91Ó°ÊÓ

Antibodies, or immunoglobulins, play a crucial role in the immune system. They are specialized proteins designed to identify and neutralize foreign objects like bacteria and viruses. An antibody is typically composed of four polypeptide chains: two heavy (H) chains and two light (L) chains. These chains are linked by disulfide bonds, creating a Y-shaped molecule. Each antibody has constant (C) regions and variable (V) regions. The constant region determines the antibody's class and does not vary much among antibodies of the same class. The variable region, on the other hand, is unique and binds specifically to antigens, allowing the antibody to recognize a vast array of pathogens.
Understanding the structure of antibodies helps explain how they perform their functions and why they have such diversity in targeting invaders.

B cells are a type of white blood cell fundamental to the immune response. They mature in the bone marrow and are responsible for producing antibodies. When a B cell encounters its specific antigen, it gets activated and differentiates into plasma cells. These plasma cells then produce large quantities of antibodies. Each B cell is programmed to make only one specific antibody, enabling the immune system to target and remember a wide range of pathogens.
Additionally, some activated B cells become memory cells, which can survive for years. These memory B cells quickly produce antibodies if the antigen reappears, providing long-term immunity.

Immunoglobulins (Ig) are categorized into five main classes: IgA, IgD, IgE, IgG, and IgM. Each class has a different structure and function in the immune system:

• IgA: Mostly found in mucous membranes (e.g., respiratory and gastrointestinal tracts) and secretions like saliva and tears. It prevents the colonization of pathogens.
• IgD: Functions mainly as a receptor on B cells that have not been exposed to antigens. Its role in the body remains less understood compared to other immunoglobulins.
• IgE: Primarily involved in allergic reactions and defending against parasitic infections. It binds to allergens and triggers histamine release from mast cells and basophils.
• IgG: The most abundant antibody in blood and extracellular fluid. It provides the majority of antibody-based immunity against pathogens and can cross the placenta to protect the fetus.
• IgM: The first antibody type produced in response to an infection. Highly effective at forming complexes with antigens and initiating their removal.

The constant region of an antibody determines its class, which is why antibodies from the same individual can show a variety of constant domains.

Passive immunity occurs when a person receives antibodies produced by another individual or organism. This can happen naturally, such as when a fetus receives antibodies through the placenta, or artificially through antibody-containing treatments. Passive immunity provides immediate but temporary protection.
Since the antibodies are not produced by the person's own immune system, they are not as long-lasting as active immunity, where the body makes its own antibodies in response to an infection or vaccination. Passive immunity does not lead to the formation of memory cells, so once the externally supplied antibodies are degraded, the individual is susceptible to the infection again. It's a crucial mechanism during emergencies, where immediate defense is necessary.