11.1 Body Defence
Immunity is the body's ability to resist infection caused by pathogens (microorganisms such as bacteria, viruses, and fungi) or foreign substances.
Three Lines of Defence
- First Line of Defence (Non-specific): Physical and chemical barriers that prevent pathogens from entering the body.
- Skin: Physical barrier with keratin; secretes sebum (antimicrobial) and sweat (contains lysozyme, acidic pH 3–5).
- Mucous Membranes: Line the respiratory, digestive, and urogenital tracts; secrete mucus to trap pathogens. Cilia sweep trapped pathogens away.
- Chemical Secretions: Lysozyme in tears, saliva, and nasal secretions destroys bacterial cell walls; hydrochloric acid in the stomach destroys ingested pathogens.
- Second Line of Defence (Non-specific): Fights pathogens that breach the first line.
- Phagocytosis: Process where phagocytes (neutrophils and monocytes/macrophages) engulf and digest pathogens using lysosomal enzymes.
- Inflammation: Tissue response to injury or infection characterized by redness, heat, swelling, and pain. Damaged cells release histamine, causing vasodilation and increased capillary permeability so phagocytes migrate to the infected area.
- Fever: Increases body temperature to inhibit pathogen growth and accelerate phagocyte activity.
- Third Line of Defence (Specific): Targeted immune response mediated by lymphocytes when pathogens survive the first two lines.
- Antigens: Foreign molecules (usually proteins or polysaccharides) that stimulate an immune response.
- Antibodies: Y-shaped proteins produced by B lymphocytes (plasma cells) that specifically bind to antigens to neutralize or destroy them.
- T Lymphocytes: Directly attack infected cells (cytotoxic T cells) or stimulate B lymphocytes and phagocytes (helper T cells).
11.2 Actions of Antibodies
Antibodies destroy pathogens through several mechanisms:
- Neutralisation: Antibodies bind to bacterial toxins or viruses, preventing them from attaching to host cells.
- Agglutination: Antibodies bind to multiple pathogens simultaneously, clumping them together for easier ingestion by phagocytes.
- Precipitation: Antibodies bind to soluble antigens, forming insoluble complexes that precipitate out of solution for phagocytosis.
- Opsonisation: Antibodies coat pathogens, marking them as targets to enhance recognition and engulfment by phagocytes.
- Lysis: Antibodies combine with antigens and activate complement proteins, which rupture (lyse) the pathogen's cell membrane.
11.3 Types of Immunity
Immunity is categorized into Active Immunity (body produces its own antibodies) and Passive Immunity (body receives external antibodies).
1. Active Immunity
- Naturally Acquired Active Immunity: Achieved when a person recovers from a natural infection. The body produces memory B and T cells for long-lasting protection.
- Artificially Acquired Active Immunity: Induced by vaccination (injection of dead or weakened pathogens / toxoids / mRNA). Stimulates antibody production and memory cell formation without causing disease.
2. Passive Immunity
- Naturally Acquired Passive Immunity: Immediate, temporary protection acquired when antibodies pass from mother to fetus via the placenta (IgG) or to an infant through colostrum/breast milk (IgA).
- Artificially Acquired Passive Immunity: Acquired by injecting antiserum containing ready-made antibodies (e.g., antitetanus serum, antivenom). Provides immediate but temporary protection with no memory cell formation.
11.4 Health Issues Related to Immunity
- Acquired Immunodeficiency Syndrome (AIDS): Caused by the Human Immunodeficiency Virus (HIV), which attacks and destroys helper T lymphocytes, crippling the immune system and leaving the patient vulnerable to opportunistic infections.
- Autoimmune Diseases: Occur when the immune system fails to distinguish self-antigens from non-self antigens, attacking the body's own tissues (e.g., Systemic Lupus Erythematosus (SLE), Rheumatoid Arthritis).
- Allergies: Hypersensitive immune responses to harmless environmental substances (allergens, such as pollen or peanuts), triggering excessive release of histamine.