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B cell Activation
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B cell Activation

Most antigens require input from Helper T cells* to activate naïve B cells; this is referred to as Thymus-Dependent activation. 1. Naïve B cells bind and internalize antigen. 2. Within the B cell, the antigen is complexed with MHC II and displayed on the B cell surface. 3. When a Helper T cell recognizes and interacts with the antigen-MHC II complex, the B cell is activated. 4. The B cell proliferates and differentiates, giving rise to memory cells, which participate in subsequent immune responses, and, plasma cells, which secrete large quantities of antibodies that circulate in the blood to fight infection.
  • Thymus-independent B cell activation occurs when antigens, such as polysaccharide bacterial products, directly activate B cells. Write that thymus-independent B cell activation results in minimal class switching.
Details of B and T cell interactions:
Two key signals are required
  • The first signal involves MHC recognition by the T cell, and is necessary for the second signal.
— The B cell displays the antigen-MHC II complex, and that it is recognized by the T-cell receptor (TCR) and CD4 proteins of the Helper T cell.
  • The second signal involves co-stimulation by B and T cell proteins:
— Binding of B7-2 with CD-28 triggers cytokine activation. — Binding of CD40 with CD40L (L is for ligand) triggers class switching and affinity maturation.
  • Cytokines released by the Helper T cell influence class switching from IgM/IgD to the other isotypes (IgA, IgG, and IgE).

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