Introduction
Infectious diseases are caused by pathogens such as bacteria, viruses, fungi, and parasites. The human body has developed multiple defense mechanisms to protect against these pathogens. This study note will cover the key aspects of the body's defense against infectious diseases as outlined in the International Baccalaureate (IB) Biology syllabus. The main topics include the role of the skin, blood clotting, white blood cells, antibiotics, and antibiotic resistance.
6.3.1 Skin
Physical Barrier
The skin is the largest organ in the human body and serves as a formidable barrier against pathogens. It provides:
- Tough Physical Barrier: The outermost layer of the skin, the epidermis, is composed of tightly packed cells that are continuously shed, removing potential pathogens.
- Microorganisms: The skin is home to a variety of harmless microorganisms that outcompete potential pathogens for resources and space.
Chemical Protection
The skin also offers chemical defenses:
- Sebum: Produced by sebaceous glands, sebum maintains a low pH on the skin surface, inhibiting the growth of harmful microorganisms.
- Sweat: Contains lysozyme, an enzyme that breaks down bacterial cell walls.
Mucous Membranes
Mucous membranes line areas of the body that are exposed to the external environment, such as the respiratory, digestive, and reproductive tracts. They provide additional protection through:
- Mucus: Produced by goblet cells, mucus traps pathogens and particles, which are then expelled or swallowed.
- Lysozyme: Present in mucus, lysozyme provides antibacterial properties.
Regular hand washing and maintaining good hygiene can significantly enhance the protective functions of the skin and mucous membranes.
6.3.2 Blood Clotting
Mechanism of Blood Clotting
When the skin is cut, blood clotting prevents pathogens from entering the body. The process involves:
- Platelet Activation: Platelets adhere to the site of injury and release clotting factors.
- Clotting Cascade: A series of reactions that convert the plasma protein fibrinogen into fibrin.
- Fibrin Mesh: Fibrin strands form a mesh that traps blood cells, forming a clot.
Key Factors
- Thrombin: An enzyme that converts fibrinogen to fibrin.
- Calcium Ions: Essential for the clotting cascade.
$$ \text{Prothrombin} \rightarrow \text{Thrombin} \ \text{Fibrinogen} \rightarrow \text{Fibrin} $$
NoteVitamin K is crucial for the synthesis of clotting factors. A deficiency in Vitamin K can impair blood clotting.
6.3.3 White Blood Cells
White blood cells (WBCs) are essential components of the immune system. They can be classified into two main types: phagocytes and lymphocytes.
Phagocytes
- Macrophages and Neutrophils: Engulf and digest pathogens through a process called phagocytosis.
- Mechanism:
