Immunity is the capacity of an organism to resist infection or disease by recognizing and neutralizing pathogens or toxins.
Explanation
Immunity encompasses the mechanisms that protect an organism from infection and disease. There are two broad categories: innate and adaptive. Innate immunity provides an immediate, non‑specific response through physical barriers such as skin and mucous membranes, chemical defenses like antimicrobial peptides and the complement system, and cellular components like neutrophils, macrophages and natural killer cells. Adaptive immunity develops over days and is highly specific; it involves lymphocytes that recognize particular antigens. B lymphocytes produce antibodies that neutralize pathogens and facilitate their clearance, while T lymphocytes destroy infected cells and regulate immune responses. Once activated, adaptive responses generate memory cells that allow rapid and robust reactions upon subsequent exposure to the same antigen. Immunity can also be classified as active or passive. Active immunity arises when exposure to an antigen stimulates the host’s own immune system, either through natural infection or vaccination. Passive immunity results from the transfer of antibodies from another source, such as maternal antibodies crossing the placenta or antiserum therapy. The effectiveness of immunity can be influenced by genetic factors, nutrition, age and underlying health conditions. Dysregulation of immune responses can lead to immunodeficiency, allergies or autoimmune diseases.
Immune responses in practice
After recovery from varicella (chickenpox), most people develop long‑lasting immunity mediated by memory B and T cells that prevent reinfection. Newborns receive maternal immunoglobulin G (IgG) through the placenta, providing temporary protection during the first months of life. Vaccinations induce active immunity without causing disease; for example, the measles, mumps and rubella vaccine stimulates the production of specific antibodies and memory cells. Innate immune mechanisms include the ciliated epithelium in the respiratory tract, which traps and expels pathogens, and phagocytic cells that engulf bacteria invading a wound. Booster immunizations restore waning immunity by re‑exposing the immune system to the antigen.
Immunity reflects the coordinated action of innate and adaptive defenses that keep organisms healthy. Understanding the components and dynamics of immune responses informs the development of vaccines and therapies. Maintaining robust immunity depends on adequate nutrition, vaccination and avoidance of immunosuppressive factors.
Related Terms: Innate immunity, adaptive immunity, vaccination, antibodies, immune response