Immunity and Infectious Diseases Overview

Immunity to Infections

• Immunity to infections involves innate and adaptive immunity, along with immune memory
• Innate immunity includes physical and chemical barriers, interferon (IFN) system, complement system, phagocytosis/opsonization, and natural killer (NK) cells
• Adaptive immunity includes neutralizing antibody, antibody-mediated phagocytosis, antibody-dependent cell-mediated cytotoxicity (ADCC), and cytotoxic T lymphocytes (CTL)
• Immune memory involves resistance for long periods after infection, along with memory cells

Infectious Diseases

• Infectious diseases cause a substantial number of deaths annually, with cardiovascular conditions causing more deaths than infectious diseases
• Infectious diseases cause an estimated 14.9 million deaths annually, while cardiovascular conditions account for 16.7 million deaths
• Respiratory infections account for 3.96 million deaths annually
• HIV/AIDS accounted for 2.77 million deaths in 2023
• Diarrheal diseases cause 1.8 million deaths annually
• Tuberculosis causes 1.56 million deaths annually
• Vaccine-preventable childhood diseases contribute to 1.12 million deaths annually

Immunity to Viruses

• The innate response to viral infections includes antiviral effects caused by interferon (IFN), lysis of infected cells by NK cells and inflammation
• The antibody-mediated response involves virus neutralization and antibody-dependent cell-mediated cytotoxicity (ADCC) by NK cells, macrophages and neutrophils
• The cell-mediated response involves cytotoxic T lymphocytes (CTLs, CD8 T cells)
• Viruses can evade the immune system by infecting immune cells

Immunity to Bacteria

• Innate responses include antimicrobial peptides, complement-mediated lysis, phagocytosis, involving pattern recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs), and opsonization
• Adaptive responses involve antibody-mediated responses like agglutination, blocking of toxins, and neutralizing toxins, phagocytosis, and cytotoxic T lymphocytes (CTLs) for intracellular infections.
• Gram-positive bacteria are resistant to lysis by the complement system, and Gram-negative bacteria are susceptible to neutralizing antibodies, phagocytosis and the complement system

Immunity to Parasites

• Innate responses to parasitic infections include complement-mediated lysis, phagocytosis (involving pattern recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs)), and opsonization.
• Adaptive responses to parasitic infections include antibody-mediated responses like agglutination and blocking, phagocytosis by antibodies (Ab-mediated), and antibody-dependent cell-mediated cytotoxicity (ADCC)
• IgE/mast cell/eosinophil responses are also involved in adaptive immunity to parasites

Immunity to Fungi

• Innate responses to fungal infections include the body's barriers like skin and normal bacterial flora, phagocytosis by neutrophils, and complement-mediated lysis and opsonization
• Adaptive responses are not well characterized but may involve antibodies and likely T cell response

Viral Immune Evasion

• Viruses can infect immune cells, such as B cells, T cells, and macrophages, to escape the immune system.
• Some viruses encode homologs of the body's interferon receptors, such as vaccinia virus.
• Adenovirus E3 protein blocks the transport of MHC to the cell surface, preventing the presentation of viral antigens to the immune system.