AQA A level Biology
AQA A level Biology
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Topic 2: Cells
Topic 2: Cells
The immune system
The immune system
Pathogens and non-specific defense
Pathogens and non-specific defense
A pathogen is a microorganism that can cause disease. Examples include bacteria, viruses, fungi, and parasites. Our body defends itself against pathogens through several non-specific mechanisms:
Hydrochloric acid in the stomach destroys pathogens quickly.
Mucus in the airways traps pathogens, which are then wafted upwards by cilia to be swallowed.
The skin acts as a protective barrier against pathogens, though breaks in the skin can allow entry.
If a pathogen bypasses these barriers, it encounters phagocytes (a type of white blood cell) which perform phagocytosis to engulf and destroy it:
The phagocyte is attracted to and binds with the pathogen.
It engulfs the pathogen, enclosing it within a phagosome (a specialised vesicle).
Lysosomes within the phagocyte fuse with the phagosome.
Lysozyme enzymes are released, digesting the pathogen.
Antigens from the pathogen are displayed on the phagocyte’s surface, turning it into an antigen-presenting cell.
Specific defense: Cellular and humoral immunity
Specific defense: Cellular and humoral immunity
The specific immune response is tailored to destroy a particular strain of pathogen. This process begins when the immune system distinguishes between foreign antigens (proteins on the pathogen's surface) and the body’s own cells. Each pathogen has unique antigens, but within the same species, pathogens have identical antigens. Variations within pathogens, like viruses, can result in new strains due to mutations in their DNA, altering the structure and shape of their proteins—a process called antigenic variation.
Cell-Mediated Immunity (T-Cells)
Helper T (TH) cells detect antigens on antigen-presenting cells and become activated.
Activated TH cells then stimulate cytotoxic T (TC) cells to divide rapidly by mitosis, producing clones with identical receptors that bind to the pathogen.
TC cells destroy infected cells by releasing perforin, a protein that creates holes in the pathogen’s cell membrane.
Some TH cells become memory T cells, enabling a faster response if the pathogen re-infects.
Humoral Immunity (B-Cells)
B-cells that encounter pathogens display the pathogen’s antigens on their surface, becoming antigen-presenting cells.
Activated TH cells stimulate these B-cells to divide into plasma cells or memory cells.
Plasma cells secrete antibodies specific to the pathogen’s antigens, forming an antigen-antibody complex that marks the pathogen for destruction.
Antibodies help in pathogen destruction through agglutination, where they cause pathogens to clump together, making them easier for phagocytes to engulf.
In humoral immunity, antibodies circulate in the blood, allowing them to reach pathogens throughout the body.
During a primary infection, the immune response is slow, and symptoms may occur. However, if reinfection occurs, memory B and T cells respond rapidly, producing large quantities of antibodies to neutralize the pathogen before symptoms appear.
Antibody structure
Antibody structure
Antibodies are proteins shaped like a Y. The tips of the Y, called the variable region, are specific to each antigen, allowing antibodies to bind only to that particular antigen. The rest of the antibody is the constant region, which is identical across antibodies and helps attract other immune cells.
Different forms of immunity
Different forms of immunity
Passive Immunity: Antibodies from an external source provide short-lived immunity, as no B cells are stimulated.
Active Immunity: The body produces its own antibodies, providing long-lasting immunity. This can be achieved through vaccination, where inactive or weakened pathogens are introduced, stimulating an immune response and the production of memory cells. These memory cells allow the body to respond quickly if it encounters the active pathogen in the future.
Herd Immunity: When a large portion of a population is vaccinated, the spread of the pathogen is limited as there are fewer hosts available, protecting even those who are unvaccinated.
Definitions
Definitions
Active immunity: ability to destroy a pathogen before symptoms show as a result of previous exposure to antigens from that pathogen causing the stimulation of B cells and antibody production.
Agglutination: the clumping together of antigens or pathogens as a result of being bound to antibodies.
Antibody: a protein produced by a B cell that binds to an antigen.
Antigen: a foreign protein that stimulates an immune response.
Antigen-antibody complex: when an antibody binds to a complementary antigen.
Antigen binding site: region on an antibody to which an antigen binds.
Antigenic variability: when the antigens on a pathogen change as a result of genetic mutations.
Antigen-presenting cell: phagocyte or B cell that that has come into contact with an antigen or pathogen and attaches the antigens to its cell surface membrane.
B cell (B lymphocyte): type of white blood cell involved in the specific immune response that produce antibodies.
Cellular immunity: immunity controlled by T cells.
Clonal selection: the activation of one type of B cell during an immune response.
Clones: two or more genetically identical cells or organisms.
Cytotoxic T cell: T cell capable of destroying a specific pathogen using the perforin protein.
ELISA (enzyme-linked immunosorbent assay) test: technique that allows the detection of an antigen in a sample through the binding of an antibody with an attached enzyme, capable of changing the colour of a substrate.
Helper T cell: T cell responsible for activating immature T and B cells and phagocytes.
Herd immunity: when the majority of a population is immune to a pathogen (normally through vaccination) the pathogen has fewer hosts to be transmitted through, so making it less likely that people not immune will be infected.
Humoral response: immunity that arises as a result of the antibodies produced by B cells.
Immune system: the organs and processes of the body that enable to body to defend itself against pathogens.
Immunology: the study of the immune system and immunity.
Lysozymes: hydrolytic enzymes found in lysosomes.
Memory cell: specific B cells that remain in the blood for a long time after infection by a pathogen and are capable of producing monoclonal antibodies complementary to an antigen on that pathogen.
Monoclonal antibody: an antibody produced by a specific B cell or many clones of that cell.
Passive immunity: the ability to defend that body against a pathogen as a result of the introduction of antibodies from another individual.
Pathogen: microorganism that causes disease.
Phagocyte: type of white blood cell that engulfs and digests pathogens as part of nonspecific immunity.
Phagocytosis: when a phagocyte engulfs a pathogen and surrounds it in a vesicle.
Plasma cell: activated B cell that produces many antibodies capable of destroying a specific pathogen.
Primary immune response: the reaction of lymphocytes to first infection by a pathogen. The response is relatively slow and produces relatively few antibodies.
Protein: functional molecule made up of one or more polypeptides and prosthetic groups.
Secondary immune response: the reaction of memory lymphocytes to second infection by a pathogen. The response is relatively fast and produces large numbers of antibodies, so that the pathogen is destroyed before symptoms show.
T cell (T lymphocyte): type of white blood cell part of the specific immune response that has receptors on the surface.
Toxin: chemical capable of damaging cells or disrupting cellular processes.
Vaccine: a medical treatment that contains antigens found on a dead or weakened pathogen, stimulating production of plasma cells and memory cells.
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