Introduces innate immunity as the body's fast, non-specific first line of defence, then covers external physical, chemical and biological barriers and their disruption. It goes on to the humoral complement system with its three activation pathways, C3 and the membrane attack complex, and to cellular defences such as phagocytosis, phagocyte types, killing mechanisms, NK cells and neutrophil extracellular traps. It closes with inflammation, systemic effects and interferons, and includes three memory mnemonics for barriers, complement pathways and phagocytes.
Topic 47 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 9 sections, about 987 words (7 min of reading), with 14 flashcards.
In this note
Innate immunity
Understanding Immunity
Innate Immunity: The Body’s First Line of Defense
External Barriers: Keeping Pathogens Out
Internal Defenses: Battling Invaders That Breach Barriers
Humoral Defenses: Blood Clotting and Complement System
Cellular Defenses: Immune Cells at Work
Inflammation: Coordinating a Local Response
MNEMONICS
Understanding Immunity
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: The Body’s First Line of Defense
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: Keeping Pathogens Out
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.
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