Edexcel International A Level Biology - Immunity

Opportunistic infections are diseases that only affect people with weakened immune systems, such as those with AIDS. They are caused by pathogens that usually don't cause illness in healthy people.

HIV stands for Human Immunodeficiency Virus. It is a virus that attacks the immune system, making it harder to fight off infections.

AIDS stands for Acquired Immune Deficiency Syndrome. It is a condition that develops when HIV weakens the immune system so much that it can no longer fight off infections.

Factors like the number of existing infections, the strain of HIV, age, and access to healthcare can all affect how quickly HIV progresses into AIDS and how long someone with AIDS will survive.

Vectors are living organisms, like insects, that carry pathogens and transmit them between hosts.

Pathogens can enter the body through body openings like the mouth, eyes, and urinary tract. They can also enter through breaks in the skin.

Inhalation occurs when we breathe in droplets containing pathogens released by an infected person coughing, sneezing, or talking. This can lead to infections like flu, measles, or tuberculosis.

Ingestion occurs when we eat or drink contaminated food or water. This can lead to infections like food poisoning.

HIV targets and destroys helper T cells (also called CD4 T-lymphocytes), compromising the body's immune response.

The initial stage of HIV infection often involves mild, flu-like symptoms that may go unnoticed.

HIV's viral DNA can remain dormant for years before reactivating and actively destroying helper T cells.

Helper T cells are crucial for the body's specific immune response, stimulating B cells, antibody production, and phagocytosis.

A decline in helper T cell count due to HIV infection leads to weakened immunity and susceptibility to opportunistic infections.

Opportunistic infections are diseases that typically cause minor issues in healthy individuals but become severe in those with weakened immune systems.

AIDS develops when an HIV-infected individual begins to suffer from frequent opportunistic infections and their helper T cell count drops below a critical threshold.

The time it takes for HIV to progress to AIDS varies between individuals, but the progression follows a pattern of increasingly severe infections.

Pathogens spread through direct contact when an infected individual's skin, body fluids, or mucous membranes come into contact with a healthy individual.

Inanimate objects, such as bedding, towels, or surfaces, can harbor pathogens. A healthy individual may touch the object, then touch their face, transferring the pathogen.

Pathogens enter the body through breaks in the skin, such as open wounds, scratches, or needle sharing. This provides a direct route to the bloodstream.

Pathogens spread through the air in droplets released by coughing, sneezing, or talking. These droplets can be inhaled by a healthy individual.

A living organism, such as an insect, carries pathogens from an infected individual to a healthy individual.

The skin provides a physical barrier against pathogens. Any damage to the skin can expose the underlying tissue to infection, making it vulnerable to pathogens.

The body's blood clotting mechanism helps prevent pathogens from entering via damaged skin. This process takes time, so some pathogens may still enter before a clot forms.

Harmless microorganisms that live on the skin and in the gut are called flora. They compete with pathogens for resources, keeping their numbers low and reducing their ability to infect the body.

Hydrochloric acid in the stomach, also known as gastric juices, creates an acidic environment that is unfavorable to many pathogens ingested with food and drink. However, some pathogens may survive and infect the gut wall.

Lysozyme, an enzyme found in secretions like tears, saliva, and mucus, damages bacterial cell walls, causing them to burst.

Active immunity occurs when a person's immune system produces antibodies after being exposed to an antigen, either through an infection or vaccination.

Passive immunity involves the transfer of pre-made antibodies from one person to another, providing temporary protection without the body needing to produce its own antibodies.

Vaccines introduce weakened or inactive versions of pathogens, or just the antigens, into the body to trigger the production of antibodies and memory cells, leading to long-term immunity.

Antigenic variation is when pathogens change their surface antigens over time, making it harder for the immune system to recognize and fight them. This can lead to reinfection or vaccine failure.

Antigenic changes are often caused by mutations in the pathogen's genetic material. This can result in new strains emerging, requiring new vaccine formulations.

Antibodies are Y-shaped proteins that bind specifically to antigens, triggering the immune response.

Antibodies can be either membrane-bound, attached to lymphocytes, or secreted into the blood.

The variable regions of antibodies are complementary to the epitope of the antigen, allowing them to bind.

Antibodies prevent pathogens from binding to host cells by blocking their receptors.

Antibodies can neutralize toxins produced by pathogens by binding to them.

Agglutination is the clumping of pathogens due to antibody binding, making them easier for phagocytes to engulf.

Antibodies cause agglutination, which reduces the spread of pathogens and makes them easier for phagocytes to ingest.

Antibodies can bind to toxins produced by pathogens, neutralizing them and preventing them from harming the host.