Understanding the Immune System: Innate and Acquired Immunity

Posted on Feb 27, 2025 in Biology

Understanding the Immune System

The immune system combats pathogens (infectious agents including viruses, bacteria, protozoa that cause injuries and diseases). Key features include:

  • Recognizing external agents (antigens).
  • Neutralizing these agents.
  • Memory and acquired immunity.

Specific or Innate Immunity

  • Present before exposure to pathogens.
  • Provides a fast response.
  • Acts as the first barrier of defense.

Defense Mechanisms

External Defenses

  • Skin:
    • Epidermis: Protective function, continuous shedding, sweat glands and secretions.
    • Dermis:
      • Loose connective tissue (fibroblasts).
      • Defense cells (macrophages, mast cells, lymphocytes).
      • Dense connective tissue (elastin and collagen).
    • Hypodermis: Adipose tissue (temperature regulation and energy reserves).
  • Mucous Membranes: Layers of epithelial cells lining the respiratory tract.
  • Mouth Breathing: Moves dust particles or germs (cough, sneeze).
  • Gastrointestinal Mucosa
  • Bacterial Microbiota: Mutualistic relationship with 1014 live bacteria in the body.

Internal Defenses

  • Inflammatory response.
  • Fever.
  • Fight against tumors.
  • Phagocytosis.
Inflammatory Response

Mast cells (tissues) and basophils (blood) attract eosinophils and neutrophils. This releases histamine, causing inflammation.

Fever

Bacteria/Virus → Exogenous Pyrogens → Endogenous Pyrogens → Hypothalamus (responsible for body temperature) → Temperature rises (1-4 °C).

Combatting Tumors

Cell division:

  • Stimulation → proliferation genes (proto-oncogenes).
  • Inhibition → tumor suppressor genes (oncogenes).

Carcinogenic cells can proliferate without control mechanisms, and all daughter cells will form a tumor. Natural Killer cells of the immune system target these cells.

Phagocytosis

Mast cells and basophils → histamine → attracts eosinophils and neutrophils → vasodilation (allows passage of blood cells to damaged tissue).

The Complement System

A set of approximately 20 enzymatic proteins that act in a chain reaction, providing a fast response amplifier.

Antigen-Antibody Union (or bacterium) → activates (C3) →

  • Recognizes bacteria (C3b + C5) (opsonization).
  • Activates mast cells (C3 → C5a) → histamine → inflammation.

C5b binds to (C6 + C7 + C8 + C9) → Lysis of the bacteria → complex C5b-C9 creates a pore in the bacterial membrane, allowing intracellular material to leak out.

Specific or Acquired Immunity

  • Mechanisms activated upon recognizing the intrusion of an infectious agent.
  • Efficacy increases with repeated immune responses.
  • Components involved:
    • Cells:
      • B lymphocytes
      • T-Lymphocytes (Helpers, cytotoxic, suppressor)
    • Chemicals: Immunoglobulins or antibodies (Ig).

Immunoglobulins or Antibodies

Infectious Agent: antigen + antibodies → Union specifically (agglutination). The union makes the Antigen-Antibody complex remain fixed and acts as a signal for elimination by activating different types of white blood cells.

Types of Antibodies
  • IgG: Main type found in the blood, responsible for secondary immune responses.
  • IgM: Responsible for primary immune responses, 5 units, 10 binding sites.
  • IgA: Present in secretions (milk, mucous, saliva).
  • IgE: Binds to mast cells and basophils, stimulating histamine release. Responsible for allergies.
  • IgD: Determined by the heavy chain.
Ig Structure

Genes encoding Ig undergo mutations, increasing the diversity of the variable region. The presence of an antigen selects the antibody that best combines with it. Encoded by 3 genes (2 light and 1 heavy). Binding increases variability.

Cellular Immunity

  • B cells (produce antibodies).
  • T-Lymphocytes:
    • Helpers (Th): stimulate.
    • Cytotoxic or Killer.
    • Suppressor (Ts): inhibit T and B Lymphocytes.