8. The immune system: Adaptive immunity

• innate immune response is rapid and often effective but is not always sufficient to eliminate infection
• innate immunity has no memory
• another arm of the immune system is needed to clear more serious infections and respond to pathogens in a faster, better manner with memory

• initiated when circulating T lymphocytes recognize specific antigens on the surface of dendritic cells or other antigen presenting cells
• these T cells proliferate and differentiate into antigen specific effector cells
• T cells usually target intracellular pathogens
• B cells that recognize antigens differentiate into plasma cells and secrete anitgen-specific antibodies
• antibodies usually target extracellular microorganisms and their toxins

• immature B cells in bone marrow, B cells mature in lymphoid tissue
• express IgM and IgD - antigens bind and activate B cells
• once activated undergo clonal expansion - differentiate into plasma cells that secrete antibodies
• some responses are T-cell independent
• usually directed at extracellular microorganisms and toxins

a protein (immunoglobulin) produced by plasma cells (B cells) that recognize a target molecule (antigen) to be foreign

protect against bacteria, viruses, enhance phagocytosis, neutralize toxins (crosses placental barrier)

protects against infection on mucous membranes

protects against early phase of infection; comprises anti-A and anti-B antibodies of ABO blood groups, can lyse microbes

serves as receptors for activation of B cells

invovled in allergic responses, triggers release of histamine

• 5-10 days
• IgM>IgG
• limited affinity to antigen

• 1-3 days
• mostly IgG
• greater affinity for antigen

• variable regions are inherited as gene segments
• how an almost unlimited repertoire of antibodies get generated from a limited set of genes

• T lymphocytes mature in thymus, migrate to lymphoid tissues
• become activated when encountering antigen presented by antigen presenting cell
• once activated, multiply and differentiate - cytotoxic T cells and helper T cells
• once antigen cleared, most cells undergo apoptosis, others become memory cells
• usually directed at intracellular pathogens

• T helper cells (CD4):
• - Th1 cells = involved in control of intracellular pathogens, produce cytokines and chemokines to attract other T cells and marcrophages
• -Th2 cells = activates B cells into antibody producing plasma cells
• cytotoxic T cells (CD8): kill infected cells
• regulatory T cells: suppress activation of immune response, play a role in tolerating self-antigens

antigens are presented on the cellular surface in conjunction with major histocompatibility complex (MHC) molecule

• present peptide fragments from proteins in the cytosol (intracellular)
• recognized by CD8 cytotoxic T cells

• present peptides derived from phagosomes (extracellular)
• recognized by CD4 T helper cells