lyse foreign cells.

2 & 3 ) The third line of defense is the immune response. It is different from the first two lines because it targets specific antigens (any molecule, usually a protein or polysaccharide, that can be identified as foreign). It may be a toxin (injected into the blood by the sting of an insect, for example), a part of the protein coat of a virus, or a molecule unique to the plasma membranes of bacteria, protozoa, pollen or other foreign cells.

The major histocompatibility complex or MHC is the mechanism by which the immune system is able to differentiate between self and nonself cells. It is a collection of glycoproteins (proteins with carbohydrates) that exist on the membranes of body cells. The proteins of an individual are unique (originating from 20 genes, each with more than 50 alleles each), making it highly unlikely for two people (excluding identical twins) to possess cells with the same set of MHC molecules.

The primary agents of the immune response are lymphocytes, leukocytes that originate in the bone marrow but concentrate in lymphatic tissues such as the lymph nodes, the thymus gland and the spleen.

B Cells are lymphocytes that originate and mature in the bone marrow (like B for bone). B cells respond to antigens. The plasma membrane surface of B cells are characterized by specialized antigen receptors called antibodies, which have the following properties:

Are proteins
Each antibody is specific to a particular antigen
5 classes of immunoglobulins (antibodies): IgA, IgD, IgE, IgG, IgM. Each class is associated with a particular activity.
Each class of antibodies is a variation of the basic Y-shaped protein that consists of constant regions and variable regions. Variable regions are sequences of amino acids that differ among antibodies and give them specificity to antigens.
Antibodies inactivate antigens by binding to them.

Inactivation is followed by macrophage phagocytosis.
In addition, by binding to the surface antigens of nonself cells, antibodies stimulate complement proteins to bring about the lysis of pathogens.

When B Cells encounter antigens that specifically bind to their antibodies, the B cells proliferate, producing two kinds of B cells:

Plasma Cells

B cells that release their specific antibodies which then circulate through the body, binding to antigens.

Memory Cells

Long-lived B cells do not release their antibodies in response to the immediate antigen invasion, but instead circulate in the body and respond quickly to eliminate any subsequent invasion by the same antigen.
This mechanism provides immunity to many diseases after the first occurrence.

T cells are lymphocytes that originate in the bone marrow, but mature in the thymus gland (like T for thymus). Like B cells, the plasma membranes of T cells have antigen receptors. However, these receptors are not antibodies, but recognition sites for molecules displayed by nonself cells (see answer 4 for how to distinguish between self and nonself cells)

When T cells encounter nonself cells, they divide and produce two kinds of cells:

Cytotoxic T cells (or killer T cells) that recognize and destroy nonself cells by puncturing them, causing them to lyse.
Helper T cells that stimulate the proliferation of B cells and cytoxic T cells.

When an antigen binds to a B cell or when a nonself binds to a T cell, the B cell or T cell begins to divide, producing numerous daughter cells, all identical copies of the parent cell. This process is called clonal selection, since only the B or T cells that bears the effective antigen receptor is "selected" and reproduced to make identical copies of itself. Clonal selection results in the proliferation of B cells and T cells that will engage a specific, invading antigen.

The cell-mediated response uses mostly T cells and responds to any nonself cell, including cells invaded by pathogens. When a nonself cell binds to a T cell, the T cell undergoes clonal selection, initiaing the following chain of events:

T cells produce Cytotoxic T cells, which destroy nonself cells
T cells produce helper T cells
Helper T cells bind to macrophages. Macrophages that have engulfed pathogens display aberrant plasma membrane markers.
Helper T cells produce interleukins to stimulate a proliferation of T cells and B cells. When helper T cells bind with macrophages, they release interleukins, or communication chemicals “between leukocytes.” The interleukins initiate a sequence of positive-feedback events that result in the proliferation of interleukins, macrophages, helper T cells, B cells, and cytotoxic T cells.

The humoral response or antibody-animated response involves most cells and responds to antigens or pathogens that are circulating in the lymph or blood ("humor" is a medieval term for body fluid).

B cells produce plasma cells, which in turn release antibodies that bind with antigens or antigen-bearing pathogens
B cells produce memory cells, which provide future immunity.
Macrophage and helper T cells stimulate B cell production. In many cases, the antigen will not directly stimulate the proliferation of B cells. Instead, the antigen or antigen-bearing pathogen must first be engulfed by a macrophage. T cells then bind to the macrophage in a cell-mediated response. Interleukins secreted by the helper T cells stimulate the production of B cells.

4. Self and nonself cells are distinguished as follows:

MHC markers on the plasma membrane of cells distinguish between self and nonself cells.
When a body cell is invaded by a virus, foreign cell or by any antigen, the body cell displays a combination of self and nonself markers. T cells interpret this mixed display of markers as nonself.
Cancer cells of tissue transplant cells, or other cells that display aberrant markers are recognized as nonself by T cells.

0 Comment: