Immunity

• An antigen is a cell-surface molecule that stimulates an immune response
• Usually a (glyco)protein, sometimes a (glyco)lipid or polysaccharide
• The immune system recognizes antigens as "self" or "non-self", enabling identification of cells from other organisms of the same species, pathogens, toxins, and abnormal body cells
• Phagocytosis destroys pathogens through the following steps:
• Phagocyte moves towards the pathogen via chemotaxis
• Phagocyte engulfs the pathogen via endocytosis to form a phagosome
• Phagosome fuses with a lysosome (phagolysosome)
• Lysozymes digest the pathogen
• Phagocyte absorbs the products from pathogen hydrolysis
• Antigen-presenting cells (APCs) play a role in enhancing recognition by T H cells, which cannot directly interface with pathogens/antigens in body fluid
• Two differences between specific and nonspecific immune responses:
• Nonspecific responses (inflammation, phagocytosis) are the same for all pathogens, while specific responses (B & T lymphocytes) are complementary to the pathogen
• Nonspecific responses are immediate, while specific responses have a time lag
• The two types of specific immune responses are cell-mediated and humoral
• The process of the cell-mediated response involves:
• Complementary T H lymphocytes binding to foreign antigen on APC
• Releasing cytokines that stimulate clonal expansion of complementary T H cells and cytotoxic T cells
• The process of the humoral response involves:
• Complementary T H lymphocytes binding to foreign antigen on antigen-presenting T cells
• Releasing cytokines that stimulate clonal expansion of complementary B lymphocytes
• B cells differentiating into plasma cells that secrete antibodies with complementary variable regions to antigens
• An antibody is a protein secreted by plasma cells with a quaternary structure consisting of 2 'light chains' and 2 longer 'heavy chains'
• Binding sites on the variable region of light chains have a specific tertiary structure complementary to an antigen
• The rest of the molecule is known as the constant region
• Antibodies lead to the destruction of a pathogen by forming an antigen-antibody complex that results in agglutination, enhancing phagocytosis
• Monoclonal antibodies are antibodies produced from a single clone of B cells
• Memory cells are specialized T H/B cells produced from the primary immune response that can divide rapidly by mitosis if the organism encounters the same pathogen again
• The primary immune response has a slower rate of antibody production, longer time lag, lower concentration of antibodies, and the pathogen is usually not destroyed before symptoms appear
• Antigen variability is caused by random genetic mutations that result in different primary structures of antigens, making it difficult to develop vaccines containing all antigen types
• Antigen variability affects the incidence of disease by making memory cells no longer complementary to the antigen, leading to individuals being susceptible to catching the disease multiple times
• Passive and active immunity both involve antibodies and can be natural or artificial
• Passive immunity examples: antibodies in breast milk/placenta (natural) and anti-venom (artificial)
• Active immunity examples: humoral response to infection (natural) and vaccination (artificial)
• Passive immunity lacks memory cells and antibodies are not replaced when broken down, while active immunity produces memory cells for long-term immunity
• Vaccination involves triggering a primary immune response by introducing a dead/inactive form of a pathogen or antigen, leading to the production of memory cells for a rapid secondary response
• Herd immunity is achieved by vaccinating a large proportion of the population to reduce available carriers of the pathogen and protect individuals who have not been vaccinated
• Ethical issues surrounding vaccines include production involving animals, potential dangerous side effects, fatal clinical tests, and the debate between compulsory vaccination and opt-out options
• The structure of HIV includes genetic material (2 x RNA) and viral enzymes surrounded by a capsid, with a viral envelope derived from the host cell membrane and GP120 attachment proteins on the surface
• HIV results in the symptoms of AIDS by binding to CD4 receptors on T H cells, replicating inside them, leading to a decrease in T H cells, and causing individuals to be unable to destroy other pathogens, resulting in secondary diseases/infections
• Antibiotics are ineffective against viruses because they often work by damaging murein cell walls, which viruses lack, and viruses replicate inside host cells, making it difficult to target them without harming normal cells