IB BIO - 6.3 Defense against infectious disease

• 6.3.1 

  • Define pathogen.

• 6.3.1 

  • Define pathogen.

A pathogen is a biological agent, such as a virus, bacterium, fungus, protozoan, or parasite, that causes disease in its host organism. Pathogens have the ability to invade tissues, replicate within host cells, and disrupt normal physiological functions, leading to the development of infectious diseases. Pathogens can be transmitted from one host to another through various routes, including direct contact, airborne droplets, contaminated food or water, and vectors such as mosquitoes or ticks.

• 6.3.2 

  • Explain why antibiotics are effective against bacteria but not against viruses.

• Antibiotics are medications that specifically target and kill bacteria or inhibit their growth. They are effective against bacteria because they exploit unique features of bacterial cells that are not present in human cells. Here's why antibiotics are effective against bacteria but not against viruses:

  • 1. Structural Differences: Bacteria are prokaryotic cells, meaning they lack a nucleus and other membrane-bound organelles found in eukaryotic cells like human cells. Antibiotics typically target structures or processes that are unique to bacterial cells, such as their cell walls or protein synthesis machinery. For example, antibiotics like penicillin interfere with the synthesis of bacterial cell walls, causing them to weaken and rupture. Human cells lack cell walls, so antibiotics targeting this structure do not harm human cells.

  • 2. Viral Replication: Viruses are not considered living organisms; they are composed of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. They lack cellular structures and metabolic machinery. Viruses replicate by hijacking the machinery of host cells, using them to produce new virus particles. Since viruses rely on host cells for replication, they do not have their own cellular structures or metabolic processes that can be targeted by antibiotics.

  • 3. Antibiotics Mechanism of Action: Antibiotics work by interfering with specific bacterial processes or structures, such as cell wall synthesis, protein synthesis, DNA replication, or metabolic pathways. These targets are absent in viruses because viruses do not carry out these processes independently. Therefore, antibiotics have no effect on viruses.

  • 4. Selective Toxicity: Antibiotics are designed to have selective toxicity, meaning they target bacterial cells while sparing human cells. This selectivity is based on differences in cellular structures or biochemical pathways between bacteria and humans. Since viruses do not have their own cellular machinery, antibiotics cannot selectively target them without harming host cells.

• In summary, antibiotics are effective against bacteria because they exploit unique features of bacterial cells that are not present in human cells. However, antibiotics are not effective against viruses because viruses rely on host cells for replication and do not have their own cellular structures or metabolic processes that can be targeted by antibiotics.

• 6.3.3 

  • Outline the role of skin and mucous membranes in defence against pathogens.

• The skin and mucous membranes serve as physical and chemical barriers that play a crucial role in the body's defense against pathogens. Here's an outline of their roles in defense:

  • 1. Physical Barrier:

    •    - Skin: The skin is the body's largest organ and acts as a physical barrier that prevents pathogens from entering the body. The outermost layer of the skin, called the epidermis, consists of closely packed cells and a tough, waterproof protein called keratin. This barrier prevents pathogens from penetrating into deeper tissues.

    •    - Mucous Membranes: Mucous membranes line the respiratory, digestive, urinary, and reproductive tracts and provide a physical barrier against pathogens. These membranes are moist and covered with a layer of mucus, which traps pathogens and prevents them from entering the body's internal environment.

  • 2. Chemical Barrier:

    •    - Skin: The skin produces antimicrobial substances, such as sweat and sebum (oil), which contain antimicrobial peptides and fatty acids that inhibit the growth of bacteria and fungi on the skin's surface.

    •    - Mucous Membranes: Mucous membranes secrete various antimicrobial substances, including enzymes, antibodies, and antimicrobial peptides, into the mucus layer. These substances help to neutralize or kill pathogens that come into contact with the mucous membranes.

  • 3. Normal Flora:

    •    - The skin and mucous membranes are inhabited by beneficial microorganisms known as normal flora or microbiota. These microorganisms compete with potential pathogens for nutrients and space, preventing the colonization and overgrowth of harmful bacteria, fungi, and viruses.

    •    - Normal flora also produce antimicrobial substances that help to inhibit the growth of pathogens and maintain a balanced microbial community on the skin and mucous membranes.

  • 4. Immune Response:

    •    - If pathogens breach the physical and chemical barriers of the skin and mucous membranes, the body's immune system is activated to mount an immune response. Immune cells, such as macrophages, neutrophils, and dendritic cells, patrol the skin and mucous membranes, detecting and engulfing pathogens.

    •    - In addition, specialized immune cells, such as mast cells and lymphocytes, are present in the underlying connective tissue of mucous membranes and respond to the presence of pathogens by releasing inflammatory mediators and antibodies to neutralize the invaders.

• In summary, the skin and mucous membranes serve as the body's first line of defense against pathogens by providing physical and chemical barriers, hosting beneficial microorganisms, and initiating immune responses when necessary. Their combined efforts help to prevent the entry and colonization of pathogens, thereby protecting the body from infection and disease.

• 6.3.4 

  • Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.

• Phagocytic leukocytes, such as neutrophils and macrophages, play a crucial role in the body's immune response by ingesting and destroying pathogens. Here's an outline of how phagocytic leukocytes ingest pathogens in the blood and body tissues:

  • 1. Chemotaxis:

    •    - When tissues are injured or infected, immune cells release signaling molecules called chemokines and cytokines. These molecules attract phagocytic leukocytes to the site of infection through a process called chemotaxis.

    •    - Phagocytes detect the concentration gradient of these signaling molecules and migrate towards the source of infection.

  • 2. Recognition and Adherence:

    •    - Upon reaching the site of infection, phagocytes recognize and adhere to pathogens through specific interactions between cell surface receptors on the phagocytes and molecular patterns present on the surface of the pathogens. This process is known as recognition and adherence.

    •    - Pathogen-associated molecular patterns (PAMPs), such as bacterial cell wall components like lipopolysaccharides (LPS) or fungal cell wall components like beta-glucans, are recognized by pattern recognition receptors (PRRs) on the surface of phagocytes.

  • 3. Engulfment:

    •    - Once adhered to the pathogen, the phagocyte extends pseudopodia (cytoplasmic projections) around the pathogen, enclosing it within a phagocytic vesicle called a phagosome.

    •    - The process of engulfing the pathogen into the phagocyte is called phagocytosis.

  • 4. Formation of Phagosome:

    •    - Inside the phagocyte, the phagosome fuses with lysosomes, which are membrane-bound organelles containing digestive enzymes.

    •    - This fusion forms a phagolysosome, which is a vesicle containing both the ingested pathogen and lysosomal enzymes.

  • 5. Degradation and Destruction:

    •    - Within the phagolysosome, the lysosomal enzymes degrade the pathogen, breaking down its components into smaller molecules.

    •    - The acidic pH inside the phagolysosome helps to activate the lysosomal enzymes and facilitate their enzymatic activity.

    •    - Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are also generated within the phagolysosome, contributing to the destruction of the pathogen.

  • 6. Exocytosis of Residual Material:

    •    - After digestion is complete, the residual material, including degraded pathogen components and waste products, is expelled from the phagocyte through exocytosis.

    •    - The phagocyte can then continue its surveillance for other pathogens or undergo apoptosis if necessary.

• In summary, phagocytic leukocytes ingest pathogens in the blood and body tissues through a series of steps, including chemotaxis, recognition and adherence, engulfment, formation of phagosome, degradation and destruction of the pathogen, and exocytosis of residual material. This process helps to eliminate pathogens and contribute to the body's immune defense against infections.

• 6.3.5 

  • Distinguish between antigens and antibodies.

• Antigens and antibodies are both key components of the immune system, but they have distinct roles and functions:

  • 1. Antigens:

    •    - Antigens are molecules that can stimulate the immune system to produce an immune response.

    •    - They are typically proteins, polysaccharides, lipids, or nucleic acids found on the surface of pathogens, such as bacteria, viruses, fungi, and parasites, as well as on foreign substances, such as toxins, allergens, and transplanted tissues.

    •    - Antigens are recognized by the immune system as non-self, triggering the activation of immune cells and the production of antibodies to neutralize or eliminate the antigen.

  • 2. Antibodies:

    •    - Antibodies, also known as immunoglobulins, are specialized proteins produced by B lymphocytes (B cells) in response to the presence of antigens.

    •    - Antibodies are Y-shaped molecules composed of four polypeptide chains—two heavy chains and two light chains—linked together by disulfide bonds.

    •    - Each antibody molecule has a specific binding site, called the antigen-binding site or paratope, which recognizes and binds to a specific antigen with high specificity.

    •    - Antibodies play several important roles in the immune response, including:

      •      - Neutralizing pathogens by binding to their surface antigens and preventing them from infecting host cells.

      •      - Facilitating the clearance of pathogens by promoting their phagocytosis by phagocytic cells, such as macrophages and neutrophils.

      •      - Activating the complement system, a group of proteins that enhance the immune response by promoting inflammation, opsonization (coating of pathogens for phagocytosis), and cell lysis.

      •      - Marking pathogens for destruction by other immune cells, such as natural killer (NK) cells, through a process called antibody-dependent cell-mediated cytotoxicity (ADCC).

• In summary, antigens are molecules that stimulate the immune system to produce an immune response, while antibodies are specialized proteins produced by the immune system in response to antigens. Antibodies bind to specific antigens with high specificity and play essential roles in neutralizing pathogens, promoting their clearance, and enhancing the immune response against infections.

• 6.3.6 

  • Explain antibody production.

• Antibody production, also known as humoral immune response, is a complex process orchestrated by the immune system in response to the presence of antigens. Here's an overview of how antibody production occurs:

  • 1. Antigen Recognition:

    •    - The process begins when antigens from pathogens, such as bacteria, viruses, or other foreign substances, are encountered by immune cells called antigen-presenting cells (APCs), such as dendritic cells, macrophages, or B cells.

    •    - APCs engulf and process the antigens, breaking them down into smaller fragments, which are then displayed on their cell surface bound to major histocompatibility complex (MHC) molecules.

  • 2. Activation of B Cells:

    •    - Antigen-presenting cells, particularly dendritic cells, present the processed antigens to naïve B cells in secondary lymphoid organs, such as lymph nodes or the spleen.

    •    - The interaction between the antigen and the B cell receptor (BCR) on the surface of the B cell, along with additional signals from helper T cells, stimulates the activation of the B cell.

  • 3. Clonal Expansion and Differentiation:

    •    - Upon activation, the B cell undergoes clonal expansion, rapidly proliferating to form a large population of identical B cells, known as a clone.

    •    - Some of the activated B cells differentiate into plasma cells, which are specialized antibody-producing cells. Plasma cells are responsible for the bulk of antibody production during the primary immune response.

  • 4. Antibody Production:

    •    - Plasma cells synthesize and secrete large quantities of antibodies specific to the antigen encountered by the B cell.

    •    - Antibodies are Y-shaped proteins composed of four polypeptide chains—two heavy chains and two light chains—linked together by disulfide bonds. Each antibody molecule has two identical antigen-binding sites, located at the tips of the Y-shaped arms.

  • 5. Antibody Secretion and Circulation:

    •    - The antibodies produced by plasma cells are released into the bloodstream and circulate throughout the body, where they can bind to and neutralize antigens.

    •    - Antibodies may also exit the bloodstream and enter other bodily fluids, such as mucus, saliva, or breast milk, providing additional protection at mucosal surfaces or passing immunity to offspring through breastfeeding.

  • 6. Memory B Cells:

    •    - Some activated B cells differentiate into memory B cells, which are long-lived cells that persist in the body after the resolution of the primary immune response.

    •    - Memory B cells are capable of rapidly responding to subsequent encounters with the same antigen, mounting a faster and more robust secondary immune response.

• In summary, antibody production is a coordinated process involving the activation, proliferation, and differentiation of B cells, leading to the generation of plasma cells that produce and release antibodies specific to the encountered antigen. Antibodies play crucial roles in immune defense by neutralizing pathogens, promoting their clearance, and providing long-term immunity through memory B cells.

• 6.3.7 

  • Outline the effects of HIV on the immune system.

• HIV (Human Immunodeficiency Virus) is a virus that attacks the immune system, specifically targeting CD4-positive T lymphocytes (CD4 cells), which are crucial for coordinating the body's immune response against infections. Here's an outline of the effects of HIV on the immune system:

  • 1. CD4 Cell Depletion:

    •    - HIV primarily targets CD4 cells, which play a central role in coordinating the immune response by activating other immune cells and secreting cytokines.

    •    - HIV infects and replicates within CD4 cells, leading to their destruction and depletion over time.

    •    - As the number of CD4 cells declines, the body's ability to mount an effective immune response against pathogens diminishes, making the individual more susceptible to infections and opportunistic diseases.

  • 2. Impaired Immune Function:

    •    - The progressive depletion of CD4 cells weakens the immune system's ability to defend against infections and diseases.

    •    - Individuals with HIV may experience recurrent or persistent infections, such as bacterial pneumonia, fungal infections (e.g., candidiasis), tuberculosis, and viral infections (e.g., cytomegalovirus, herpes simplex virus).

    •    - Opportunistic infections, which are caused by organisms that typically do not cause disease in healthy individuals but can take advantage of a weakened immune system, become increasingly common as HIV progresses.

  • 3. Decline in Immunological Memory:

    •    - HIV also affects the function of memory T cells, which are responsible for maintaining immunological memory and providing long-term protection against previously encountered pathogens.

    •    - With the decline in CD4 cells and memory T cells, the body's ability to mount an effective immune response upon re-exposure to pathogens is compromised, leading to recurrent or chronic infections.

  • 4. Loss of Immune Regulation:

    •    - HIV infection disrupts the balance of the immune system, leading to dysregulation of immune responses and chronic inflammation.

    •    - Persistent immune activation and inflammation contribute to tissue damage and the development of non-infectious complications, such as cardiovascular disease, neurocognitive disorders (e.g., HIV-associated neurocognitive disorders), and metabolic abnormalities.

  • 5. Development of AIDS:

    •    - Without effective treatment, HIV infection progresses to AIDS (Acquired Immunodeficiency Syndrome), which is characterized by severe immune suppression and the onset of opportunistic infections or AIDS-defining illnesses.

    •    - AIDS-defining illnesses include severe fungal infections, disseminated tuberculosis, certain cancers (e.g., Kaposi's sarcoma, non-Hodgkin lymphoma), and opportunistic infections caused by pathogens like Pneumocystis jirovecii and Toxoplasma gondii.

• In summary, HIV infection leads to the progressive depletion of CD4 cells, impaired immune function, increased susceptibility to infections and opportunistic diseases, loss of immunological memory, dysregulation of immune responses, chronic inflammation, and the development of AIDS if left untreated. Effective antiretroviral therapy (ART) can suppress viral replication, restore immune function, and prevent the progression to AIDS, improving the prognosis and quality of life for individuals living with HIV.

• 6.3.8 

  • Discuss the cause, transmission and social implications of AIDS.

• AIDS (Acquired Immunodeficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus), which is transmitted through specific bodily fluids and can lead to severe immune suppression and a range of opportunistic infections and cancers. Here's a discussion of the cause, transmission, and social implications of AIDS:

  • 1. Cause:

    •    - AIDS is caused by infection with HIV, a retrovirus that targets and destroys CD4-positive T lymphocytes (CD4 cells), which are critical for coordinating the body's immune response.

    •    - HIV primarily infects CD4 cells by binding to the CD4 receptor and a coreceptor, typically CCR5 or CXCR4, on the surface of these cells.

    •    - Once inside CD4 cells, HIV replicates and produces new virus particles, leading to the destruction of CD4 cells and progressive immune suppression.

  • 2. Transmission:

    •    - HIV is transmitted through the exchange of specific bodily fluids containing the virus, including blood, semen, vaginal fluids, rectal fluids, and breast milk.

    •    - The main routes of HIV transmission include:

    •      - Unprotected sexual intercourse (vaginal, anal, or oral) with an HIV-infected partner.

    •      - Sharing contaminated needles or syringes for injecting drugs.

    •      - Vertical transmission from an HIV-positive mother to her child during pregnancy, childbirth, or breastfeeding.

    •      - Occupational exposure to HIV-infected blood or body fluids among healthcare workers.

    •    - HIV transmission does not occur through casual contact, such as hugging, kissing, sharing food or utensils, or through insect bites.

  • 3. Social Implications:

    •    - AIDS has profound social implications, affecting individuals, families, communities, and societies at large:

    •      - Stigma and Discrimination: People living with HIV/AIDS often face stigma, discrimination, and social rejection due to misconceptions, fear, and prejudice surrounding the disease. This stigma can lead to social isolation, loss of employment, and barriers to accessing healthcare and support services.

    •      - Impact on Relationships and Families: HIV/AIDS can strain relationships and families, leading to issues related to disclosure, trust, caregiving responsibilities, and financial burdens. Children orphaned by AIDS may face challenges in accessing education, healthcare, and social support.

    •      - Healthcare Burden: The HIV/AIDS epidemic places a significant burden on healthcare systems, particularly in resource-limited settings, where access to prevention, testing, treatment, and care services may be limited. Addressing the healthcare needs of people living with HIV/AIDS requires comprehensive and integrated approaches, including prevention, testing, treatment, and support services.

    •      - Economic Consequences: HIV/AIDS can have far-reaching economic consequences, affecting productivity, workforce participation, and household incomes. The loss of breadwinners due to AIDS-related illnesses can exacerbate poverty and socioeconomic disparities, particularly in communities with limited resources and social support networks.

    •      - Global Health Challenges: HIV/AIDS remains a global health challenge, disproportionately affecting vulnerable populations, such as women, adolescents, men who have sex with men, people who inject drugs, sex workers, prisoners, and migrants. Addressing the social determinants of health, promoting human rights, and fostering community engagement are essential for an effective response to the HIV/AIDS epidemic.

• In summary, AIDS is caused by HIV infection and is transmitted through specific bodily fluids. The social implications of AIDS include stigma and discrimination, impact on relationships and families, healthcare burden, economic consequences, and global health challenges. Addressing these social issues requires comprehensive strategies that address the structural, economic, and cultural factors contributing to HIV/AIDS stigma and discrimination, promote access to prevention and care services, and support the rights and dignity of people living with HIV/AIDS.