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Immune System Part 2
● Adaptive Defenses
○ Specifically attack invaders that breach the first lines of defense
○ Protect against abnormal cells and pathogens
○ Amplify inflammation
○ Must be primed to foreign substance through exposure
○ Recognize specific invaders
○ Systemic response
○ “Memory”- recognize known antigens and response activated faster
○ Two types
■ Humoral(antibody-mediated)
■ Cellular(cell-mediated)
● Humoral Immunity (B lymphocytes)
○ Produce antibodies
○ Bind to target cell, inactivate or mark for destruction
● Cellular Immunity (T lymphocytes)
○ Kill infected cell
○ Release chemicals that enhance inflammatory response
● Antigen
○ Mobilize and provoke the immune system
○ Recognize molecules that are “non-self”
○ Antigenic determinants
■ Mobilize several different lymphocyte populations

■ Different antibodies
■ Microplastics have little to no immunogenicity
○ Incomplete antigens are not immunogenic but may be if attached to body
proteins( allergic reaction)
● Self antigen: MHC Proteins
○ Protein molecules on the surface of cells not antigenic to self but antigenic to
others in transfusions or grafts
○ MHC Glycoproteins
■ Genes of MHC are unique to the individual
■ Have groove holding self or foreign antigen
■ Lymphocytes only bind antigens on MHC proteins
● Lymphocytes
○ Origin: Red bone marrow
○ B cells mature in red bone marrow
■ T lymphocytes mature in Thymus
○ Maturation
■ Immunocompetence- recognize one specific type of antigen
■ Self tolerance- unresponsive to own antigens
■ Seeding- enter circulation to encounter antigen
○ Activation
■ Antigen encounter develops cell
■ Active cell
○ Proliferation

■ Clones itself
■ Effector cells- fight infection
■ Memory cells- respond to same antigen faster the second time
● Antigen Receptor Diversity
○ Genes determine which foreign substances the immune system will recognize
■ Evolution produces variety of genetic knowledge of microbes
■ Variety of receptors and gene shuffling result in many combinations
● Antigen presenting cells
○ Engulf antigens and present fragments of antigens to T cells for recognition
○ Dendritic cells ( most effective antigen presenter)
○ Macrophages
○ B lymphocytes
● T cells mature in thymus under negative and positive selection pressure
○ Positive- T cells that can recognize self MHC proteins and those that can’t self
destruct
○ Negative selection- prompts apoptosis of T cells that bind to self antigens
displayed by self MHC
■ Clonal deletion ensures self- tolerance
● Humoral Immune Response
○ B cells provoke humoral response
○ Produce antibodies for specific antigens
● B Cell activation
○ B cells are activated when antigens bind to surface receptors cross linking them

○ Triggers receptor-mediated endocytosis of cross linked antigen receptor
complexes and start cloning
○ Clone cells become plasma cells, antibody secreting effector cells
■ Secrete specific antibodies
■ Antibodies circulate inactivating and marking invaders for destruction
○ Clone cells become memory cells
■ Immunological memory to exposure to same invader
● Immunological Memory
○ Primary response- cell proliferation and differentiation upon exposure to antigen
for the first time
○ Secondary response- re-exposure to same antigen
● Active and Passive Humoral Immunity
○ Active
■ Natural- contact with pathogen
■ Artificial- vaccination
○ Passive
■ Natural- antibodies passed from mother to baby
■ Artificial- injection of exogenous antibodies (gamma globulin)
● Antibodies
○ Bind to antigen detected by B cells
○ Plasma B cells can switch from making one type of antibodies to another while
maintaining specificity
○ IgM is initially released and IgG is released in the secondary response

● Cellular Immune Response
○ T cells provide defense against intracellular antigens
■ Cancer
○ Some T cells directly kill cells
● Types of T cells
○ CD4- become helper T cells that activate B cells, other T cells, and
macrophages-> direct adaptive immune response
○ CD8- become cytotoxic T cells that are capable of destroying cells harboring
foreign antigens
○ Helper cells and cytotoxic cells are activated
○ CD4 and CD8 are naive cells
● Class 1 MHC Proteins
○ Displayed by all nucleated cells
○ Recognized by CD8 and cytotoxic T cells
○ Endogenous proteins (cancer response)
● Class 2 MHC Protein
○ Recognized by APC
○ CD4 cells and Helper T cells
○ Exogenous proteins ( pathogen)
● Activation and Differentiation of T cells
○ Antigen Binding
○ Co stimulation
■ Anergy- T cells becomes tolerant to antigen without co stimulation

○ Proliferation and Differentiation- activated T cells differentiate and proliferate in
response to cytokines
● Helper T cells
○ Amplify innate defenses
○ Play central role in adaptive immune response, both humoral and cellular
● Once primed by APC presentation of antigen, helper T cells
○ Help activate B cells and other T cells
○ Induce T and B cell proliferation
● Activation of B cells
○ Helper T cells activate B cells via MHC 2 receptors
● Activation of CD8 cells
○ Co stimulated by Helper T cells to become cytotoxic cells
● Cytotoxic T cells
○ Attack and kill infected cells
■ Cancer, viruses, parasites
○ Identify foreign antigens on MHC 1 and binds to target cell
○ Release perforins and granzymes by exocytosis
○ Perforins insert into target cell membrane and form pores
○ Granzymes enter through pores and activate other enzymes that trigger apoptosis
● Cytotoxic T cells and Natural Killer cells use the same mechanism
● Regulator T cell
○ Slows or stops immune activity
○ Prevents autoimmune reaction and secretes inhibitory cytokines

○ Suppressed self reactive lymphocytes
○ Specific suppression of immune response



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