Acquired immunity is a specific defense mechanism that develops after birth. Unlike the First Line of Defence→ and Second Line of Defence→, which are non-specific, acquired immunity targets particular pathogens and creates immunological memory. This document outlines the types of acquired immunity and discusses various disorders arising from immune system malfunction.
Acquired immunity is specific defense against a particular pathogen developed after birth. It can be acquired in two main ways: actively or passively, each having natural and artificial methods. The artificial provision of immunity is known as immunization.
| Feature | Active Immunity | Passive Immunity |
|---|
| Source of Antibodies | Body produces its own antibodies. | Antibodies are received from an external source. |
| Onset of Protection | Slow (takes days or weeks to develop). | Immediate. |
| Duration of Protection | Long-lasting, sometimes for life. | Short-term (weeks to months). |
| Memory Cells | Produced, providing long-term memory. | Not produced. |
The body is stimulated to produce its own antibodies and memory cells against a specific antigen.
Natural Active Immunity
- Acquisition: Develops as a result of a natural infection. The body is exposed to an infectious agent and manufactures its own antibodies.
- Duration: Highly effective and typically long-lasting, often providing lifelong protection.
- Example: A person who has had measles develops lifelong immunity and will not get the disease again.
Artificial Active Immunity (Vaccination)
- Acquisition: Achieved by injecting or orally administering a small amount of an antigen, known as a vaccine.
- Mechanism: The vaccine stimulates the body to produce antibodies and memory cells without causing the disease.
- Booster Shots: A second (or subsequent) dose is often given to stimulate a much quicker and more robust secondary immune response, ensuring long-lasting protection.
Antibodies are transferred from one individual to another, providing immediate but temporary protection.
Natural Passive Immunity
- Acquisition: Antibodies are passed naturally from mother to offspring.
- Mechanisms:
- Antibodies cross the placenta from mother to fetus, protecting the baby until its own immune system matures.
- Antibodies are transferred through colostrum (the first milk from mammary glands), which the baby absorbs through its gut.
Artificial Passive Immunity
- Acquisition: Antibodies formed in one individual (or animal) are extracted and injected into another.
- Purpose: Used for immediate protection when a person is exposed or likely to be exposed to a potent toxin or pathogen.
- Examples: Antivenom for snake bites, antibodies against tetanus (ATS), diphtheria, and rabies.
These are conditions where the immune system functions improperly, either by overreacting, failing to react, or attacking the body's own tissues.
- Definition: A defective or hypersensitive immune response to normally harmless substances called allergens (e.g., pollen, dust, certain foods).
- Mechanism:
- Exposure to an allergen causes the body to produce large quantities of a special antibody, Immunoglobulin E (IgE).
- IgE binds to immune cells called basophils.
- Upon re-exposure to the allergen, the basophils release inflammatory chemicals, primarily histamine.
- Effects of Histamine: Causes allergic symptoms like increased capillary leakiness, swelling, mucus secretion, and inflammation, leading to conditions like hay fever, eczema, and asthma.
- Treatment: Antihistamine drugs block the effects of histamine. Vaccination (allergy shots) can also be an effective long-term treatment.
- Definition: A condition where the immune system mistakenly attacks the body's own healthy cells, tissues, and organs.
- Mechanism: The body produces autoantibodies (antibodies against its own components), which begin to destroy them.
- Examples:
- Type 1 Diabetes: The immune system destroys the insulin-secreting cells of the pancreas.
- Anemia (some types): Antibodies destroy the person's own red blood cells.
- Vitiligo: An autoimmune disorder causing patches of skin to lose their pigment.
- Treatment: There is currently no cure. Treatments focus on suppressing the autoimmune response with drugs.
- Definition: The recipient's immune system recognizes a transplanted organ or tissue as foreign and mounts an immune response to destroy it.
- Mechanism: The recipient's immune cells recognize the Major Histocompatibility Complex (MHC) antigens on the donor's cells as foreign.
- Role of T Cells:
- Helper T cells are activated upon contact with the graft's MHC antigens.
- They then either stimulate cytotoxic T cells to directly attack the transplant or release lymphokines (signaling molecules) that recruit other immune cells like macrophages to the site to destroy the tissue.
- Role of B Cells:
- Helper T cells can also activate B cells.
- The B cells then produce antibodies against the donor's MHC antigens, which can damage the graft via the complement system or by marking it for destruction by phagocytes.
- Prevention: Transplant recipients are given immunosuppressive drugs to dampen their immune response and enhance tolerance of the new organ. However, this leaves them more vulnerable to infections.