International A Level Biology / Immunity

Antigens, antibodies and antigen-presenting cells

Edexcel International A Level Biology (YBI11), chapter 18: Immunity, specification point 6.9. Free lesson.

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Antigens trigger immune responses, while antibodies bind to specific antigens and help the body deal with pathogens. Knowing the antibody structure and how macrophages, T helper cells and B cells work together helps you explain immune responses accurately.

What an antigen is

An antigen is a molecule that triggers an immune response. It is often a protein or glycoprotein, which is a protein with sugar chains. Antigens may be on a pathogen's surface or free, such as a toxin. Your own cells' antigens normally do not trigger an immune response. When describing what an antibody binds to, or is specific to, write antigen, not pathogen or bacterium.

How an antibody is built

An antibody is a Y-shaped protein made of four polypeptide chains, so it has quaternary structure. Its two long chains are heavy chains and its two shorter chains are light chains. Disulfide bridges hold the chains together. The constant region is the stem and lower arms. Receptors on phagocytes bind to it. The variable region is at the tips of the arms. Its amino acid sequence differs between antibodies, giving it a different shape. Each variable region forms an antigen-binding site with a shape complementary to one particular antigen. There are two sites, one on each arm.…

How antibodies help the body

Antibodies bind to antigens and help in three ways. They do not kill pathogens themselves. Instead, they make pathogens easier for phagocytes to engulf, or block viruses and toxins from binding to their targets. Agglutination: each antibody can bind antigens on two pathogens. This clumps pathogens together, so they cannot spread as easily and are easier to engulf. Neutralisation: an antibody binds to a virus or toxin and stops it binding to a body cell or its target. Opsonisation: antibodies coat a pathogen as markers. Phagocyte receptors bind to the antibodies' constant regions,…

How macrophages lead to antibody production

Antibody production involves a macrophage, a T helper cell and a B cell. The correct B cell is activated and becomes a plasma cell, which releases antibodies that match the antigen.

How to explain antibody action in an exam

For a question about how antibodies help phagocytes destroy bacteria, link antibody binding to antigens with two actions and their effect on phagocytosis. The antibodies do not destroy the bacteria themselves. Plasma cells are packed with rough endoplasmic reticulum and Golgi because antibodies are proteins made for export.

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