The body protects itself from internal infections primarily through the immune system, a complex network of cells, tissues, organs, and processes.
The Immune System: The Body's Internal Defense Force
The immune system is designed to identify and eliminate pathogens (harmful microorganisms like bacteria, viruses, fungi, and parasites) and other threats from inside the body. It accomplishes this through a combination of innate and adaptive immune responses.
1. Innate Immunity: The First Line of Defense
The innate immune system provides immediate, non-specific protection. It's the body's first responder to any perceived threat. Key components include:
- Physical Barriers: Though primarily external, some physical barriers protect internal environments. For example, mucus membranes lining the respiratory and digestive tracts trap pathogens. Cilia (tiny hairs) in the respiratory tract sweep mucus and trapped pathogens out.
- Chemical Barriers: Internal chemical defenses include stomach acid, which kills ingested pathogens, and antimicrobial proteins like lysozyme (found in tears and saliva, but also internally).
- Internal Defenses:
- Phagocytes: Cells like macrophages and neutrophils engulf and destroy pathogens through phagocytosis. They circulate in the blood and tissues, constantly seeking out invaders.
- Natural Killer (NK) Cells: These cells target and kill infected or cancerous cells by releasing cytotoxic granules.
- Inflammation: This process recruits immune cells to the site of infection and promotes healing. Signs of inflammation include redness, swelling, heat, and pain. The inflammatory response is triggered by chemicals released from damaged tissues and immune cells.
- Complement System: A group of proteins in the blood that can directly kill pathogens, enhance phagocytosis, and promote inflammation.
2. Adaptive Immunity: Targeted and Long-Lasting Defense
The adaptive immune system is slower to respond but provides highly specific and long-lasting protection. It "learns" to recognize specific pathogens and mounts a tailored response. It relies on two main types of lymphocytes (white blood cells):
- B Cells: These cells produce antibodies, proteins that bind to specific antigens (molecules on the surface of pathogens). Antibody binding can neutralize pathogens, mark them for destruction by phagocytes, or activate the complement system. B cells can also develop into memory B cells, which provide long-term immunity to specific pathogens.
- T Cells: There are several types of T cells:
- Helper T Cells: These cells "help" other immune cells, such as B cells and cytotoxic T cells, become activated. They release cytokines, signaling molecules that coordinate the immune response.
- Cytotoxic T Cells (Killer T Cells): These cells directly kill infected or cancerous cells by recognizing specific antigens on their surface.
- Regulatory T Cells: These cells help to suppress the immune response after the infection is cleared, preventing autoimmunity (the immune system attacking the body's own tissues).
3. Lymphatic System: A Crucial Supporting Role
The lymphatic system plays a vital role in immune function by:
- Filtering Lymph: Lymph nodes contain immune cells that filter lymph fluid, trapping pathogens and presenting them to immune cells.
- Transporting Immune Cells: The lymphatic system transports immune cells throughout the body.
- Removing Waste: The lymphatic system also removes waste products from tissues.
How Memory Works:
After encountering a pathogen, the adaptive immune system creates memory cells (memory B cells and memory T cells). If the body encounters the same pathogen again, these memory cells will quickly recognize it and mount a faster and stronger immune response, often preventing illness altogether. This is the basis of vaccination.
In summary, the body protects itself from infection internally through a layered and complex immune system encompassing innate and adaptive immunity, working in concert with the lymphatic system to defend against a vast array of pathogens.
