Immune System: Body's Defenses Against Pathogens

Infection: Pathogen Penetration and Host Response

Infection is the penetration of a pathogen into the body of an animal or plant, causing changes in organs and their functions. It is essential for the microbe to possess pathogenicity (the ability to cause disturbances in the body), which depends on its invasiveness and toxin production. For an infection to develop, the host organism must be receptive to the microbes (i.e., susceptible to invasion).

Organic Defense Mechanisms: Protecting the Body

Nonspecific Defense (External Barriers)

These are the body's first lines of defense, acting as external barriers:

• Structural: Skin and mucous membranes. These are common entry points for pathogens when compromised.
• Mechanical: Tears, urine, coughing, and sneezing. These systems help remove microorganisms and particles, preventing contagion.
• Biochemical: Saliva, semen, gastric juice, and fatty acids. These substances act as barriers, neutralizing pathogens.
• Ecological: In the respiratory, digestive, urinary, and reproductive tracts, beneficial microorganisms (natural flora) compete against pathogens, preventing their entry. This natural flora also secretes antibiotic proteins.

Internal Barriers (Fast Action)

Phagocytes are key defense mechanisms involved in internal barriers:

• Monocytes: These are leukocytes that, after circulating for some time, migrate to tissues (e.g., lungs, liver) to become macrophages.
• Macrophages: Large cells capable of phagocytosis, forming part of the reticuloendothelial system. They can also extend pseudopodia to engulf foreign particles.
• Neutrophils: Infected tissues release substances that attract them, causing them to leave blood vessels via amoeboid movement towards the site of infection.

Inflammation: A Localized Immune Response

Inflammation is a local reaction to the presence of pathogens or tissue injury. It occurs when infected tissue cells release histamine and serotonin, causing the area to become inflamed and red. Inflammation results from increased capillary permeability, while redness is due to increased blood flow. Other effects include local heat and pain. Blood brings phagocytic cells to the area, which form vacuoles. The collection of leukocytes and microbial debris forms pus, which can either be reabsorbed or expelled.

The Complement System: Enhancing Immunity

The complement system is a group of about 20 plasma proteins synthesized in the liver, activated by various triggers. These proteins complement and enhance the immune response, mediate inflammation, cause lysis of invading cells by rupturing their plasma membranes, and tag foreign cells for recognition by macrophages and antibodies.

Complement activation can occur via two main pathways:

• The classical pathway, triggered by antigen-antibody complexes.
• The alternative pathway, which does not require antibody production.

Specific Defense: Adaptive Immune Response

This is the adaptive immune response, which distinguishes and detects foreign molecules (antigens), subsequently developing a targeted response for their destruction. There are two main types:

• Humoral immunity: Involves B cells and antibodies.
• Cellular immunity: Involves T cells.