# Cells of the immune system

The body's defence against disease relies on a complex internal system known as the **immune system**, which is an "army of cells" protecting us from pathogens. This system distinguishes between the body's own "self" cells and "non-self" foreign invaders or abnormal cells, primarily identified by molecules called **antigens**.

The immune system's cells, known as **white blood cells (leucocytes)**, all originate from **stem cells in the bone marrow**. These cells are broadly categorized into two main groups: **phagocytes** and **lymphocytes**.

#### 1. Phagocytes (Non-Specific Immune Response)

Phagocytes provide an **immediate, non-specific immune response**, meaning they react in the same way to all microorganisms. They are "scavengers" that remove dead cells and invasive microorganisms.

* **Types and Function:**
  * **Neutrophils:** These are the most common type of phagocyte, making up about 60% of white blood cells. They are short-lived, travel in the blood, and can squeeze through capillary walls into infected tissues. During infection, they are released in large numbers. They are attracted to the site of infection by chemical stimuli (chemotaxis) released by pathogens or damaged cells. After engulfing and destroying pathogens, they die, forming pus.
  * **Macrophages:** Larger than neutrophils, these are long-lived phagocytes found predominantly in organs like the lungs, liver, spleen, kidney, and lymph nodes. They develop from monocytes, which circulate in the blood. Macrophages are the principal "rubbish-collecting cells" of the body.
* **Mode of Action (Phagocytosis):**
  1. A phagocyte recognizes foreign antigens on a pathogen.
  2. Its cytoplasm engulfs the pathogen, enclosing it in a **phagocytic vacuole**.
  3. A **lysosome** (an organelle containing digestive enzymes called **lysozymes**) fuses with the phagocytic vacuole.
  4. The lysozymes break down and destroy the pathogen.
  5. After digestion, the phagocyte "presents" the pathogen's antigens on its surface, becoming an **antigen-presenting cell (APC)**, to activate other immune cells.

#### 2. Lymphocytes (Specific Immune Response)

Lymphocytes are the key cells of the **specific (adaptive) immune response**, which targets particular pathogens and provides long-term immunity. They are generally smaller than phagocytes and have a large, round nucleus.

* **Maturation:**
  * **B-lymphocytes (B-cells):** These mature in the bone marrow and then spread throughout the body, concentrating in lymph nodes and the spleen. Each B-cell is genetically programmed to make one specific type of antibody molecule.
  * **T-lymphocytes (T-cells):** These migrate from the bone marrow to the **thymus gland** to mature. The thymus gland is active from birth to puberty, after which it typically shrinks. During maturation in the thymus, any T-lymphocytes that would react to the body's own "self" cells are destroyed.
* **Mode of Action (Specific Response Stages):**
  1. **Antigen Recognition & Activation:**
     * T-cells have specific **T-cell receptors** on their surface. They are activated when these receptors bind to complementary foreign antigens presented by APCs (like macrophages).
     * B-cells are covered with specific antibodies that act as receptors. When a B-cell's antibody meets a complementary antigen, it binds to it. This, along with signals from helper T-cells, **activates the B-cell** (known as **clonal selection**).
  2. **Clonal Expansion:** Activated B-cells and T-cells rapidly divide by **mitosis** to form a large clone of identical cells (known as **clonal expansion**).
  3. **Differentiation and Effector Functions:**
     * **B-cells differentiate into:**
       * **Plasma cells:** These are short-lived, highly active cells that secrete large quantities of specific antibodies (also called **monoclonal antibodies**). Plasma cells are adapted for this function with extensive rough endoplasmic reticulum, Golgi bodies, and mitochondria.
       * **Memory B-cells:** These are long-lived cells that persist in the body, providing **immunological memory**.
     * **T-cells differentiate into:**
       * **Helper T-cells (TH cells):** These are crucial for coordinating the immune response. They release chemical signals called **cytokines** that stimulate the division and activation of B-cells, cytotoxic T-cells, and phagocytes. **HIV** specifically infects and destroys helper T-cells, leading to a severely weakened immune system (AIDS).
       * **Cytotoxic T-cells (Killer T-cells):** These cells directly "search for and kill" abnormal body cells, such as those infected with pathogens or cancer cells. They attach to the antigens displayed on the infected cell's surface and release toxic substances.
       * **Memory T-cells:** Similar to memory B-cells, these are long-lived and provide rapid, strong responses upon re-exposure to the same antigen.

#### 3. Antibodies

**Antibodies (immunoglobulins)** are Y-shaped proteins produced by plasma cells. They consist of four polypeptide chains (two heavy, two light) held together by disulfide bridges.

* **Structure and Specificity:** The "variable regions" at the tips of the antibody's arms have a unique tertiary structure that forms a highly specific binding site, complementary to one particular antigen. The "constant regions" are the same in all antibodies and can bind to receptors on other immune cells like phagocytes.
* **Functions of Antibodies:**
  * **Agglutination:** Antibodies can bind to two pathogens simultaneously, clumping them together. This makes them easier for phagocytes to engulf and reduces their spread.
  * **Neutralizing Toxins:** Antibodies (called antitoxins) bind to and inactivate toxins produced by pathogens, preventing them from harming host cells.
  * **Preventing Pathogen Binding:** Antibodies can block the cell surface receptors that pathogens need to attach to and infect host cells.
  * **Opsonisation:** Antibodies coat bacteria, acting as "markers" or "labels" that make them more easily recognized and engulfed by phagocytes.
  * **Lysis:** Some antibodies, along with other molecules, can create holes in bacterial cell walls, causing them to burst due to osmosis.

#### 4. Immune Response Phases

The body's immune response to an antigen occurs in two main phases:

* **Primary Immune Response:** This is the body's **first exposure** to a specific antigen. It is **slow** because there are few B-cells capable of producing the necessary antibody, and symptoms of the disease may appear during this time. This response leads to the production of plasma cells and **memory cells**.
* **Secondary Immune Response:** This occurs upon **re-exposure** to the same antigen. Due to the presence of memory cells from the primary response, this response is much **faster, stronger, and longer-lasting**. Memory cells rapidly divide into plasma cells, quickly producing a large amount of specific antibodies, often preventing symptoms from developing.

#### 5. Antibodies vs. Antibiotics

It is important to distinguish between antibodies and antibiotics. **Antibodies** are proteins produced by the body's immune system cells. **Antibiotics** are medicinal drugs, produced outside the body, that are specifically designed to **kill or inhibit the growth of bacteria**. They target structures or metabolic pathways unique to bacteria, such as cell wall synthesis or protein synthesis. Antibiotics are **ineffective against viruses** because viruses are non-cellular and use the host cell's machinery for replication, lacking the bacterial structures that antibiotics target. The overuse of antibiotics contributes to **antibiotic resistance** in bacteria.