Summary
Highlights
Our body possesses a powerful defense mechanism, the immune system, which protects us from mechanical injuries, germs, and foreign particles. This complex system ensures we don't fall ill from the daily encounter with numerous disease-causing organisms.
The immune system is divided into innate and adaptive immunity. Innate immunity acts as the body's non-specific first line of defense, preventing intruders from entering and eliminating them if they do, without differentiating between pathogens.
The innate system includes physical barriers like skin and mucous linings that trap pathogens. Chemical barriers, such as lysozyme in the eyes and stomach acid, kill pathogens. Normal flora also compete with pathogens, acting as a barrier.
Inflammation, mediated by mast cells releasing histamine, rushes blood and leukocytes (white blood cells) to problem areas. Leukocytes are the body's cellular army, with broad access throughout the body. Sometimes, this response can be triggered by harmless particles, leading to allergic reactions.
Among the innate immune cells are phagocytes, including neutrophils, which patrol the body and quickly reach infection sites, engulfing pathogens and dying to form pus. Macrophages are larger cells that reside in specific locations, consuming up to 100 pathogens and even detecting and killing rogue cells like cancer cells.
Natural Killer Cells (NKCs) detect and destroy body cells that have gone rogue or are virally infected by checking for the Major Histocompatibility Complex (MHC) protein. Dendritic cells, found in areas exposed to the environment, link innate and adaptive immunity. They engulf pathogens and present their antigens to adaptive immune cells, specifically T cells.
Adaptive immunity is more efficient, differentiating between specific pathogens. It comprises T lymphocytes (T-cells), which handle cell-mediated immune responses post-infection, and B lymphocytes (B-cells), which manage humoral immune responses when pathogens enter but haven't caused disease.
Helper T-cells, activated by dendritic cells or macrophages, form effector T-cells that call other white blood cells, and memory T-cells that record antigen information. Cytotoxic T-cells perform 'mercy killings' for heavily infected and dying cells that have lost the battle against pathogens.
B-cells produce antibodies that specifically bind to pathogen antigens, tagging them for destruction by macrophages. B-cells also generate memory B-cells upon encountering an antigen. Both memory B-cells and T-cells maintain a record of past infections, strengthening future immune responses.
The innate immune response is quicker but non-specific, acting within hours. If it cannot contain the threat, it calls upon the adaptive immune system, which takes days to mount a response but provides specific, long-lasting immunity. Our immune system is thus crucial for our daily health.