Pathophysiology: Immunity, Infection, and Neurological Disorders

Viral vs. Bacterial Infection: Mechanisms and Body Response

Viruses and bacteria differ in their structure and mechanisms of infection (Rogers, 2020). Viruses are smaller and simpler than bacteria, and consist of genetic material (either DNA or RNA) encased within a protein coat called a capsid. Some viruses also have an outer lipid envelope. Viruses cannot reproduce independently; they require a host cell to replicate. The virus injects its genetic material into the host cell, hijacking the host's machinery to produce viral proteins and replicate viral particles. A fully assembled viral particle is called a virion (Rogers, 2020).

Viruses can be classified based on their genetic material into DNA viruses and RNA viruses. DNA viruses integrate their DNA into the host cell's genome. RNA viruses use the host's ribosomes to translate viral RNA into proteins. Some RNA viruses, such as retroviruses, reverse-transcribe their RNA into DNA, which then integrates into the host genome (Rogers, 2020).

Bacteria are single-celled prokaryotes that can live and reproduce independently. They have a more complex structure with a rigid cell wall, a plasma membrane, and cytoplasm containing DNA, ribosomes, and various enzymes necessary for metabolic processes. Bacteria can infect the host by releasing toxins or invading tissues. Gram-positive and Gram-negative bacteria have different cell wall structures that influence how they interact with the host and how susceptible they are to antibiotics.

Viruses cause damage by directly invading and destroying host cells. The viral life cycle involves the virus entering the host cell, taking over its machinery, and replicating within the cell. This often leads to the lysis (bursting) of the cell as new virions are released to infect additional cells. The damage caused by viruses can lead to inflammation, cell death, and the disruption of normal cellular functions. For example, influenza viruses target respiratory cells, while HIV attacks immune cells, leading to immunodeficiency.

Bacteria damage the body through several methods, including the release of exotoxins and endotoxins. Exotoxins are potent, secreted toxins that disrupt cellular functions, while endotoxins are components of the bacterial cell wall that trigger strong inflammatory responses when bacteria are lysed. Bacteria can also invade and colonize tissues, creating localized infections such as abscesses, or spreading systemically through the bloodstream, causing conditions like sepsis. For instance, Staphylococcus aureus can produce toxins that cause toxic shock syndrome, while Escherichia coli can release endotoxins leading to severe diarrhea and kidney damage.

For viruses, the body primarily relies on cytotoxic T cells and natural killer (NK) cells. Once a virus infects a host cell, viral proteins are displayed on the cell surface through major histocompatibility complex (MHC) class I molecules. Cytotoxic T cells recognize these viral peptides and destroy the infected cell to prevent viral replication. Interferons—signaling proteins produced by infected cells—play a critical role in alerting neighboring cells to the presence of a virus and enhancing their antiviral defenses. NK cells can also target and destroy infected cells by recognizing abnormal surface markers (Rogers, 2020).

For bacteria, the immune system uses several mechanisms. Phagocytosis, carried out by macrophages and neutrophils, is a key process: these immune cells engulf and digest bacteria using enzymes contained in their lysosomes. The immune system also produces antibodies that specifically bind to bacterial antigens, marking them for destruction through opsonization. The complement system can lyse bacterial cells by creating pores in their membranes, particularly targeting bacteria that are not efficiently phagocytosed. Helper T cells aid the immune response by activating macrophages and promoting antibody production by B cells (Rogers, 2020).

Innate and Adaptive Immunity

The innate immune system is the body's first responder against foreign invaders. Inflammation is a component of innate immunity and is brought about by the presence of pathogens or tissue damage. When a pathogen breaches the skin or mucous barriers, pattern recognition receptors on immune cells—macrophages, neutrophils, and dendritic cells—recognize pathogen-associated molecular patterns, which prompts the inflammatory response (Rogers, 2020).

Neutrophils are the first responders to infection; they migrate to the site of infection and destroy pathogens. Macrophages are phagocytic cells that engulf and digest pathogens; they also release cytokines to amplify inflammation and recruit other immune cells. Dendritic cells bridge the innate and adaptive systems by capturing antigens and presenting them to T cells, thereby activating the adaptive response (Rogers, 2020).

Cytokines—for example, interleukins and tumor necrosis factor-alpha (TNF-α)—are proteins that mediate the inflammatory response by promoting vasodilation and increasing vascular permeability, which allows immune cells to access infected tissue. Histamine is released by mast cells to increase blood flow and capillary permeability, supporting the migration of immune cells to the infection site. The innate immune response is rapid, occurring within minutes to hours of infection. It does not require prior exposure to a pathogen and is not pathogen-specific. The primary goal is to contain and eliminate pathogens before they spread, initiate repair of damaged tissues, and set the stage for the adaptive immune response if needed.

The adaptive immune system is slower to respond but provides specific and long-lasting protection against pathogens. It is activated when innate immunity is insufficient to eliminate the threat. Adaptive immunity relies on lymphocytes—B cells and T cells—and functions through antigen recognition, antigen presentation, and the production of antibodies.

When activated by an antigen, B cells differentiate into plasma cells that produce antibodies. These antibodies neutralize pathogens or mark them for destruction. There are two main types of T cells: helper T cells (CD4+) and cytotoxic T cells (CD8+). Helper T cells stimulate both B cells and cytotoxic T cells; cytotoxic T cells directly kill infected cells. Antibodies bind specifically to antigens on pathogens, neutralizing them or marking them for destruction. Adaptive immunity also uses cytokines such as interferons and interleukins to regulate immune cell function.

The adaptive immune response is slower, taking days to weeks to fully activate after initial exposure. However, it is specific to the invading pathogen and forms memory cells, allowing a faster and more robust response upon subsequent encounters. The goal of adaptive immunity is to eradicate specific pathogens and develop immunological memory, ensuring future infections by the same pathogen are dealt with more efficiently.

The innate and adaptive immune systems are distinct but highly interconnected. Dendritic cells, for instance, are part of the innate system but play a pivotal role in activating the adaptive system by presenting antigens to T cells. Cytokines produced during innate immunity can also influence the activation and differentiation of B and T cells in the adaptive response (Rogers, 2020). The innate immune system responds quickly and non-specifically to contain the infection, while the adaptive immune system provides a slower, highly specific response with the added benefit of long-term immunity through memory cells.

Antibody-Mediated Immune Response

The first step in the antibody-mediated immune response is antigen recognition. Antigens are molecules—usually proteins or polysaccharides—found on the surface of pathogens such as bacteria, viruses, and toxins. When a pathogen enters the body, B cells with membrane-bound antibodies (B cell receptors, or BCRs) circulate in the bloodstream and lymphatic system, searching for their specific antigen. Each BCR is highly specific to a particular antigen due to the unique structure of its variable region, which contains hypervariable loops that form the antigen-binding site. This allows the antibody to fit precisely with its corresponding antigen, similar to a lock and key. Once the BCR binds to an antigen, the B cell is activated. Activated B cells then proliferate and differentiate into plasma cells that secrete antibodies, which are released into the bloodstream and lymphatic system to seek out and neutralize pathogens (Rogers, 2020).

Antibodies can block the activity of pathogens by binding to the surface of viruses or bacterial toxins. For example, when antibodies coat the surface of a virus, they prevent it from binding to host cell receptors, neutralizing its ability to infect cells. Antibodies can also mark pathogens for destruction through opsonization, effectively "tagging" them for recognition by phagocytic cells such as macrophages and neutrophils. Antibodies can additionally trigger the complement system—proteins in the blood that assist in destroying pathogens (Rogers, 2020).

Humoral immunity, mediated by antibodies produced by B cells, is the main defense against extracellular pathogens such as bacteria and free-floating viruses. Circulating antibodies bind to antigens, neutralize them, and facilitate their removal through phagocytosis or complement-mediated destruction. Cell-mediated immunity uses T cells rather than antibodies; however, helper T cells help activate B cells to produce antibodies in the first place. Cytotoxic T cells target and destroy infected cells by recognizing antigens, which is essential against intracellular pathogens because antibodies alone cannot reach a pathogen once it has infected a cell (Rogers, 2020).

Cell-mediated immunity does not involve antibodies directly, but it works in concert with humoral immunity to clear infections. For example, antibodies can mark infected cells for destruction by cytotoxic T cells or natural killer cells through antibody-dependent cellular cytotoxicity. The ultimate goal of the antibody response is the complete elimination of the pathogen. Once antibodies have neutralized pathogens, marked them for destruction through opsonization, or activated the complement system, immune cells ingest and destroy the pathogens. Memory B cells formed during the immune response ensure that future encounters with the same pathogen are met with a more rapid and effective response.