Immunity - Session 3

Antibodies are specialized proteins produced by the immune system to identify and neutralize foreign objects like bacteria, viruses and other pathogens. Antibodies are also known as immunoglobin. Each antibody is specific to a particular antigen. Do you know what is an antigen? Antigen is a substance that the immune system recognizes as foreign.

Antibodies have a Y shaped structure. Antibodies are composed of four polypeptide chains. There are two identical heavy chains and two identical light chains. The tips of the antibody is known as variable region. The variable region is unique to each antibody and allow for the specific binding to antigens. The stem of the antibody is known as constant region. It is less variable and determines the class or isotype of the antibody.

There are different types of antibodies. IgG is the most abundant antibody in blood and extracellular fluid. It is important for long term immunity. IgA antibody is found in mucous membranes, saliva, tears and breast milk. It is crucial for mucosal immunity. It protects mucosal surfaces by neutralizing pathogens and preventing their attachment and entry.

The antibody IgM is the first antibody produced in response to an infection. It is primarily found in the blood and lymphatic fluid. It is effective in agglutinating and neutralizing pathogens. It strongly activates the complement system. It also serves as a receptor on B cells.

The IgE antibody is found in low concentrations in the blood. It is involved in allergic reactions and responses to parasitic infections. This antibody binds to allergens and triggers histamine release from mast cells and basophils, leading to inflammation and other allergy symptoms. It also provides immunity against parasitic worms.

The IgD antibody is found on the surface of B-cells and in low concentrations in the blood. It functions mainly as an antigen receptor on naive B-cells that have not been exposed to antigens. It plays a role in initiating B-cell activation and differentiation. Immunoglobin D, Immunoglobin E and Immunoglobin G antibodies are monomers. Immunoglobin A antibody is a dimer. Immunoglobin M antibody is a pentamer.

Monoclonal antibodies are highly specific antibodies produced by identical immune cells that are clones of a unique parent cell. These antibodies are designed to bind to a single epitope on an antigen. The production of monoclonal antibodies involves several key steps. These antibodies are primarily developed through a technique called hybridoma technology.

First of all, the target antigen is identified and prepared. This antigen will be used to stimulate an immune response in a host animal, typically a mouse. The host animal is immunized with the antigen, usually in combination with an adjuvant to enhance the immune response. Multiple immunizations are often required to ensure a strong and specific immune response.

After the host animal has developed a sufficient immune response, B-cells producing the desired antibody are harvested from the spleen. These B-cells are then fused with myeloma cells to create hybridoma cells. Myeloma cells are a type of cancer cell that can divide indefinitely. This fusion is facilitated by a chemical agent like polyethylene glycol.

The fused cells are grown in a selective medium. This allows only the hybridoma cells to survive, as myeloma cells lack the necessary enzyme to survive in this medium unless fused with B-cells. Hybridoma cells are screened to identify those producing the desired monoclonal antibody. Screening is typically done using enzyme linked immunosorbent assay or other binding assays.

Positive hybridomas are cloned by limiting dilution, ensuring that each well contains a single hybridoma cell. These clones are then expanded to produce a monoclonal cell line. The selected hybridoma cells are cultured to produce large quantities of the monoclonal antibody. This can be done in tissue culture flasks or bioreactors. The antibodies are harvested from the culture supernatant and purified using techniques such as affinity chromatography. This isolates antibodies that specifically bind to the antigen.

Monoclonal antibodies have diverse applications in medicine, diagnostics and research. Monoclonal antibodies can target specific antigens on cancer cells. Some monoclonal antibodies bind to specific receptors on the surface of cancer cells, blocking the receptors from interacting with their natural ligands. This can inhibit the signaling pathways that promote cancer cell growth and survival.

Monoclonal antibodies can recruit immune cells to attack cancer cells. A specific antibody binds to antigens on the surface of a target cell, such as a cancer cell. This binding occurs via the antigen binding region of the antibody. The effector cells such as the natural killer-cells recognize and bind to the Fc region of the antibody that is attached to the target cell. This recognition is mediated by Fc receptors on the surface of the natural killer-cells.

Binding of the Fc receptor to the antibody Fc region activates the effector cells. This activation triggers a series of intracellular signaling events within the effector cell. Activated effector cells release cytotoxic molecules, such as perforin and granzymes. Perforin forms pores in the target cell membrane, allowing granzymes to enter the target cell. Granzymes, once inside the target cell, initiate a cascade of enzymatic reactions that result in the apoptosis of the target cell. This process effectively eliminates the antibody coated cell.

Monoclonal antibodies can modulate the immune response to treat autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. Monoclonal antibodies can neutralize pathogens directly or enhance the immune response. For example, Palivizumab is used to prevent respiratory syncytial virus in high risk infants. Monoclonal antibodies are used in various diagnostic tests to detect the presence of specific antigens.