Anatomy & Physiology

Anatomy & Physiology Study Guide activation seldom occurs through direct lymphocyte–antigen interaction, and foreign compounds or pathogens entering a tissue commonly fail to stimulate an immediate immune response. Antigen Pr sentation Antigen presentation occurs when an antigen–glycoprotein combination capable of activating T cells appears in a plasma membrane. The structure of these glycoproteins is genetically determined. The genes controlling their synthesis are located along one portion of chromosome six in a region called the major histocompatibility complex (MHC) . These membrane glycoproteins are called MHC proteins , or human leukocyte antigens (HLAs). Individuals have different amino acid sequences and MHC protein shapes. Two major classes of MHC proteins are known: Class I and Class II. An antigen bound to a Class I MHC protein acts as a red flag that in effect signals the immune system to kill it. An antigen bound to a Class II MHC protein tells the immune system to get rid of it. Antigen-presenting cells (APCs) are specialized cells responsible for activating T cell defenses against foreign cells (i.e., bacteria) and foreign proteins. Inactive T cells contain receptors to help differentiate between Class I or Class II MHC proteins. The presence of specific antigens is detected on the binding sites of receptors. If an MHC protein contains any antigen other than the specific target of a particular kind of T cell, the T cell remains inactive. If the MHC protein contains the antigen that the T cell is programmed to detect, binding will occur. This process is called antigen recognition because the T cell recognizes that it has found an appropriate target. Antigen recognition simply prepares the cell for activation. Before activation can occur, a T cell must bind to the stimulating cell at a second site. The vital secondary binding process is called co-stimulation . B Cells B cells respond to antigens by producing specific antibodies. Each kind of B cell carries its own particular antibody molecules in its plasma membrane. If corresponding antigens appear in the interstitial fluid, they will interact with these antibodies. When binding occurs, the B cell prepares to undergo activation. This preparatory process is called sensitization . Since B cells migrate throughout the body, pausing briefly in one lymphoid tissue or another, sensitization typically occurs within the lymph node nearest the site of infection or injury. Memory B cells perform the same role in antibody- mediated immunity that memory T cells perform in cell-mediated immunity. These cells (Memory B) are not considered first responders. Instead, they will stay in reserve until needed for subsequent injuries or infections that involve the same antigens. Antibody Structure Two parallel pairs of polypeptide chains, one heavy and one light, create an antibody molecule. Each chain contains both constant segments and variable segments . The specificity of an antibody molecule depends on the structure of the variable segments of the light and heavy chains. The free tips of the two variable segments form the antigen binding sites of the antibody molecule. The Antigen-Antibody Complex When an antibody molecule binds to its corresponding antigen molecule, an antigen-antibody complex is formed. Once the two molecules are in position, hydrogen bonding and other weak Achieve Page 288 of 368 ©2018