5 Cell recognition and the immune system

HIV (how it causes AIDS)

*** is a virus that causes acquired immune deficiency syndrome (AIDS). *** may not immediately cause AIDS following infection since *** can enter dormancy for several years. *** causes AIDS by killing or interfering with the function of helper T cells, reducing a person's ability to respond to infections by other diseases. All aspects of immunity are affected, including the generation of memory cells. *** does not directly cause death, but by reducing the effectiveness of the immune system, a person is open to secondary infections that can be fatal.

Antibodies

********** are the proteins produced by B lymphocytes that mediate humoral immunity. The ********** circulate in the bloodstream and attach to antigens that are complementary, stimulating an immune response to the cells presenting that antigen.

Passive immunity

This is the introduction of antibodies from an external source leading to immunity but there is no direct contact with a pathogen. For example, antibodies passed from a mother to her foetus during pregnancy. No memory cells are formed so it is short-term immunity.

Active immunity

This is the production of antibodies by the immune system as a result of direct contact with a pathogen. Memory cells are formed so it provides long term immunity. Natural active immunity is a result of a natural infection and immune response. Artificial active immunity - vaccination - is the result of introduction of the disease in an individual, usually without producing symptoms.

Humoral immunity

When T lymphocytes respond to an infection some stimulate B lymphocytes to begin to divide. This is part of the next stage of immunity. It involves antibodies. There are lots of different types of B lymphocytes and each one produces an antibody that is specific to a single antigen. The antibodies are soluble in blood and tissue fluid and so are easily transported around the body. When an antigen enters the blood or tissue fluid there will be one B cell that has an antibody on its surface. The antibody's shape exactly fits and is complementary to the antigen and becomes attached to it. The antigen enters the B cell by endocytosis and is presented on the surface. Helper T cells bind to the antigen and stimulate B cells to divide by mitosis and form clones that all produce antibodies.

Herd immunity

When immunization levels in a population are high, ________ provides protection from infection for at-risk persons who cannot be immunized. When a large proportion of the population is immune to a disease a ___________ effect develops. This protects individuals that are not immune by making it unusual for a vulnerable individual to come in contact with the pathogen as it cannot survive in the wider immune population.

Lymphocytes

White blood cells that are involved in the body's immune system. They are involved in specific responses.

Human immunodeficiency virus (HIV)

A pathogen that can lead to the disease acquired immune deficiency syndrome (AIDS).

Ethical issues (associated with vaccination programmes)

A successful vaccination programme will give a community immunity to a disease, but it rarely eliminates a disease completely. The gene that encodes the antigen can mutate to produce variations in the antigens. Vaccination against a disease does not give immunity against all variations to the antigen. Influenza is a virus with an antigen that changes rapidly, which means any immunity against it is temporary and a person can catch Influenza several times in their lifetime. What other reasons might there be for why a vaccination may not completely eliminate a disease? Vaccines are beneficial to populations, but there are ethical issues to be considered: -Production and testing of vaccines may be done using animals. -The risks and benefits of the vaccine need to be balanced. -Before a vaccine can be used widely it must be tested on humans. -Should vaccination be compulsory? On what grounds should people be able to opt out? -Vaccination can be expensive.

Phagocytes

A type of white blood cell that ingests invading microbes. Non-specific and are the next line of defence after the skin.

Vaccine

A way of introducing a pathogen or an antigen into the body. The pathogen may be inactive or dead to avoid causing the disease.

Vaccination programmes

A way to give a large proportion of the population immunity to a particular disease. A successful vaccination programme is dependent on a number of factors which are the: -cost of the vaccine -severity of side-effects -ease of production, transport, storage, and administration of the vaccine -number of people who need to be vaccinated for herd immunity.

AIDS

Acquired immune deficiency syndrome

Antibodies (structure and function)

Antibodies are made of four polypeptide chains which form a Y-shaped molecule. Every antibody has a specific binding site that is complementary to a single antigen. Antibodies do not breakdown an antigen - they form an antigen-antibody complex. Antibodies serve as markers to phagocytes to destroy the attached cells. The antibodies can cause cells to clump together to make it easier for phagocytes to find them (agglutination).

Antigen

Any part of a cell or molecule that an organism recognises as non-self and which stimulates an immune response. They are usually proteins in the cell-surface membrane.

HIV treatment

As HIV is a virus, antibiotics are ineffective against it and treatment of HIV and AIDS is difficult. Antibiotics work in different ways, one of which is to prevent bacteria from making cell walls. Without cell walls bacteria cannot control the entry of water and the cell will swell and burst. As viruses do not have a cell wall, and are reproduced within a host cell, they are unaffected by antibiotics. HIV can be treated using antiretroviral drugs. These drugs keep levels of the virus at very low levels in the blood by stopping it from reproducing. This reduces the impact of HIV on a person's immune system.

Antibody levels

Can indicate whether a person has been exposed to a pathogen.

Indirect therapy

Drugs are attached to monoclonal antibodies such as a cytotoxic drug. The antibody directs the drug towards the cells displaying a particular antigen rather than towards other cells.

B lymphocytes

Mature in bone marrow. They are involved in producing antibodies found in the blood. This is the humoral immune response.

T lymphocytes

Mature in the thymus gland. They are involved in cell-mediated immunity.

Pathogen

Microorganisms that cause infection or disease.

Pregnancy testing

Monoclonal antibodies in home pregnancy kits are specific to the hormone human chorionic gonadotrophin.

Direct therapy

Monoclonal antibodies, specific to antigens found on the surface of cancerous cells, target and then destroy the cells as part of an immune response. For example, herceptin targets breast cancer cells.

ELISA (enzyme-linked immunosorbent assay) test

On the outside of the virus is a lipid envelope embedded with attachment proteins. Inside is a protein capsid. HIV is a retrovirus because it contains reverse transcriptase, an enzyme that catalyses the production of DNA from RNA. Along with reverse transcriptase there are other enzymes and RNA inside the protein capsid. Viruses cannot replicate themselves. Instead they instruct a host cell to produce the components they need. HIV binds to the protein CD4, most frequently on helper T cells. The capsid fuses with the cell-surface membrane and the enzymes and RNA enter the host cell. The reverse transcriptase converts the RNA into DNA, which moves into the nucleus and is inserted into the host cell's DNA. The cell will then produce the components of the virus. An HIV infection, like many others, can be identified using an *****. The ***** test uses antibodies to identify the presence and the quantity of a specific protein or antigen, such as the antigen on the coat of HIV.

Lysosomes (role in phagocytosis)

Once a foreign molecule or cell has been engulfed by a phagocyte in a phagosome, ********* within the phagocyte move to the phagosome. The ********* fuse with the phagosome releasing lysozymes. These are enzymes that destroy cells by breaking down cell walls through hydrolysis. Once the pathogen has been broken down the products are absorbed into the cytoplasm of the phagocyte.

White blood cells

Part of the body's defence system. They protect the body by ingesting pathogens and by producing antibodies and antitoxins. They're called lymphocytes are involved in more specific immune responses.

Skin

Part of the defence mechanisms of the body and it prevents pathogens from entering the body.

Diagnosis

Particular antigens are targeted by antibodies to measure levels of that antigen in the body.

Phagocytosis

Phagocytes engulf foreign cells and molecules inside vesicles formed from the cell-surface membrane. 1) Phagocyte recognises foreign antigen 2) Pathogen engulfed/ingested 3) Enclosed in vacuole/vesicle/phagosome 4) Vacuole fuses/joins with lysosome 5) Lysosome contains enzymes 6) Pathogen digested/ molecules hydrolysed

Cell-mediated immunity

T lymphocytes recognise antigen-presenting cells. There are a huge number of different T lymphocytes, each of which recognises a different antigen. 1) Pathogens invade body cells or are ingested by phagocytes. 2) Antigens are presented on the cell's surface. 3) Receptors on helper T cells fit onto the antigens. 4) This activates the T cells to multiply rapidly. 5) The cloned cells follow four different paths. They: -develop into memory cells that enable a rapid response to repeat infections -stimulate phagocytes to engulf cells by phagocytosis. -stimulate B cells to divide and secrete their antibody -activate cytotoxic T cells to produce perforin that perforates the cell-surface membrane of the antigen-presenting cell, causing the cell to die.

Agglutination

The antibodies can cause cells to clump together to make it easier for phagocytes to find them.

Identifying foreign cells

The body can identify cells and molecules that belong to itself and foreign cells and molecules. All cells have proteins on the surface that lymphocytes are able to recognise. There are millions of types of lymphocytes and each one is able to recognise a different pathogen. During development lymphocytes in the foetus only encounter self cells and will be able to recognise the proteins on the surface of self cells. These cells are suppressed or die so that the only remaining lymphocytes respond to foreign material.

Primary immune response

The next stage of defence after the non-specific immune response is the ******* ****** ******** which involves T lymphocytes.

Vaccination

The process of producing active immunity. Individuals are exposed to a small amount of vaccine which triggers the primary immune response and the creation of memory B cells. This triggers a rapid secondary immune response if the pathogen is ever detected again. People can be immunised for a particular disease by introducing small amounts of dead or inactive pathogens.

Monoclonal antibodies

There are many types of antigens, each of which induces a different B cell to divide and produce clones. The different B cells all produce different antibodies. It is medically useful to be able to produce clones of a single type of antibody. This is the production of ********** **********.

Memory cells

These live for decades. They do not produce antibodies but circulate in the bloodstream. If they encounter the same antigen they will divide and produce more plasma and ****** cells. The ****** cells enable large amounts of antibodies to be released rapidly in response to a future infection. This is the secondary immune response.

Plasma cells

These only survive for a few days and they secrete thousands of antibodies into the blood. These antibodies lead to the destruction of antigens. This is the primary immune response.