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Antigens: Substances that are recognized by the immune system and induce an immune reaction.

Cell-mediated Immune Responses

JoVE 10896

The cell-mediated immune system is the host’s primary response against invasive bacteria and viruses that cause intracellular infections. It is also essential for fighting against and destroying cancer cells. Furthermore, the cell-mediated immune system plays a role in the rejection of organ transplants or graft tissue.

Phagocytic cells of the innate immune system, such as macrophages or dendritic cells, are the first to recognize a foreign particle. These cells engulf the foreign particle and digest it. Small molecules of the foreign particle, called antigens, remain intact and are presented at the surface of the phagocytic cell. The presentation is facilitated by proteins of the major histocompatibility complex (MHC), which binds the antigen and protrude from the cell. The phagocytic cell is therefore also called an antigen presenting cell (APC). The MHC-antigen complex activates cells of the adaptive immune system, which eventually fight the source of the foreign particle. T cells are a type of lymphocyte that are named after their location of maturation—the thymus. In the thymus, precursor T cells differentiate into two main types, CD4+ and CD8+ T cells. These cell types are named after the surface receptor that determines the cell’s function. All T cells carry T-cell receptors, but the coreceptor CD4

 Core: Biology

Antibody Structure

JoVE 10898

Antibodies, also known as immunoglobulins (Ig), are essential players of the adaptive immune system. These antigen-binding proteins are produced by B cells and make up 20 percent of the total blood plasma by weight. In mammals, antibodies fall into five different classes, which each elicits a different biological response upon antigen binding.

Antibodies consist of four polypeptide chains: two identical heavy chains of approximately 440 amino acids each, and two identical light chains composed of roughly 220 amino acids each. These chains are arranged in a Y-shaped structure that is held together by a combination of covalent disulfide bonds and noncovalent bonds. Furthermore, most antibodies carry sugar residues. The process of adding sugar side chains to a protein is called glycosylation. Both the light chain and heavy chain contribute to the antigen binding site at each of the tips of the Y structure. These 110-130 amino acids are highly variable to allow recognition of an almost unlimited number of antigens. This region is also called the variable region and is part of the antigen binding fragment. Each arm of the Y-shaped unit carries an identical antigen binding site. Antibodies can crosslink antigens: when one arm binds to one antigen and the other arm binds to a second, structurally identical antigen. Crosslinking is facilitated by the f

 Core: Biology

Cross-reactivity

JoVE 10900

The ability of a single antibody to recognize multiple structurally similar epitopes is an important immune defense strategy that enables the host to efficiently defend against many potentially threatening pathogens. However, cross-reactivity also elicits allergy symptoms against related allergens. It is increasingly important to understand the principles of cross-reactivity, as antibodies are actively being developed as therapeutic modalities for diverse diseases, including cancer. Antibodies can initiate an immune response by binding to specific structures on the surface of pathogens or other foreign elements. By definition, anything that is bound by an antibody, and subsequently elicits an immune response, is called an antigen. Often, antigens are proteins on the surface of viruses, bacteria, fungi, and protozoan invaders. The specific sequence of amino acids that is recognized by the antibody is called an epitope. Most epitopes are only 5-6 amino acids long. As such, a single antigen may present several distinct epitopes. Cross-reactivity occurs when two distinct epitopes are structurally similar, and hence are recognized by the same antibody. A major benefit of antibody cross-reactivity is that it provides cross-protective immunity to related pathogens. When a circulating antibody recognizes a viral or bacterial pathogen that it has encountered previ

 Core: Biology

Antibody Generation: Producing Monoclonal Antibodies Using Hybridomas

JoVE 10499

Source: Frances V. Sjaastad1,2, Whitney Swanson2,3, and Thomas S. Griffith1,2,3,4
1 Microbiology, Immunology, and Cancer Biology Graduate Program, University of Minnesota, Minneapolis, MN 55455
2 Center for Immunology, University of Minnesota, Minneapolis, MN 55455
3 Department of Urology, University of Minnesota, Minneapolis, MN 55455
4 Masonic…

 Immunology

Vaccinations

JoVE 10903

Vaccination is the administration of antigenic material from pathogens to confer immunity against a specific microorganism. Vaccination primes the immune system to recognize and mount an immune response faster and more effectively if the real pathogen is encountered. Vaccinations are one of the most efficient ways to protect both individual humans and the general public from disease. A growing anti-vaccination skepticism risks the successes of vaccination programs that helped reduce and, in some instances, eradicate fatal diseases. Vaccines can be administered via oral and intranasal routes, as well as by injection into the muscle (intramuscular), the fat layer under the skin (subcutaneous), or the skin (intradermal). Vaccines contain antigens derived from a specific pathogen. Those containing “dead” antigens, which are intact but unable to replicate, are referred to as inactive vaccines. By contrast, subunit vaccines contain only parts of the pathogen. Some vaccines contain the live pathogen in a weakened (attenuated) form. An attenuated pathogen stimulates the immune system without causing severe disease. Vaccines often contain adjuvants, chemicals that enhance the immune response against the pathogen. When a vaccine is administered, antigen-presenting immune cells (APCs), such as dendritic cells or macrophages, engulf the antigen from the v

 Core: Biology

Affinity and Avidity

JoVE 10899

Antibodies bind to toxins or substances on the surface of cells, bacteria, viruses, or fungi. The substance is called an antigen, and the precise binding site is the epitope. The strength of the antibody-epitope interaction is called affinity. When an antibody binds an antigen by multiple epitopes, the cumulative strength of the interaction is called avidity. The strength of the interaction influences the elicited immune response. By definition, everything that an antibody can bind to is called an antigen. An antigen can be from another organism, a foreign particle such as a toxin, drug or a physical intruder (e.g., splinter), or the body’s own tissue. The exact point of contact where the antibody binds is called the epitope of the antigen. The strength with which an antibody binds to an epitope is called its affinity. When the body encounters an antigen for the first time, only some of the available antibodies in the body bind the antigen by chance. The affinity of the antibody is likely low. However, the adaptive immune system earns its name by reacting adaptively to antigens that the organism encounters during its lifetime. Once an antigen has been recognized for the first time, a complex selection process leads to the production of antibodies with higher affinity against this specific antigen. Hence, the affinity of the antibody for a particul

 Core: Biology

Humoral Immune Responses

JoVE 10897

The humoral immune response, also known as the antibody-mediated immune response, targets pathogens circulating in “humors,” or extracellular fluids, such as blood and lymph. Antibodies target invading pathogens for destruction via multiple defense mechanisms, including neutralization, opsonization, and activation of the complement system. Patients that are impaired in the production of antibodies suffer from severe and frequent infections by common pathogens and unusual pathogens. B lymphocytes, also called B cells, detect pathogens in the blood or lymph system. Although B cells originate in the bone marrow, their name is derived from a specialized organ in birds in which B cells were first discovered, the bursa of Fabricius. After release from the bone marrow, B cells mature in secondary lymphoid tissues, such as the spleen, lymph nodes, tonsils and mucosa-associated lymphoid tissue throughout the body. B cells bind to specific parts of a pathogen, called antigens, via their B cell receptors. In addition to antigen binding, B cells require a second signal for activation. This signal can be provided by helper T cells or, in some cases, by the antigen itself. When both stimuli are present, B cells form germinal centers, where they proliferate into plasma cells and memory B cells. All cells that are derived from a common ancestral B c

 Core: Biology
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