Adaptive Immunity

ADAPTIvE ImmunITy Adaptive Immunity The adaptive immune responses share two key characteristics: • They are specific (i.e. they can differentiate between different types of pathogens and respond accordingly) • They are adaptive (i.e. they produce a heightened response upon re-exposure – there is immunological memory) Antigen Recognition Antibodies Antigens are substances that the body recognise as foreign and that can elicit an immune response Antibodies are proteins produced by B lymphocytes that are specific to a given antigen (they are also called immunoglobulins) Antigens are presented to lymphocytes via identification markers on the surface of native cells (MHC molecules) • MHC I markers are found on all body cells (except RBCs) and present endogenous antigens (cell-mediated response) • MHC II markers are on innate immune cells (macrophages) and present exogenous antigens (humoral response) Variable region (binds to the antigen) Light chain (×2) Heavy chain (×2) Constant region (site for opsonisation) Role of Lymphocytes Humoral Immunity (targets ‘non-self ’) • B cells each produce one specific type of antibody • Macrophages or dendritic cells present antigen fragments (via MHC II markers) to helper T lymphocytes (TH cells) • TH cells release cytokines and activate the antigen-specific B cells (which rapidly divide to form many plasma cells) • The plasma cells make antibodies specific to the antigen • A small proportion of B cell clones differentiate into long-lasting memory B cells (for long-term immunity) Cell Mediated Immunity (targets ‘self ’) • Infected cells present antigens on their MHC I markers • Antigens are recognised by cytotoxic T cells (and TH cells) • Cytotoxic T lymphocytes (TC cells) bind to the infected cell and trigger its destruction (via perforating enzymes) • TH cells stimulate the formation of memory TC cells • TC cells can target virus-infected cells and tumor cells • Suppressor T cells regulate the action of TC cells in order to prevent sustained T cell activation (i.e. autoreactivity) B Cell Pathogen Virus Macrophage + TH Cell antibodies Infected cell + TC cell Lysed cell Immune System Disorders Immunodeficiency • HIV is a retrovirus that infects helper T cells (TH cells) • It is usually transmitted via the exchange of bodily fluids (e.g. sex, breastfeeding, transfusions, injections, etc.) • HIV is integrated into the genome of infected TH cells • After a prolonged period of inactivity, it becomes active and lyses the TH cell as it begins to spread • This results in an inability to produce antibodies and a general loss of immunity (disease is called AIDS) Hypersensitivity • Allergens are substances that trigger an immune response despite not being inherently harmful (e.g. peanut allergy) • When a B cell is activated by an allergen, it makes large quantities of allergen-specific antibodies (IgE) • These IgE antibodies bind to mast cells and ‘prime’ them • Upon re-exposure to the allergen, the sensitised mast cells release large quantities of histamine (causes inflammation) • This inflammatory response is called an allergic reaction