Immune response: phagocytosis, antibodies (overview)

The immune system is vital for protecting the body from disease. It distinguishes between 'self' (the body's own cells) and 'non-self' (foreign invaders like bacteria, viruses, fungi, and parasites). The immune response involves a coordinated effort of various cells and molecules.

There are two main types of immunity:

• Non-specific (Innate) Immunity: This is the body's first line of defense. It provides immediate, general protection against a wide range of pathogens. It does not distinguish between different types of pathogens and does not lead to long-term immunity. Examples include physical barriers (skin, mucous membranes), chemical barriers (stomach acid, tears), and phagocytic cells.
• Specific (Adaptive) Immunity: This is a more targeted and sophisticated defense mechanism. It develops in response to specific pathogens and involves specialized cells that 'remember' previous encounters. This memory allows for a faster and stronger response upon re-exposure, leading to long-term immunity.

Phagocytosis is a fundamental process in non-specific immunity, carried out by specialized white blood cells called phagocytes. These cells act as 'scavengers', engulfing and digesting foreign particles, pathogens, and cellular debris.

Key Phagocytes:

• Neutrophils: These are the most abundant type of white blood cell. They are usually the first responders to an infection and have a short lifespan. They are highly phagocytic.
• Macrophages: These are larger, longer-lived phagocytes found in tissues throughout the body. They develop from monocytes (another type of white blood cell) and are crucial for clearing pathogens and presenting antigens to specific immune cells.

Steps of Phagocytosis:

1. Chemotaxis: Phagocytes are attracted to the site of infection by chemical signals released by damaged cells or pathogens.
2. Adherence: The phagocyte attaches to the surface of the pathogen. This can be enhanced by opsonization (coating of the pathogen by antibodies or complement proteins).
3. Ingestion (Engulfment): The phagocyte extends its cell membrane (pseudopods) around the pathogen, enclosing it in a vesicle called a phagosome.
4. Digestion: The phagosome fuses with a lysosome (an organelle containing digestive enzymes) to form a phagolysosome. The enzymes within the lysosome break down and destroy the pathogen.
5. Elimination: Indigestible material is expelled from the cell.

Antibodies are Y-shaped proteins, also known as immunoglobulins (Ig), produced by B lymphocytes (B cells) in response to specific antigens. Antigens are molecules (usually proteins or polysaccharides) found on the surface of pathogens or other foreign substances that can trigger an immune response.

Key Characteristics of Antibodies:

• Specificity: Each antibody is highly specific and can only bind to a particular antigen or a small group of closely related antigens. This is due to the unique shape of its antigen-binding sites.
• Production: When a B cell encounters its specific antigen, it becomes activated, proliferates, and differentiates into plasma cells. Plasma cells are antibody-producing factories, releasing large quantities of antibodies into the bloodstream and lymphatic system.
• Memory: Some activated B cells differentiate into memory B cells, which persist in the body for a long time. Upon subsequent exposure to the same antigen, these memory cells can quickly differentiate into plasma cells and produce antibodies, leading to a faster and stronger secondary immune response.

Antibodies do not directly destroy pathogens, but rather act as markers or neutralizers, facilitating their removal by other immune mechanisms. Their main functions include:

• Agglutination: Antibodies can bind to multiple pathogens, causing them to clump together (agglutinate). This makes it easier for phagocytes to engulf and clear them from the body.
• Neutralisation: Antibodies can bind to toxins produced by bacteria or to the surface proteins of viruses, preventing them from attaching to host cells and causing damage. This effectively neutralizes their harmful effects.
• Opsonisation: Antibodies can coat the surface of pathogens, making them more recognizable and palatable for phagocytes. This process, called opsonisation, enhances phagocytosis.
• Activation of Complement System: Antibodies can activate a group of proteins called the complement system, which can directly lyse (burst) bacterial cells or enhance inflammation and phagocytosis.
• Prevention of Adhesion: By binding to surface antigens, antibodies can prevent pathogens from adhering to host cells, which is often the first step in infection.