Immune system

• Cell Recognition in the immune system involves various defence mechanisms against pathogens, including physical and chemical barriers like the skin, mucous membranes, tears, and saliva
• Inflammation is a non-specific inflammatory response to the region invaded by a pathogen
• The specific immune response involves recognizing 'foreign' cells through specific molecules found on the surface of cells, usually proteins like glycoproteins or glycolipids
• Antigens are the surface molecules used by the body to identify cells, viral particles, and toxins
• Phagocytes, like white blood cells, have surface proteins that act as receptors and bind to the proteins (antigens) on the surface of pathogens, enabling their engulfment and digestion
• Antigens can be self (not stimulating an immune response) or non-self (stimulating an immune response) depending on whether they are produced by the organism's own body cells or not
• Some pathogens exhibit antigen variability, changing the antigens on their surface frequently due to genetic mutations, which can evade the immune system's response
• Phagocytes, like neutrophils and macrophages, carry out phagocytosis, the process of recognizing and engulfing pathogens, with slight differences in their modes of action
• Neutrophils are short-lived cells that move towards pathogens, engulf them, and digest them using lysosomes, forming phagosomes and releasing lysozymes
• Neutrophils die, leading to pus formation as a sign of dead neutrophils
• Macrophages are larger than neutrophils and are long-lived cells
• Macrophages move into organs like the lungs, liver, spleen, kidney, and lymph nodes
• Macrophages are produced in the bone marrow, travel in the blood as monocytes, and develop into macrophages once they settle in organs
• Macrophages play a crucial role in initiating an immune response and display antigens of pathogens on their surface through the major histocompatibility complex
• T lymphocytes are smaller than phagocytes and have a large nucleus that fills most of the cell
• T lymphocytes are produced in the bone marrow before birth and include T cells and B cells
• Immature T lymphocytes mature in the thymus and have specific cell surface receptors called T cell receptors
• Activated T lymphocytes divide when they encounter their specific antigen presented by host cells, leading to the differentiation into helper T cells and killer T cells
• Helper T cells release cytokines that stimulate various immune responses, while killer T cells destroy infected cells by secreting toxic substances
• Antigen-presenting cells help recruit other immune system cells by presenting antigens from pathogens
• Antigen-presenting cells include macrophages, dendritic cells, and sometimes neutrophils
• B lymphocytes produce millions of types of cells, each capable of making one type of antibody molecule
• When an antigen enters the body, specific B lymphocytes recognize and bind to it, leading to clonal expansion and differentiation into plasma cells and memory cells
• During clonal selection, B-lymphocytes that respond to an antigen are stimulated to divide by mitosis
• Clonal selection results in large numbers of identical B-lymphocytes being produced over a few weeks
• Some B-lymphocytes become plasma cells that secrete specific antibody molecules into the blood, lymph, or linings of the lungs and the gut
• Plasma cells are short-lived, but the antibodies they secrete stay in the blood for a longer time
• Other B-lymphocytes become memory cells that remain circulating in the blood for a long time
• During a primary immune response, B-lymphocytes form two types of cells: plasma cells and memory cells
• Antibodies are globular glycoproteins called immunoglobulins with a quaternary structure represented as Y-shaped
• Each antibody has a different variable region with an antigen-binding site specific to one antigen
• Antibodies can bind to more than one bacterium or virus at the same time, causing groups of the same pathogens to become clumped together in a process known as agglutination
• Memory cells form the basis of immunological memory and can last for many years or even a lifetime
• During a secondary immune response, memory cells recognize the antigen, divide quickly, and differentiate into plasma cells to produce antibodies and more memory cells