The adaptive immune system

 The adaptive immune system, also known as the acquired or specific immune system, is one of the two main branches of the immune system in vertebrates (the other being the innate immune system). The adaptive immune system is highly specialized and provides the host with the ability to mount specific responses to particular pathogens, such as bacteria, viruses, and fungi, as well as abnormal cells, including cancer cells.

Here are key features of the adaptive immune system:

1. **Specificity:** The adaptive immune system is highly specific. It can recognize and selectively target particular antigens, which are molecules on the surface of pathogens or other foreign substances.

2. **Memory:** Perhaps one of its most remarkable features is immunological memory. Once the adaptive immune system encounters a specific pathogen, it "remembers" it. If the same pathogen enters the body again, the immune system can mount a faster and more effective response.

3. **Diversity:** The adaptive immune system is diverse, with a wide range of cells and molecules that can recognize an extensive array of antigens. This diversity is essential for recognizing and combating the vast array of potential pathogens.

4. **Cellular and Humoral Immunity:**

   - **Cellular immunity:** Involves immune cells called T lymphocytes (T cells), which directly attack infected or abnormal cells.

   - **Humoral immunity:** Involves immune molecules such as antibodies (produced by B lymphocytes or B cells) that can neutralize or tag pathogens for destruction by other immune cells.

5. **Lymphocytes:** Lymphocytes are key cells of the adaptive immune system. There are two main types:

   - **T lymphocytes (T cells):** Involved in cell-mediated immunity, directly attacking infected or abnormal cells.

   - **B lymphocytes (B cells):** Involved in humoral immunity, producing antibodies that can recognize and neutralize pathogens.

6. **Antigen Presentation:** Antigens from pathogens are presented to T cells by antigen-presenting cells (APCs), such as macrophages and dendritic cells. This process is crucial for activating T cells.

7. **Major Histocompatibility Complex (MHC):** MHC molecules play a crucial role in antigen presentation. They are expressed on the surface of cells and present antigens to T cells.

The adaptive immune response typically takes some time to develop, but once established, it provides a more targeted and powerful defense against specific pathogens. Vaccination works by stimulating the adaptive immune system to produce a memory response, offering protection against future infections.