Viruses and the Immune System: Defense Mechanisms

Posted on Jan 28, 2025 in Biology

Viruses: Structure and Cycles

Viruses are supramolecular complexes composed of proteins, nucleic acids, and, in some cases, lipids. They are considered genetic factors in transit. When a virus is outside the cell, it is called a virion. Viruses are intracellular parasites, not cells. They can be classified by the type of nucleic acid (RNA in retroviruses, DNA in adenoviruses), by the presence or absence of an envelope, and by the type of host they infect (bacteria in bacteriophages, other cells in other viruses).

When viruses infect bacteria, they can undergo two cycles:

  • Lytic cycle: The virus injects its genetic material into the bacterium, merging with the host’s DNA. It then uses the bacterium’s biochemical machinery to generate copies until the bacterium ruptures, releasing new viruses.
  • Lysogenic cycle: The virus injects its genetic material into the bacterium, binding to the bacterial DNA. As the bacterium grows and divides, the viral segment is also replicated.

Immunology: Understanding Immune Responses

Immunology is the study of immunity and the cellular and molecular events that occur when the body encounters microorganisms and other macromolecules. The immune system has evolved to protect the host against macromolecules from both the external and internal environment.

It can recognize different antigens produced by macromolecules and direct immune responses against these antigens, minimizing damage to the host.

Lines of Defense Against Pathogens

First Line of Defense

  • Physical mechanisms: Lining epithelia, ciliated epithelia
  • Chemical mechanisms: Secretions, enzymes, acidic substances, defensins, antimicrobial peptides
  • Microbiological mechanisms: Normal flora

Second Line of Defense

  • Phagocytes and the complement system: Against extracellular pathogens
  • Macrophages: Against intracellular bacteria
  • NK cells and interferons: Against virus-infected cells

Third Line of Defense

  • Lymphocytes: Specific and adaptive mechanism that creates immune memory

Types of Immunity

Natural or innate immunity: Defense mechanisms against microbes or foreign macromolecules that are present in the body prior to exposure and are not stimulated by these exposures.

Innate Immune Responses

  • Existing mechanisms develop before infection.
  • Responses are very fast.
  • Reactions are always the same to different infectious agents.
  • There is no antigen specificity.

Innate Defenses

Innate defenses are responsible for:

  • Physical and chemical barriers (epithelia and antimicrobial substances)
  • Phagocytic cells and NK cells
  • Complement pathway
  • Cytokines

External Barriers

For microorganisms to invade animals, they must penetrate the skin through natural orifices such as the mouth, anus, respiratory, urogenital, and digestive tracts.

The skin of mammals is a mechanical barrier due to its thickness, the process of keratinization, and the peeling of the outer layers.

Defensive barriers include the mucus lining the epithelia.

First Line of Defense in the Body

  • Chemical: Lysozyme (antibacterial function) in tears, saliva, and nasal secretions; ear wax; sebaceous gland secretions; acids and digestive enzymes; intestinal defensins; spermine (bactericidal function) in semen
  • Physical: Ciliated epithelia, lining epithelia
  • Microbiological: Normal bacterial flora (prevents the development of other microbes that are deposited on it)

The secretions of the sebaceous glands and sweat create an acidic pH.

Acidic secretions of the vaginal epithelium and the digestive tract are unfavorable for the development of microorganisms.

In the respiratory mucosa, microbes and other foreign particles are trapped in the mucus and are eliminated by coughing and sneezing.

Immune Pathways

  • Classical pathway: Not triggered without a prior specific immune response.
  • Alternative pathway: Does not require the existence of a prior specific immune response.

Complement Actions

  1. Increased vascular permeability
  2. Smooth muscle contraction
  3. Mast cell degranulation
  4. Assistance in phagocytosis
  5. Activation and chemotaxis of neutrophils
  6. Lysis of bacteria
  7. Lysis of foreign cells

Inflammation and Phagocytosis

  1. Capillary vasodilation
  2. Increased permeability (retraction of endothelial cells)
  3. Infiltration of blood cells (adhesion and rolling on the endothelium, diapedesis, chemotaxis)
  4. Production and release of active molecules