Ch 7 Humoral Immune Responses Flashcards
What are the signals that induce B cell responses to protein antigens and polysaccharide antigens?
The signals that induce B cell responses to protein antigens include binding of the protein to membrane immunoglobulin (Ig) on the B cell, and subsequent signals delivered by helper T cells, which include secreted cytokines that bind to cytokine receptors on the B cell, and CD40 ligand on activated helper T cells, which binds to CD40 on the B cell. The signals that induce B cell responses to a polysaccharide antigen are generated by the binding of the polysaccharide, which is polyvalent, to two or more membrane Ig molecules on the B cell, thereby cross-linking two or more B cell receptors, and activating signal transduction pathways. Complement fragments bound to antigens engage the complement receptor CR2 on B cells, which generate signals that increase B cell activation. This is especially important for polysaccharide and other nonprotein antigens.
What are some of the differences between primary and secondary antibody responses to a protein antigen?
Secondary antibody responses develop more quickly and are of greater magnitude than primary immune responses. Secondary responses to protein antigens also differ from primary responses in that the antibodies produced are higher-affinity IgG, IgA, or IgE antibodies, whereas low-affinity IgM antibodies are mainly produced in the primary response.
How do helper T cells specific for an antigen interact with B lymphocytes specific for the same antigen? Where in a lymph node do these interactions mainly occur?
B cells express membrane Ig molecules that bind intact proteins and facilitate their endocytosis. The internalized proteins are processed into peptides, and the peptides are bound to class II MHC molecules and displayed on the B cell surface. Helper T cells may recognize a peptide-MHC complex presented by a B cell, leading to activation of the T cell. Thus, a B cell and a T cell recognize different parts of the same protein in sequence. The B cell recognition occurs first and is independent of the T cell, and the T cell recognition is second and requires B cell presentation of a peptide fragment of the antigen. After antigen recognition and activation, the two cell types migrate toward each other in response to chemokines. The initial B-T interactions occur at the interface of the B and T cell zones of lymph nodes or spleen, just outside the follicles.
What is affinity maturation? How is it induced, and how are high-affinity B cells selected to survive?
Affinity maturation is the increase in the average affinity of antibodies for a protein antigen that occurs as an immune response develops over time. It occurs in the germinal center reaction, where signals from follicular helper T cells, including cytokines and CD40 ligand, induce point mutations in the variable-region genes of the heavy-chain and light-chain loci. B cells in which these mutations result in increased affinity of the antibodies they produce have a selective advantage for binding to the antigen displayed by follicular dendritic cells. These B cells receive signals through the B cell receptor that prevent apoptotic death, and thus the highest-affinity B cells are selected to survive and develop into plasma cells.
What are the characteristics of antibody responses to polysaccharides and lipids? What types of bacteria stimulate mostly these types of antibody responses?
Antibodies produced in response to T-independent polysaccharide and lipid antigens are predominantly IgM antibodies of relatively low affinity. These antigens are inefficient at generating long-lived plasma cells and memory B cells, because of the absence of helper T cell signals, so the IgM response to TI antigens wanes relatively quickly.
