Understanding Infection and the Immune Response

What are Pathogens?

Pathogens are microorganisms that can cause disease in plants and animals. The main types of pathogens are:

• Bacteria - Unicellular organisms with a cell wall but no nucleus. Examples include Salmonella and Staphylococcus.
• Viruses - Small infectious agents made of genetic material inside a protein coat. Examples include influenza and HIV.
• Fungi - Organisms like mushrooms and yeasts that obtain nutrients from other organisms. Examples include athlete's foot fungus.
• Protists - Unicellular eukaryotic organisms like plasmodium (malaria).

How Pathogens Spread Disease

Pathogens can be transmitted in various ways:

• Through air via coughing/sneezing (e.g. influenza virus)
• Via food and water contaminated with pathogens (e.g. Salmonella)
• Direct contact with infected surfaces or bodily fluids
• Animal/insect vectors that transmit pathogens to humans

The Body's Defences

The body has several lines of defence against pathogens:

1. Physical barriers like skin, mucus membranes, and stomach acid prevent pathogen entry.
2. The immune system includes:
   • White blood cells like lymphocytes and phagocytes that destroy pathogens:
   • Phagocytosis - process where phagocytes engulf and digest pathogens
   • Antibody production - lymphocytes produce antibodies that bind to and mark pathogens for destruction
   • Memory cells - enable a rapid immune response if the same pathogen is encountered again

Worked Example: Immune Response

1. A pathogen enters the body
2. Phagocytes like neutrophils engulf and digest some pathogens by phagocytosis
3. Lymphocytes detect the remaining pathogens as foreign and produce antibodies
4. Antibodies bind to antigens on the pathogen surface, marking them for destruction
5. Memory cells remain, allowing a faster response to future exposure to the same pathogen

Disease Prevention

• Vaccination - exposing the body to weakened or inactive pathogens triggers immunity without causing disease
• Antibiotic drugs - kill bacterial pathogens or prevent their growth, but are not effective on viruses
• Challenges - Antibiotic resistance is increasing due to overuse and evolution of resistant bacterial strains
• New drugs and treatments are continuously developed to combat pathogens and diseases

Plant Diseases

Plants also suffer from pathogenic infections, caused by bacteria, viruses, and fungi. These can impact agriculture and crop yields. Methods to control plant diseases include chemical fungicides, plant breeding for resistant varieties, and biological control using other organisms.