Viruses and Viral Replication

Overview

• Basic animal physiology and homeostasis are maintained by organ systems, including the immune system.
• The immune system defends against pathogens and maintains overall health.

Viruses

• The study of viruses includes their discovery, detection methods, replication steps, and the use of vaccines.

Viral Replication

• Viruses utilize either DNA or RNA as their genetic code.
• Viral genomes are characteristically small, containing only key genes.
• The genetic structure can be single/double-stranded, linear/circular, or segmented.
• DNA viruses rely on the host's cellular mechanisms, responsible for diseases like chickenpox and herpes.
• RNA viruses utilize unique enzymes and are prone to rapid evolution, causing diseases such as hepatitis C and measles.
• Viral infection is host and cell-specific, relying on viral receptors to enter and replicate.

Steps of Viral Infection

• A virus must "take over" a cell to replicate effectively.
• Productive viral infections involve attachment, replication, assembly, and release.
• Attachment involves the virus binding to a host cell membrane's specific receptor.
• Replication depends on the viral genome’s mechanism.
• Release occurs when the host cell dies, or the virus exists from the cell by budding.

Viruses and Disease

• Viruses underlies numerous human ailments, ranging from mild to severe.

Vaccines for Prevention

• Vaccines are the primary method to combat viral diseases.
• Vaccines are developed with weakened live viruses, killed viruses, or molecular subunits.
• Attenuation is the process used to make live vaccines in the laboratory.
• The polio vaccine was a turning point in vaccine use.
• Use of live vaccines have associated risks like reversion to a disease-causing state, causing potential epidemics.

Innate Immunity

• The body's innate defenses include physical barriers, chemical barriers, the inflammatory response, and the complement system.

External and Chemical Barriers

• Physical barriers like skin, mucus, and hairs hinder pathogens.
• Chemical barriers, such as skin, mucus, and highly acidic stomach environments, defend against pathogens.
• Beneficial microorganisms compete with disease-causing organisms.
• Pathogens still breach through abrasions, punctures, or overwhelming mucus/cilia.

Internal Defenses

• Internal defenses include the inflammatory response, phagocytosis, natural killer cells, and the complement system.
• Cytokines activate immune responses and are released when a pathogen is recognized as foreign.
• Inflammation symptoms are localized redness, swelling, heat, and pain, signaling white blood cell attraction.
• Neutrophils constitute the majority of white blood cells.
• Macrophages follow neutrophils.

Adaptive Immunity

• Adaptive immunity is the body's specific response to pathogens, involving cell-mediated and humoral responses, as well as immune tolerance.

Humoral Immune Response

• B-cells create antibodies against antigens.
• Each B-cell produces a unique antigen receptor.
• Helper T cells stimulate B cell division into memory B cells and plasma cells.
• Memory B cells are inactive until future antigen encounters.
• Plasma cells produce and secrete antibodies.
• Active immunity results from the host's response to a pathogen.
• Passive immunity comes from an external source.

Immunological Memory

• Primary immune response is the body's initial response to a previously unencountered pathogen.
• Memory cells persist after an infection and can initiate a secondary immune response.
• The secondary immune response is rapid and can prevent infection before symptoms appear.

Immune Tolerance

• Immune tolerance is the prevention of unnecessary immune responses.
• It is crucial in mucosal linings, like the upper digestive system.
• It prevents the immune system from attacking healthy tissues, focusing instead on pathogens.

Disruptions to the Immune System

• Disruptions include hypersensitivity and autoimmunity.

Immunodeficiency

• Immunodeficiency can cause a failure, insufficiency, or delay in immune response.
• Immunodeficiency may be inherited or acquired through infection, chemical exposure, malnutrition, or stress.
• Primary immunodeficiencies are present from birth.

Allergies

• Allergies produce an immediate response to a harmless antigen.
• Upon initial exposure, IgE antibodies are created.
• Subsequent exposures induce histamine and inflammatory chemical releases.
• An allergic reaction can vary from mild to life-threatening.

Autoimmunity

• Autoimmunity occurs when antibodies attack the body's own components.
• Myasthenia gravis involves antibodies targeting muscle-cell receptors.
• Systemic lupus erythematosus produces a diffuse antibody response to DNA and proteins.
• Autoimmunity can arise from molecular mimicry.
• Post-strep infections can cause antibodies to react with heart muscle.
• Insulin-dependent diabetes can result from inflammation against pancreatic insulin-producing cells.