Body Defense Mechanisms Lecture 2 PDF

**Body Defense Mechanisms:** **1. Introduction to Body Defense Mechanisms** - **Immune System Overview**: The immune system is the body's defense network, consisting of cells, tissues, and organs working together to protect against infection and disease. It operates on multiple level...

**Body Defense Mechanisms:** **1. Introduction to Body Defense Mechanisms** - **Immune System Overview**: The immune system is the body's defense network, consisting of cells, tissues, and organs working together to protect against infection and disease. It operates on multiple levels to identify and neutralize harmful pathogens like bacteria, viruses, fungi, and parasites. - **Immune System Components**: - **Innate Immunity**: Rapid, non-specific defense that acts as the first responder. - **Adaptive Immunity**: A slower but more specific and powerful response, which has memory, allowing for faster reactions to pathogens the body has encountered before. **2. First Line of Defense: Physical and Chemical Barriers** - **Skin**: - The largest organ of the body, composed of tightly packed epithelial cells. - Sebum and sweat contain antimicrobial substances (e.g., fatty acids, lactic acid). - Desquamation (shedding of dead skin cells) removes surface microbes. - **Mucous Membranes**: - Line the respiratory, gastrointestinal, and urogenital tracts. - Mucus traps pathogens, preventing them from entering cells. - **Cilia** in the respiratory tract move mucus, loaded with trapped particles, out of the body. - **Chemical Defenses**: - **Stomach Acid (HCl)**: Kills most pathogens ingested with food. - **Lysozyme**: An enzyme found in tears, saliva, and sweat that breaks down bacterial cell walls. - **Antimicrobial Peptides**: Short proteins (like defensins) that disrupt microbial membranes. - **Flushing Mechanisms**: - Tears, urine, and mucus constantly wash away microbes. - **Example**: Blinking spreads tears across the eyes to keep the corneal surface free of pathogens. **3. Second Line of Defense: Innate Immunity** - **Phagocytes**: - **Macrophages**: Reside in tissues and engulf pathogens by phagocytosis. They also release cytokines to recruit more immune cells. - **Neutrophils**: The most abundant white blood cells, rapidly recruited to infection sites. They engulf pathogens and release enzymes and toxic substances. - **Mechanism of Phagocytosis**: Pathogens are engulfed into a phagosome, which fuses with a lysosome, where enzymes degrade the pathogen. - **Natural Killer (NK) Cells**: - Recognize cells that lack "self" markers (MHC class I molecules), typically virus-infected or cancerous cells. - Release perforins and granzymes that induce apoptosis (programmed cell death). - **Inflammatory Response**: - When tissues are injured, histamine is released from mast cells, causing blood vessels to dilate (leading to redness and heat) and become more permeable (causing swelling). - **Purpose**: Increases blood flow and recruits immune cells to the area to fight infection and begin healing. - **Clinical Example**: Swelling and redness around a cut are signs of inflammation. - **Fever**: - Caused by pyrogens, substances that reset the body's thermostat in the hypothalamus. - **Function**: Inhibits microbial growth and enhances the immune response (e.g., speeds up enzyme activity). - **Complement System**: - Comprises about 30 proteins that work together to destroy microbes. They form a **Membrane Attack Complex (MAC)**, punching holes in microbial membranes. - Complement also enhances phagocytosis through **opsonization**, making pathogens easier to engulf. - **Interferons**: - Released by virus-infected cells to warn neighboring cells, stimulating them to produce antiviral proteins. **4. Third Line of Defense: Adaptive Immunity** - **Antigen Recognition**: - **Antigens** are foreign molecules (proteins, polysaccharides) that are recognized by the immune system. - The adaptive immune system uses **antigen-presenting cells** (like dendritic cells) to display pathogen fragments to lymphocytes. - **B Cells and Antibodies**: - **B cells** mature in the bone marrow and, upon activation by an antigen, differentiate into **plasma cells** that secrete antibodies. - **Antibodies (Immunoglobulins)**: - **IgG**: Most common in blood, provides long-term immunity. - **IgA**: Found in mucous secretions (tears, saliva) to protect mucosal surfaces. - **IgM**: First antibody produced during an infection. - Functions include neutralization (blocking pathogen entry), agglutination (clumping pathogens), and opsonization (enhancing phagocytosis). - **T Cells**: - Mature in the thymus and are crucial for cell-mediated immunity. - **Helper T Cells (CD4+)**: Activate B cells and other immune cells by releasing cytokines. - **Cytotoxic T Cells (CD8+)**: Kill infected or abnormal cells by releasing perforin and granzymes. - **Memory Cells**: - After an infection, both memory B and T cells persist in the body. Upon re-exposure to the same pathogen, these cells respond more rapidly and effectively. **5. Role of the Lymphatic System in Defense** - **Lymph Nodes**: Filter lymph fluid and trap pathogens. Lymphocytes are activated here. - **Spleen**: Filters the blood, removing old red blood cells and pathogens. - **Lymphatic Vessels**: Transport lymph fluid, which carries immune cells and waste products away from tissues. - **Clinical Example**: Swollen lymph nodes during an infection indicate active immune responses. **6. Disorders of the Immune System** - **Autoimmune Diseases**: The immune system mistakenly attacks the body's own cells. - **Examples**: Multiple sclerosis (attacks nerve cells), Type 1 diabetes (attacks insulin-producing cells in the pancreas). - **Allergies**: - Hypersensitivity to non-harmful antigens (e.g., pollen, dust) triggers an exaggerated immune response. - Involves **IgE antibodies** and mast cell activation, releasing histamine, causing symptoms like swelling and itching. - **Immunodeficiency**: - **Primary** (genetic) or **Secondary** (acquired) conditions where the immune system is weakened. - **HIV/AIDS**: Targets and depletes Helper T cells, crippling the immune response. **7. Vaccination and Immune Memory** - **How Vaccines Work**: - Vaccines introduce a harmless form of an antigen, allowing the body to produce memory cells without causing disease. - Types include **inactivated**, **live-attenuated**, and **mRNA** vaccines. - **Herd Immunity**: - When a significant portion of a population is immune to a disease, it prevents the spread of infection to unvaccinated individuals. - **Example**: Polio and measles eradication efforts depend on widespread vaccination. **8. Pathogen Evasion Mechanisms: How Microbes Outsmart the Immune System** While our immune system is highly effective, many pathogens have evolved sophisticated mechanisms to evade detection and destruction. Here's how some of them do it: **1. Antigenic Variation** - **Description**: Some pathogens, especially viruses and parasites, change their surface proteins (antigens) to escape recognition by immune cells. - **Examples**: - **Influenza Virus**: Undergoes frequent mutations in its surface proteins (hemagglutinin and neuraminidase), requiring new vaccines every year. - **Trypanosoma brucei** (causes African sleeping sickness): Periodically switches its surface glycoproteins to evade the host immune response, making it hard for antibodies to keep up. **2. Hiding in Host Cells** - **Description**: Intracellular pathogens hide within the host's own cells to avoid detection by immune surveillance. - **Examples**: - **Mycobacterium tuberculosis**: Resides within macrophages, a type of immune cell, preventing lysosome fusion with the phagosome (where the bacteria are engulfed). - **Herpes Simplex Virus (HSV)**: Establishes latent infections by hiding in nerve cells, where it remains dormant and undetectable by the immune system for long periods. **3. Inhibition of Phagocytosis** - **Description**: Some bacteria produce factors that prevent phagocytes (like macrophages and neutrophils) from engulfing or destroying them. - **Examples**: - **Streptococcus pneumoniae**: Has a thick polysaccharide capsule that prevents effective phagocytosis by hiding its surface antigens. - **Staphylococcus aureus**: Produces proteins that bind to the Fc region of antibodies, preventing opsonization and phagocytosis. **4. Inhibiting the Complement System** - **Description**: Pathogens may produce proteins that inhibit the complement system, a crucial part of the innate immune response. - **Examples**: - **Neisseria gonorrhoeae**: Expresses surface molecules that bind complement regulatory proteins, preventing the complement cascade from forming membrane attack complexes (MACs). - **HIV**: Utilizes host proteins that help it evade complement-mediated destruction. **5. Disrupting Cytokine Signaling** - **Description**: Some pathogens interfere with the host's communication system (cytokines) to evade immune detection or manipulate the immune response. - **Examples**: - **Epstein-Barr Virus (EBV)**: Produces a viral homolog of IL-10, an anti-inflammatory cytokine, which suppresses the host immune response, helping the virus persist. - **Hepatitis C Virus (HCV)**: Inhibits interferon signaling, reducing the production of antiviral proteins in infected cells. **6. Molecular Mimicry** - **Description**: Some pathogens produce molecules that resemble host proteins, allowing them to blend in with the body's own cells and avoid immune detection. - **Examples**: - **Treponema pallidum** (causes syphilis): Covers itself with host proteins, making it harder for the immune system to recognize it as foreign. - **Helicobacter pylori**: Produces proteins that mimic those found in the host's stomach lining, helping it evade immune attacks. **7. Biofilm Formation** - **Description**: Many bacteria form biofilms, communities of microorganisms that are surrounded by a protective extracellular matrix, making them more resistant to immune responses and antimicrobial treatments. - **Examples**: - **Pseudomonas aeruginosa**: Forms biofilms in the lungs of patients with cystic fibrosis, protecting the bacteria from both the immune system and antibiotics. - **Staphylococcus epidermidis**: Often forms biofilms on medical devices (like catheters), shielding the bacteria from immune cells. **8. Immunosuppression** - **Description**: Some pathogens actively suppress the host\'s immune system to establish chronic infections. - **Examples**: - **HIV**: Targets and destroys **Helper T cells (CD4+ cells)**, the key coordinators of the immune response, leading to immunodeficiency. - **Measles Virus**: Causes temporary immune suppression after infection, making individuals more susceptible to secondary infections.