Understanding the Immune System and Its Defenses

Immunology

By removing serum from pustules of the udder of a cow and injecting it into human skin, an individual suffered from a mild illness but never acquired smallpox. It was the first experiment of active immunization.

1. The Body's Defenses

All organisms have developed mechanisms of defense against the invasion of pathogens. These mechanisms may be nonspecific, preventing entry into the body or destroying them quickly, or very specific, which is known as the immune response.

1.1 Nonspecific Defense Mechanisms

These act against any organism or foreign substance. They are of three types:

a) Natural Barriers: Skin and secretions from the mucosal surfaces.

• Skin is a mechanical barrier because its outermost stratum corneum is fully keratinized.
• It acts as a chemical barrier as it has a slightly acidic pH, inadequate for many microorganisms.
• These acidic secretions also prevent the development of microorganisms in the body's natural openings, protected by mucosal surfaces, as in the vagina or stomach.
• Mucus also contains bactericidal enzymes such as lysozyme, present in mucus, saliva, and tears, or spermine, which is found in semen.
• In the airways, mucus is expelled to the outside along with the remains of microorganisms and foreign substances. This expulsion is carried out through mechanisms such as coughing and sneezing.

b) The Body's Normal Microflora: Animals have a microflora formed by commensal or mutualistic organisms, which hinders the development of other microorganisms.

Human skin is populated by millions of harmless microorganisms that appear to inhibit the proliferation of potentially pathogenic microorganisms.

c) Nonspecific Cellular Response (or Secondary Barrier): It is activated if, for some reason (injury, burns, etc.), invading pathogens reach tissues.

• The affected cells themselves produce antimicrobial substances called glycoproteins, reacting by secreting interferon. Its function is to prevent the cell from synthesizing macromolecules necessary to destroy the virus or viral mRNAs.
• The cells of the affected tissues also release other substances, such as histamine, serotonin, and cyanine. Their mechanism is as follows:
  • The substances released cause dilation of blood vessels, causing an increase in blood flow to the area (redness and local heat), which comes loaded with many phagocytic cells.
  • These substances cause increased permeability of the capillaries in the area, leading to blood plasma exiting into the interstitial space, resulting in edema or lasting inflammation. This swelling causes a local pain sensation.

The main role of inflammation seems to be that phagocytes in the area are attracted chemotactically by substances released by cells.

After phagocytosis, they are disabled and die. The set of dead white blood cells and the remains of microorganisms form pus, which can be reabsorbed or expelled abroad.

When the infection is extensive, fever occurs.