Overview
Antigen-presenting cells (APCs) and natural killer (NK) cells belong to the innate immune system and link it to adaptive immunity. This topic reviews innate defences and then the characteristics and functions of dendritic cells, macrophages, B cells and NK cells.
- Immunity refers to the body’s ability to resist and fight against harmful pathogens.
- It is a complex defense system that prevents pathogens from entering the body and eliminates them if they do.
- The immune system includes specialized organs, cells, and molecules working in harmony to maintain health.
- Innate immunity is the body’s natural, non-specific defense mechanism present from birth.
- It does not target specific pathogens but responds broadly to any invader.
- This type of immunity does not improve with repeated exposure to the same pathogen.
- The innate immune response is very fast, usually activated within minutes to a few hours of infection.
External barriers
Physical Barriers
- The skin acts as a tough physical shield that pathogens cannot easily cross.
- Tears, saliva, and urine mechanically flush out microbes from body surfaces.
- The respiratory tract uses mucociliary clearance where cilia push mucus filled with trapped particles toward the throat to be expelled or swallowed.
Chemical Barriers
- The acidic environment of the stomach and vagina kills many pathogens.
- Lysozymes in saliva, tears, and other secretions break down bacterial cell walls.
- Sudden pH changes from the stomach to the intestines act as additional hostile conditions for microbes.
Biological Barriers
- The body’s natural microbiota (such as gut and vaginal flora) compete with pathogens for space and resources, preventing their colonization.
- These resident microbes may also secrete substances that are toxic to pathogens.
Disruption of Barriers
- Conditions like cystic fibrosis or primary ciliary dyskinesia impair mucociliary clearance, leading to chronic respiratory infections.