生物大分子结构与功能第16章免疫球蛋白PPT.pptx

第十六章： 免疫蛋白结构When a foreign substance - a virus, a bacterium, or even a foreign protein-invades the tissues of a higher vertebrate (like a human), the organism defends itself by what is called the immune response (免疫应答，免疫反应). The immune response is a first line of defense against infection and probably against cancer cells as well. The adaptive immune response has two facets方面:1. Humoral immune response (体液免疫应答)- Lymphatic cells called B lymphocytes synthesize specific immunoglobulin molecules that are excreted from the cell and bind to the invading substance. Binding either precipitates the foreign substance or marks it for destruction by cells called macrophages.2. Cellular immune response (细胞免疫应答) - Lymphatic cells called T lymphocytes, bearing immunoglobulin-like molecules on their surfaces, recognize and kill foreign or aberrant异常的 cells.Humans have five classes of immunoglobulins, each with different biological functions. The most abundant class is IgG, a Y-shaped protein with two heavy and two light chains. The domains near the upper ends of the Y are hypervariable within the broad population of IgGs and form two antigen-binding sites.A given immunoglobulin generally binds to only a part, called the epitope, of a large antigen. Binding often involves a conformational change in the IgG, an induced fit to the antigen.胎盘初乳Immunoglobulin Structure – Each immunoglobulin monomer consists of four chains, two heavy chains (M = 53,000 each) and two light chains (M = 23,000 each), held together by disulfide bonds. In each chain there are constant domains (identical in all antibodies of a given class) and a variable domain. Variations in the amino acid sequence (and therefore the tertiary structure) of the variable domains of the light and heavy chains confer the multitudinous多样的 specificities of antigens to different determinants.Precipitating Antigens - A large protein, a virus, or a bacterial cell has many different potential antigenic determinants on its surface that