Adaptive Immunity: T and B Cells

Overview of Adaptive Immunity

• Adaptive immunity recognizes and defends against specific invaders and their products.
• Attributes include specificity to unique antigens and memory to the same antigens.
• It is not functional at birth and involves lymphocytes.

Types of Lymphocytes

• B lymphocytes (B cells) mediate humoral immunity against extracellular pathogens and toxins outside of cells (in the blood).
• T lymphocytes (T cells) mediate cell-mediated immunity against intracellular pathogens, killing cells that are already infected.

Elements of Adaptive Immunity

• Both T and B cells require recognition of antigens.
• Antigens are molecules like surface proteins, carbs, or lipids that the body recognizes as foreign.

Types of Antigens

• Exogenous antigens are found outside a body's cells, including toxins and other parts of microbes.
• Endogenous antigens are produced by microbes inside a body's cells during reproduction.
• Autoantigens are self-made antigens and derived from our own DNA.

Epitopes

• Antigens are recognized by three-dimensional regions called epitopes.

Major Histocompatibility Complexes (MHC)

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

Types of MHC Proteins

• MHC I is present on all nucleated cells, but not red blood cells.
• MHC II is present on antigen-presenting cells (APCs) including 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
• MHC I presents the epitope if it's a nucleated cell.
• MHC I and MHC II both present the epitope if it's an antigen-presenting cell (APC).

T Cell and B Cell Receptors

• T cells and B cells recognize pathogens displayed on MHCs.
• T cells have a unique T Cell Receptor (TCR) protein for each pathogen epitope
• B cells have a unique B Cell Receptor (BCR) for each pathogen epitope.
• divisions of T/B cells occurs, once recognition occurs

Recognition and Multiplication

• Recognition usually requires an Antigen Presenting Cell (APC).
• Examples include macrophages, B cells, and Dendritic Cells
• T/B cell multiplication requires an interaction between T/B cells and APCs, called Costimulation
• Costimulation results in surface molecules on each cell and release of Cytokines.

Immune Response Cytokines

• Interleukins (ILs) signal among leukocytes.

T Cell Activation

• Involves cell-mediated attack of infected cells.

Types of T Lymphocytes

• Cytotoxic T lymphocytes (T8, Tc) directly kill other cells via perforins.
• Helper T lymphocytes (T4, Th) help make the army, multiply B cells, and cytotoxic T cells and Includes type 1 (Th1) and type 2 (Th2).
• Memory T lymphocytes remember same epitope in the future .
• Regulatory T lymphocytes involve autoimmune diseases and inhibit autoantigen attack.

Cell-Mediated Response Key Points

• Infected cells display MHC I.
• APCs display both MHC I and MHC II.
• Interleukin 12 produced by APCs causes Th to become Th1.
• Interleukin 2 is produced by both Th1 and Tc.
• Tc now attacks infected cells displaying MHC1

B Cell Activation

• Humoral immunity involves the production of ANTIBODIES to attack extracellular epitopes.

Types of Lymphocytes Used

• B lymphocytes recognize foreign epitopes and become plasma cells.
• Plasma cells produce antibodies.
• Helper T lymphocytes (T4, Th) helps multiply B cells and cytotoxic T cells
• Memory B lymphocytes remember same epitope in the future.

B-Cell Response Key Points

• APCs (dendritic and B cells) display MHC II.
• Interleukin 4 causes Th to become Th2. -Interleukin 4 also causes B cells to become Plasma cells.
• Antibodies from Plasma Cells attack exogenous antigens.

Basic Antibody Structure

• Contains 2 heavy chains and 2 light chains.
• Each chain has a variable region creating an Antigen Binding Site

Stem Characteristics

• The antibody stem of the heavy chains leads to different types of antibodies.

Antibodies

• IgM is the first antibody produced during the acute phase.
• IgG is the most common and longest-lasting antibody during chronic conditions
• IgA is associated with body secretions.
• IgE is involved in response to parasitic infections and allergies.
• IgD - exact function is not known

Antibody Functions

• Antibodies doesn't kill microbes directly and assist to get them killed
• These can have different effect: phagocytosis, lysis, agglutination(attaches microbes together) , and neutralize.

Memory

• Vaccination is one method of obtaining memory against a specific epitope.