# 9.4 Types of Acquired Immunity Acquired immunity is a specific defense mechanism that develops after birth. Unlike the and , 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. ### 1. Types of Acquired Immunity 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. | #### Active Immunity 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. #### Passive Immunity 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:* 1. Antibodies cross the *placenta* from mother to fetus, protecting the baby until its own immune system matures. 2. 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. ### 2. Disorders of the Immune System These are conditions where the immune system functions improperly, either by overreacting, failing to react, or attacking the body's own tissues. #### Allergies - *Definition:* A defective or hypersensitive immune response to normally harmless substances called *allergens* (e.g., pollen, dust, certain foods). - *Mechanism:* 1. Exposure to an allergen causes the body to produce large quantities of a special antibody, **Immunoglobulin E (IgE)**. 2. IgE binds to immune cells called *basophils*. 3. 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. #### Autoimmune Diseases - *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. #### Transplant Rejection - *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.