The immune system begins its defense with nonspecific defenses, also termed innate immunity. This system provides immediate, broad protection against a wide array of foreign invaders without needing prior exposure to a specific pathogen. Present from birth, innate immunity represents the first two lines of defense, acting instantly to block the entry of microbes or to neutralize them if they breach the initial barriers. Its responses are generalized and do not result in the immunological memory that characterizes the body’s later, specialized defensive actions.
Preventing Entry: Physical and Chemical Barriers
The first layer of defense consists of anatomical and biochemical barriers that physically and chemically prevent pathogens from entering deeper tissues. Intact skin, particularly the tough, dry epidermis, functions as an effective barrier against microbial invasion. The constant shedding of surface skin cells, called desquamation, helps remove microbes that colonize the surface.
Internal surfaces, such as the respiratory, gastrointestinal, and genitourinary tracts, are protected by mucous membranes. These membranes secrete sticky mucus that physically traps inhaled or ingested foreign particles. In the respiratory tract, specialized cells possess cilia that constantly sweep the mucus and trapped pathogens upward. This action, known as the mucociliary escalator, expels them from the body.
The body also employs chemical agents designed to kill or inhibit microbial growth on these surfaces.
Chemical Defenses
- The extremely low pH environment of the stomach (typically between 1 and 3) is highly acidic and destroys most ingested bacteria and toxins.
- Secretions like tears and saliva contain lysozyme, an enzyme that breaks down the peptidoglycan cell walls of many bacteria.
- Epithelial cells on the skin and mucosal surfaces produce small proteins called defensins, which are antimicrobial peptides that directly disrupt the membranes of invading microbes.
Internal Cellular Patrol and Attack
If a pathogen breaches the external barriers, the innate immune system mobilizes internal cellular and molecular components to patrol and attack the invader. The primary cellular response involves phagocytes, literally “eating cells,” which include macrophages and neutrophils. Neutrophils are typically the first responders to an infection site, where they actively engulf and neutralize foreign particles and cellular debris through phagocytosis.
Macrophages are larger, longer-lived phagocytes stationed in tissues, acting as sentinels and clearing agents. Both cell types recognize common microbial structures, allowing for a rapid, generalized response. After engulfing a microbe, the phagocyte fuses the ingested particle with an internal compartment containing powerful digestive enzymes and reactive oxygen species, destroying the threat.
Natural Killer (NK) cells are lymphocytes that focus on destroying the body’s own compromised cells, such as those infected with a virus or turned cancerous. They recognize these host cells not by foreign markers, but by detecting a lack of normal surface proteins (Major Histocompatibility Complex class I). This signals that the cell is in distress and must be eliminated.
The Complement System is a complex group of over 30 circulating plasma proteins. When activated, these proteins initiate a biochemical cascade that serves three main functions:
- Opsonization: Coating the surface of a pathogen, marking it for efficient ingestion by phagocytes.
- Enhancing inflammation.
- Lysis: Directly assembling into a structure that punches a hole in the microbial cell membrane, causing it to burst.
Coordinated Systemic Responses: Inflammation and Fever
The innate defenses culminate in two organized, systemic responses: inflammation and fever. Inflammation is a localized tissue reaction triggered by chemical signals released from damaged cells and immune cells, such as histamine. These signals cause small blood vessels near the injury to dilate and become more permeable, increasing blood flow to the area.
This increased blood flow brings an influx of immune cells and fluid to the site, leading to the four classic signs: redness, heat, swelling, and pain. The purpose of this reaction is to localize the infection, prevent its spread, remove the initiating agent and cellular debris, and initiate tissue repair.
Fever is a systemic response where the body’s core temperature set point is temporarily raised by the hypothalamus. This change is initiated by chemical messengers, known as pyrogens, released primarily by activated immune cells. The resulting temperature elevation is beneficial because it can directly inhibit the replication rate of many pathogens. Additionally, the increased heat accelerates host metabolic processes involved in the immune response and tissue repair, making the defense system work more quickly.