Understanding the Body's Innate Immune Defenses

The body's immune system employs various defense mechanisms to protect against pathogens and foreign invaders. Among these are the nonspecific defenses, which act with great rapidity. There are four primary types:

Inflammation: The Body's Rapid Response

If a foreign barrier is surpassed, it causes inflammation, a passive and active response consisting of pain, heat, and swelling of the affected area. The process involves:

• 1. Stimulus Input: An initial stimulus triggers the response.
• 2. Mediator Production: Affected inflammatory cells produce mediators such as histamine, bradykinin, and prostaglandins.
• 3. Tissue Action: These mediators act on the affected tissue or area, causing:
  • An increase in the number of leukocytes in the blood.
  • Vasodilation.
  • Increased capillary permeability.
  • Activation of phagocytes.
  • Chemotaxis (attraction of immune cells).

The consequences of this include increased cellular defensive actions of leukocytes (the most numerous), increased blood vasodilation, increased irrigation of the affected area, and an increased number of defensive elements, including both leukocytes and complement molecules. The increase in antibodies and capillary permeability favors the output of phagocytes, antibodies, and fibrinogen towards the infected tissues. Phagocyte activation makes them more effective, and chemotaxis is also fundamental.

Phagocytes: Cellular Defenders of Immunity

Phagocytes are a type of leukocyte. There are two main lineages of leukocytes:

• 1. Myeloid Lineage: These originate in the bone marrow and include polymorphonuclear granulocytes (such as neutrophils) and monocytes (which differentiate into macrophages and histiocytes). These are the primary phagocytic cells.
• 2. Lymphoid Lineage: Lymphocytes, which do not perform phagocytosis but produce very important specific antibodies.

The most important function of phagocytes is phagocytosis, the process of engulfing and digesting foreign particles or pathogens. The primary phagocytes include neutrophils, macrophages (which have the highest phagocytic capacity), and histiocytes.

The phagocytosis process has 4 stages:

• 1. Adhesion/Attachment: Through opsonins, phagocytes bind to the target.
• 2. Ingestion: The target is engulfed into a phagosome.
• 3. Death and Digestion: The target is broken down by enzymes.
• 4. Expulsion: Undigested remnants are expelled.

The Complement System: Rapid Immune Response

The complement system is a plasma protein system whose defensive function is carried out with great rapidity. It has 3 important functions:

• 1. Inflammation Mediator: It contributes to the inflammatory response.
• 2. Opsonization: It intervenes in the opsonization of foreign cells, facilitating the action of antibodies and phagocytes.
• 3. Cell Lysis: It causes lysis of invasive cells by breaking their plasma membrane.

For these actions to occur, the complement system must first be activated. The activation process can be initiated in 2 main ways:

• 1. Classical Pathway: Activated by the appearance of antibodies corresponding to an antigen.
• 2. Alternative Pathway: Antibody production is not necessary for activation; some components of the complement system bind directly to polysaccharides found on the surface of bacteria.

Interferons: Antiviral & Anti-Cancer Proteins

Interferons are proteins synthesized by virus-infected cells. They are known to prevent the spread of infection. They primarily carry out 2 actions:

• 1. Viral Replication Inhibition: They prevent viral replication in infected cells that have not yet been destroyed by viral action.
• 2. NK Cell Activation: They activate NK (Natural Killer) cells, which are capable of recognizing and eliminating virus-infected or cancerous cells.