C3.2-1

C3.2 Defence Against Disease

How Body Systems Recognize Pathogens and Fight Infections

• Immune systems require the ability to differentiate between self and foreign molecules.

• Antigens: Molecules triggering immune response.

  • Tumour antigens present on cancer cells due to genetic mutations.

• Pathogens recognized by collections of antigens.

Factors Influencing Disease Incidence in Populations

• Spanish flu pandemic (1918-1920) affected 1/5 of the world population, resulting in 20-50 million deaths.

  • Second wave was deadlier, especially among healthy adults (e.g., soldiers).

Key Topics

C3.2.1 Pathogens as Causes of Infectious Diseases

• Diseases are illnesses with specific symptoms; not all causes were understood historically.

  • Three types of causes: genetic, environmental, and pathogens.

• Pathogens: Disease-causing organisms that multiply in the host body.

• Types of pathogens include bacteria, fungi, protists, and viruses (e.g., tuberculosis, malaria).

C3.2.2 Primary Defence: Skin and Mucous Membranes

• Skin acts as a primary barrier against pathogens.

  • Outer layer consists of dead cells with keratin.

  • Sebum secreted by sebaceous glands lowers skin pH, inhibiting pathogens.

• Mucous membranes trap pathogens and contain lysozyme (an antibacterial enzyme).

C3.2.3 Blood Clotting Mechanism

• Clotting occurs after skin cuts to prevent blood loss and infection.

  • Involves a cascade of reactions leading to fibrin formation, sealing wounds.

C3.2.4 Innate vs Adaptive Immune System

• Innate immune system responds broadly and is unchanged throughout life.

• Adaptive immune system responds specifically and builds memory for pathogens.

C3.2.5 Infection Control by Phagocytes

• Phagocytes are white blood cells that engulf and digest pathogens through endocytosis.

C3.2.6 Lymphocytes and Antibody Production

• 25% of white blood cells are lymphocytes, found in the lymphatic system.

• Antibodies produced by lymphocytes target specific pathogens.

C3.2.7 Antigens as Recognition Molecules

• Antigens stimulate the immune response and are primarily found on pathogens.

• Immune system antibodies bind irreversibly to antigens, triggering a response.

C3.2.8 Activation of B-Lymphocytes by Helper T-Lymphocytes

• Helper T-cells activate B-lymphocytes, leading to antibody production.

C3.2.9 B-Lymphocyte Cloning and Antibody Secretion

• Activated B-lymphocytes clone and specialize into plasma cells for antibody production.

C3.2.10 Immunity and Memory Cells

• Immunity results from the retention of memory B-cells that facilitate rapid response upon reinfection.

C3.2.11 and C3.2.12 HIV Transmission and Infection

• HIV transmitted through body fluids; it attacks helper T-cells, compromising immune response, leading to AIDS.

C3.2.13 Antibiotics and Bacterial Infections

• Antibiotics selectively target bacterial processes without harming eukaryotic cells.

• Antibiotics do not affect viral infections.

C3.2.14 Antibiotic Resistance Evolution

• Increasing resistance among bacteria poses a major health threat, notably with MRSA and MDR-TB.

C3.2.15 Zoonotic Diseases

• Zoonoses transfer from animals to humans; examples include tuberculosis and rabies.

C3.2.16 Vaccines and Immunization

• Vaccines activate immune responses and can contain live, killed, or subunit pathogens.

C3.2.17 Herd Immunity

• Achieved when a significant part of the population is immune, reducing disease spread.

C3.2.18 Data Evaluation Related to COVID-19

• WHO statistics on COVID-19 reveal infection rates and the impact of vaccination efforts on public health.

C3.2 Defence Against Disease

How Body Systems Recognize Pathogens and Fight Infections

The immune system is a complex network that requires the ability to effectively differentiate between self and foreign molecules to protect the body against harmful agents.

• Antigens: These are specific molecules that trigger an immune response when detected by the immune system. They are typically present on the surface of pathogens or abnormal cells, such as tumor antigens on cancer cells resulting from genetic mutations.

• Recognition of Pathogens: The immune system identifies pathogens primarily through the diverse array of antigens they express. Each pathogen type has a unique set of antigens that facilitate their recognition.

Factors Influencing Disease Incidence in Populations

The Spanish flu pandemic (1918-1920) is a significant historical example of infectious disease impact, affecting approximately 1/5 of the world population and resulting in an estimated 20-50 million deaths globally.

• The second wave of the pandemic was notably deadlier, disproportionately affecting healthy young adults, including soldiers, a phenomenon that emphasizes the unpredictable nature of epidemic diseases.

Key Topics

C3.2.1 Pathogens as Causes of Infectious Diseases

• Diseases can be defined as illnesses exhibiting specific symptoms, and historically, the causes were not always understood.

• They can be categorized into three primary types of causes: genetic, environmental, and pathogens.

• Pathogens, defined as disease-causing organisms, have the capability to multiply within the host body, leading to illness.

• Types of pathogens include bacteria (e.g., tuberculosis), fungi, protists (e.g., malaria), and viruses (e.g., influenza).

C3.2.2 Primary Defence: Skin and Mucous Membranes

• The skin serves as the first line of defense against pathogens.

• Its outermost layer is composed of dead cells rich in keratin, which provides structural protection.

• Sebum, secreted by sebaceous glands, lowers the skin's pH, creating an inhospitable environment for many pathogens.

• Mucous membranes line various cavities and trap pathogens with mucus while also containing lysozyme, an antibacterial enzyme that aids in pathogen destruction.

C3.2.3 Blood Clotting Mechanism

• Blood clotting is a critical response following skin injuries that prevents blood loss and infection.

• The mechanism involves a cascade of reactions that lead to the formation of fibrin, a protein that forms a mesh to seal the wound effectively.

C3.2.4 Innate vs Adaptive Immune System

• The innate immune system provides immediate, non-specific defense against pathogens and remains constant throughout an individual's life.

• In contrast, the adaptive immune system develops specific responses tailored to individual pathogens and possesses immunological memory, enabling faster responses upon subsequent exposures.

C3.2.5 Infection Control by Phagocytes

• Phagocytes are a type of white blood cell that plays a vital role in the immune response by engulfing and digesting pathogens through a process known as endocytosis.

C3.2.6 Lymphocytes and Antibody Production

• Lymphocytes constitute about 25% of white blood cells and are primarily located within the lymphatic system.

• They are responsible for producing antibodies, which are proteins designed to latch onto specific pathogens and neutralize them.

C3.2.7 Antigens as Recognition Molecules

• Antigens, present on the surface of pathogens, stimulate the immune response.

• They interact with antibodies, which are produced by the immune system, binding irreversibly to these antigens and activating various immune pathways.

C3.2.8 Activation of B-Lymphocytes by Helper T-Lymphocytes

• Helper T-cells play a crucial role in activating B-lymphocytes. This interaction is pivotal for initiating the antibody production process that targets specific pathogens.

C3.2.9 B-Lymphocyte Cloning and Antibody Secretion

• Once activated, B-lymphocytes undergo clonal expansion, generating multiple identical cells that specialize into plasma cells, the primary producers of antibodies.

C3.2.10 Immunity and Memory Cells

• Immunity involves the retention of memory B-cells, which enable the immune system to mount a rapid and effective response upon re-exposure to previously encountered pathogens.

C3.2.11 and C3.2.12 HIV Transmission and Infection

• HIV (Human Immunodeficiency Virus) is transmitted through various body fluids, including blood, semen, and breast milk. It primarily attacks helper T-cells, leading to a compromised immune response and ultimately resulting in AIDS (Acquired Immunodeficiency Syndrome).

C3.2.13 Antibiotics and Bacterial Infections

• Antibiotics are therapeutic agents specifically designed to target bacterial infection processes without adversely affecting eukaryotic cells.

• It is crucial to note that antibiotics are ineffective against viral infections.

C3.2.14 Antibiotic Resistance Evolution

• The rise of antibiotic-resistant bacteria presents a significant public health challenge, with notable examples such as MRSA (Methicillin-resistant Staphylococcus aureus) and MDR-TB (Multidrug-resistant Tuberculosis), which complicate treatment options and increase health risks.

C3.2.15 Zoonotic Diseases

• Zoonoses are diseases that can be transmitted from animals to humans, highlighting the intersection of animal health and human health. Notable examples include tuberculosis and rabies.

C3.2.16 Vaccines and Immunization

• Vaccines are biological preparations that activate the immune system by introducing antigens derived from live, killed, or subunit pathogens. They are crucial for disease prevention and control in populations.

C3.2.17 Herd Immunity

• Herd immunity is achieved when a significant proportion of the population becomes immune to a particular disease, thereby reducing the overall spread and protecting those who are not immune.

C3.2.18 Data Evaluation Related to COVID-19

• According to WHO statistics on COVID-19, the pandemic has significantly affected global health, emphasizing infection rates and the critical importance of vaccination efforts in mitigating public health impacts.