Human Body Defence Mechanisms

Non-Specific Defence Mechanisms

Non-specific defence mechanisms are the body's first line of defence. Their primary function is to prevent the entry of all types of foreign substances and pathogens into the body, regardless of the specific type of threat.

• Composition: These mechanisms include physical barriers, chemical barriers, blood clotting, phagocytosis, and inflammatory responses.
• Inclusivity: Unlike specific mechanisms, non-specific responses do not differentiate between different types of invading microorganisms.

Physical and Chemical Barriers

Physical and chemical barriers provide a multi-layered defense to keep pathogens out of internal tissues.

Physical Barriers

Physical barriers consist of structural obstacles that prevent pathogens from entering the body.

• Skin: The skin covers the entire body surface. Its outermost layer consists of dead cells that are constantly worn away and replaced by new cells from below. This continuous turnover helps remove pathogens residing on the surface.
• Ciliated Epithelium of the Respiratory Tract: Cells in the epithelium are tightly packed. Pathogens are trapped by mucus, and microscopic hair-like structures called cilia beat in a coordinated fashion to move the mucus and trapped pathogens upward to be swallowed or coughed out.

Chemical Barriers

Chemical barriers involve the secretion of substances that kill or inhibit the growth of pathogens.

• Sebum: Secreted by the sebaceous glands in the skin, sebum is a natural disinfectant that can kill pathogens on the skin surface.
• Lysozyme: An enzyme found in tears (secreted by tear glands) and saliva. It effectively breaks down the cell walls of certain bacteria, such as those present on the conjunctiva of the eye.
• Gastric Juice: Secreted in the stomach, it contains concentrated hydrochloric acid ($HCl$) which kills pathogens present in ingested food.
• Vaginal Secretions: These secretions are acidic, which serves to inhibit the growth of various pathogens.

Blood Clotting

Blood clotting is a non-specific defense mechanism that seals wounds in the skin to prevent the entry of pathogens and stop blood loss.

• The Process of Clotting:
  • When a blood vessel is damaged, blood platelets and the broken vessel itself release an enzyme into the blood.
  • This enzyme catalyzes the conversion of a soluble plasma protein called fibrinogen into an insoluble fibre called fibrin.
  • The fibrin forms a dense network across the wound, trapping blood cells to form a blood clot.
  • The clot eventually dries and hardens to form a scab, protecting the area while new tissue forms beneath it.
• Temporal and Medical Factors:
  • Normal blood clotting occurs within approximately $2-5\,\text{minutes}$.
  • Haemophilia: A condition where blood clots very slowly because the individual fails to produce enough of the specific enzymes required for the clotting process.

Phagocytosis

Phagocytosis is the cellular process by which certain white blood cells, known as phagocytes, engulf and destroy pathogens.

• Mechanism:
  1. A phagocyte identifies and engulfs a pathogen.
  2. The pathogen is digested by enzymes within the phagocyte.
  3. The remains of the digested pathogen are then released from the cell.

Inflammatory Responses

Inflammation is a localized physical condition that occurs in response to injury or infection.

Physiological Changes

1. Arterioles in the infected area undergo vasodilation.
2. Capillaries in the area increase their permeability.
3. Increased blood flow brings more phagocytes to the site, which then migrate out of the capillaries and into the tissues to kill pathogens.

Signs and Symptoms of Inflammation

• Redness and Heat: Caused by the increased blood flow to the site of infection.
• Swelling and Pain: Caused by the accumulation of tissue fluid in the affected area.

Specific Defence Mechanisms

Specific defence mechanisms involve the immune system recognizing and responding to specific foreign substances called antigens.

Antigens

Antigens are substances that:

• Stimulate an immune response.
• Activate the immune system to produce antibodies.
• Combine specifically with their corresponding antibodies.

Humoral Immune Responses (HIR)

Humoral immune responses are mediated by B lymphocytes and involve the production of antibodies to attack antigens that have not yet entered host cells.

• B Lymphocytes: Formed and mature in the bone marrow.
• Activation: B lymphocytes are activated upon contact with a specific antigen and interaction with helper T cells.
• Differentiation: Activated B lymphocytes multiply and differentiate into:
  • Plasma Cells: These produce specific antibodies.
  • Memory B Cells: These "remember" the specific antigen to facilitate a faster response during future exposures.

Antibodies

An antibody is a specialized protein molecule that combines with a specific antigen to form an antigen-antibody complex.

• Actions of Antibodies:
  1. Lysis: Attaching to antigens and killing the pathogen directly.
  2. Enhancing Phagocytosis: Marking pathogens so they are more easily engulfed by phagocytes.
  3. Agglutination: Clumping pathogens together to prevent them from entering cells or reproducing.
  4. Neutralization: Acting as antitoxins to neutralize toxins secreted by pathogens.

Cell-Mediated Immune Responses (CMIR)

Cell-mediated immune responses involve T lymphocytes destroying cells already infected with pathogens or cancer cells.

• T Lymphocytes: Formed in the bone marrow and mature in the thymus gland.
• Types of T Cells:
  • Helper T Cells: Detect infected or cancer cells and activate other T cells, B cells, and phagocytes.
  • Killer T Cells: Destroy infected cells or cancer cells directly.
  • Memory T Cells: Remember the antigen for future responses.

Immunological Memory and Response Patterns

Immunological memory allows the body to respond more effectively to repeated exposures to the same pathogen.

Primary vs. Secondary Immune Response

• Primary Response:
  • Occurs on the first exposure to an antigen.
  • Characterized by a longer latent period.
  • Produces a smaller amount of antibodies and killer T cells.
  • Lasts for a shorter period of time.
• Secondary Response:
  • Occurs when the body is exposed to the same antigen again.
  • Stimulated by memory B and memory T cells.
  • Characterized by a shorter latent period (faster reaction).
  • Produces a much larger amount of antibodies and killer T cells.
  • Lasts for a longer period of time.

Principles of Vaccination

Vaccination is the artificial introduction of a vaccine into the body to induce immunity. A vaccine contains antigens but is typically modified to be harmless.

• Vaccine Composition: May contain live but weakened pathogens, killed pathogens, purified proteins, or inactivated bacterial toxins.
• Mechanism: Vaccination triggers a primary immune response, leading to the production of memory B and T cells. If the actual pathogen enters the body later, these memory cells trigger a secondary response, killing the pathogen before symptoms develop.

Active and Passive Immunity

Immunity can be categorized based on how the antibodies are acquired.

Active Immunity

• Antibodies are produced by the individual's own plasma cells.
• Natural: Acquired after recovering from an infection.
• Artificial: Acquired via vaccination for disease prevention.
• Characteristics: Slow to start but lasts for a long period.

Passive Immunity

• Antibodies are received from an external source.
• Natural: Antibodies diffusing across the placenta or through breast milk (colostrum).
• Artificial: Injection of antibodies (e.g., for immediate treatment of a disease like rabies).
• Characteristics: Provides immediate protection but lasts only for a short period.

Infectious Diseases and Transmission Pathways