Adaptive Immunity and B Cells

Adaptive Immunity and B Cells

• Adaptive immunity is slower than innate immunity, but is antigen-specific.
• Adaptive immunity has four key features: specificity, versatility, memory, and tolerance.
• Specificity: Adaptive immunity responds to a particular antigen. B and T cells have receptors for specific antigens.
• Versatility: Adaptive immunity can respond to any antigen. Different B and T cells have different receptors.
• Memory: Adaptive immunity "remembers" previous encounters with antigens. Some activated B and T cells last long term.
• Tolerance: Adaptive immunity responds to foreign substances, but ignores normal tissues. B and T cells with self-reactive receptors are deleted or not activated.

How Adaptive Immunity Features are Achieved

• Specificity is achieved because both B and T cells have receptors that recognize specific shapes of antigens.
• Versatility is achieved because different B and T cells have different receptors.
• Memory is achieved because some activated B and T cells last long term.
• Tolerance is achieved through the deletion or inactivation of B and T cells with receptors that recognize self.

B Cell Activation

• Antigens bind to specific B cell receptors.
• The antigen is internalized, digested, and combined with MHC molecules.
• The antigen-MHC complex is transported to the cell surface.

Helper T Cell Activation

• A specific helper T cell recognizes the antigen-MHC complex on the B cell surface.
• The helper T cell becomes activated.
• The activated helper T cell secretes cytokines.

Antibody Production and Responses

• Activated B cells divide—some differentiate into plasma cells.
• Plasma cells secrete antibodies.
• Other activated B cells become memory B cells.

Antibody Functions

• Antibody functions include clumping antigens, inactivating bacterial toxins, acting as opsonins to tag antigens for phagocytosis, triggering degranulation, activating the complement system, and activating B lymphocytes.

Antibody Classes

• IgG is the most common circulating antibody and is transferred across the placenta.
• IgM is the first antibody to be secreted in response to a new antigen.
• IgA crosses epithelial cells and is found in breast milk.
• IgE fights parasites and is associated with allergic reactions.
• IgD are B cell receptors.

Types of Adaptive Immunity

• Active immunity occurs when a person is exposed to a pathogen and generates memory B and T cells.
• Passive immunity occurs when antibodies are transferred from another organism; it is temporary. Examples include natural passive immunity (mother to child) and artificial passive immunity (injection of antibodies).

Coordination of Innate and Adaptive Immunity

• Innate and adaptive immune systems coordinate a response to extracellular bacteria, by recognizing, destroying, and removing pathogens from the body.

Summary

• Adaptive immunity is characterized by specific, versatile, memory function and tolerance of self.
• These features are a result of different receptors and the generation of memory cells.
• B cells are activated through antigen binding, MHC class II presentation, and activation of helper T cells, leading to the production of plasma cells that secrete antibodies.
• Antibodies bind to antigens, block their action, and activate the immune system.