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Helper T Cells and APCs

• Inactive helper T cells recognize MHC-antigen complex on antigen-presenting cells and bind with CD4 protein.
• If costimulation occurs, activated helper T cells undergo clonal selection into active and memory helper T cells.
• Active helper T cells respond within hours of costimulation by secreting cytokines, acting as a costimulator for other T cells, and enhancing proliferation of B cells and natural killer cells.

Cytotoxic T Cells and Body Cells

• Inactive cytotoxic T cells recognize MHC-antigen complex on infected or abnormal body cells and bind with CD8 protein.
• If costimulation occurs, activated cytotoxic T cells undergo clonal selection into active and memory cytotoxic T cells.
• Active cytotoxic T cells leave lymphatic tissue and secrete substances to kill infected target body cells, including:
  • Granzymes, which trigger apoptosis of target cells.
  • Perforin, which causes cell membrane cytolysis of target cells.
  • Granulysin, which destroys intracellular microbes.
  • Lymphotoxin, which fragments target cell DNA.

Types of Adaptive Immunity

• Cell-mediated immunity, which is effective against pathogens, cancer cells, and transplant tissues, involving cytotoxic T cells directly attacking invading antigens.
• Antibody-mediated immunity, which is mainly effective against extracellular pathogens, including viruses, bacteria, and fungi, involving activated B cells transforming into plasma cells that synthesize and secrete specific antibodies.

Adaptive Immunity

• Clonal selection is the process by which a lymphocyte divides and differentiates into a clone of cells that can recognize the same specific antigen as the original activated cell.
• Two major types of cells are produced from the clone:
  • Effector cells, which carry out the immune response to destroy or inactivate antigen-bearing substances.
  • Memory cells, which do not participate in the current immune response and have a long lifespan, dividing and differentiating into more effector and memory cells if the same antigen is encountered in the future.

Processing and Presentation of Antigens

• Processing of endogenous antigens involves infected cells having antigens inside the cell, including viral proteins, bacterial toxins, and abnormal proteins synthesized by cancerous cells.
• Fragments of antigen bind to MHC and are inserted into the cell membrane, signaling as an infected cell for immune response.

Cytokines

• Cytokines are small hormones that stimulate or inhibit many normal cell functions, such as growth and differentiation.
• They are secreted by lymphocytes and antigen-presenting cells for immune response.

Cell-Mediated Immune Response

• Begins with the activation of a small number of T cells by a specific antigen.
• Helper T cells with CD4 and cytotoxic T cells with CD8 have different coupling proteins for antigen-MHC complex.
• Activated T cells undergo clonal selection into active and memory T cells.

Antibody-Mediated Immune Response

• Begins with the activation of B cells by a specific antigen.
• B cells can respond to unprocessed antigens, and costimulation by helper T cell cytokine after forming an antigen-MHC complex in their own cell membrane produces a more intense response.
• Activated B cells undergo clonal selection into plasma cells and memory B cells, with plasma cells secreting antibodies specific to the antigen.

Antibodies

• Antibodies are globulin plasma glycoproteins (immunoglobulins) that form an antigen-antibody complex with a specific complementary antigen epitope.
• There are 5 classes of antibodies based on chemistry and structure, each with its own biological role:
  • IgG
  • IgA
  • IgM
  • IgD
  • IgE