Adaptive Immunity

• Chapter 16 discusses adaptive immunity, which allows the body to recognize and defend against specific invaders and their products.

Attributes of Adaptive Immunity

• Specificity is due to the recognition of unique antigens
• Memory allows for recognition of the same antigens
• This immunity is not functional at birth

Lymphocytes

• Adaptive immunity depends on the activity of lymphocytes
• The two main types are B Lymphocytes (B cells) and T Lymphocytes (T cells)

B Lymphocytes (B cells)

• Involved in Humoral immunity, attacking invaders outside of cells in the blood
• Target extracellular pathogens and toxins

T lymphocytes (T cells)

• Involved in Cell mediated immunity, attacking infected cells
• Targets intracellular pathogens

Recognition of Antigens

• Both T and B cells need to recognize antigens
• Antigens are molecules like surface proteins, carbs, and lipids that the body identifies as foreign and worthy of attack

Types of Antigens

• Exogenous antigens are found outside a body's cells, including toxins and components of microbial cell walls, membranes, flagella, and pili
• Endogenous antigens are produced by microbes replicating inside a body's cells
• Autoantigens are self-made antigens

Epitopes

• Epitopes are three-dimensional regions of antigens recognized by the body
• Epitopes make each microbe unique

Major Histocompatibility Complexes (MHC)

• Epitopes are recognized by lymphocytes when displayed as Major Histocompatibility Complexes (MHC)
• MHC's are glycoproteins found in the membranes of most cells
• These hold and position epitopes for presentation to immune cells
• A cell that presents the antigen to the lymphocytes is called an Antigen Presenting Cell (APC)

MHC proteins

• There are two types of MHC proteins
  • MHC I is present on all nucleated cells, except red blood cells
  • MHC II is present on antigen-presenting cells (APCs)
    • These include macrophages, dendritic cells, and B cells

Antigen Processing

• Autoantigens are displayed as self MHC's
• Endogenous and exogenous antigens are processed into MHC proteins to display the pathogen epitopes
  • Depending on the cell this can be either MHC I if a nucleated cell or MHC I and MHC II if an antigen-presenting cell (APC)

T Cell Receptor (TCR) Protein

• T cells and B cells recognize pathogens displayed on MHC's
• T cells possess a T Cell Receptor (TCR) protein unique for each pathogen epitope
• There is a matching TCR for every possible epitope the person can contact
• B cells also possess B Cell Receptors (BCR) unique to pathogen epitopes

Recognition and Multiplication

• Recognition usually needs an Antigen Presenting Cell (APC)
• It can be a Macrophage, B cell, or Dendritic Cell
• Multiplication of T/B cells requires interaction between T/B and APC
• This interaction is called Costimulation
• Costimulation results in surface molecules interacting and the release of Cytokines

Immune Response Cytokines

• Interleukins (ILs) signal among leukocytes

Impact of Recognition and Costimulation

• Recognition and costimulation result in mass production of T/B cells to attack foreign epitopes

Cytotoxic T lymphocyte

• One of the T lymphocytes used, it does actual killing
• It directly kills other cells with perforins to punch holes in cells

Helper T lymphocyte

• A type of T lymphocytes which helps make the army
• It helps multiply B cells and cytotoxic T cells and includes type 1 (Th1) and type 2 (Th2)

Memory T Lymphocytes

• Remember the same epitope in the future

Regulatory T lymphocyte

• It is involved with autoimmune diseases and inhibits autoantigen attack

Additional important points for cells

• Infected cells display MHC I foreign epitopes
• APC displays both MHC I and MHC II
• Interleukin 12 produced by the APC causes Th to become Th1
• Interleukin 2 is produced by both Th1 and Tc
• Tc now attacks infected cells displaying MHC1 endogenous

B Cell Activation Characteristics

• Uses attacking in the blood before getting into cells, in the plasma
• It includes Humoral Immunity by production of ANTIBODIES to attack extracellular epitopes

Types of Lymphocytes in B Cell Activation

• B lymphocytes initially recognize foreign epitopes and act as APC to become plasma cells
• Plasma cells produce antibodies
• Helper T lymphocytes (T4, Th) help multiply B cells and cytotoxic T cells
• Memory B lymphocytes remember the same epitope in the future

B-cells Additional Important Points

• APC's (both dendritic and B) display MHC II
• Interleukin 4 causes Th to become Th2
• Interleukin 4 also causes B cell to become Plasma cells
• Antibodies from Plasma cells attack exogenous antigens

Antibody Structure

• Basic structure consists of 2 heavy chains, 2 light chains
• Each chain has a variable region creating an Antigen Binding Site
• The site is epitope specific
• The stem of the heavy chains leads to different types of antibodies

Stem Creates Different Antibodies

• Different Antibody types exist: IgM, IgG, IgA, IgE, and IgD
• IgM is the first antibody produced (acute)
• IgG is the most common and longest-lasting (chronic)
• IgA is associated with body secretions (mucous memb)
• IgE is involved in response to parasitic infections and allergies
• IgD exact function is not known

Effect

• Antibodies facilitate phagocytosis, lysis, agglutination/clumping, and neutralization
• Antibodies do not actively kill microbes
• Instead they assist in killing

Method of obtaining Memory

• One way of obtaining memory against a specific epitope is vaccination