T-dependent versus T-independent antigens
Protein antigens are T-dependent. A B cell that binds a protein antigen internalizes it, displays peptides on MHC class II, and presents them to a CD4 helper T cell. The helper cell then delivers two things: contact-dependent signals through CD40 ligand binding CD40 on the B cell, and cytokines that direct class switching. Polysaccharide antigens are T-independent; they can activate B cells without help, but the response is limited to low-affinity IgM with little memory and little switching.
What CD4 help supplies to the B cell
- CD40 ligand engagement of CD40, which is required for survival, proliferation, and germinal center formation
- Cytokines such as IL-4, IL-5, and IL-21 that direct class switching to IgG, IgA, or IgE
- Signals that drive somatic hypermutation and selection of high-affinity variants in the germinal center
- Support for memory B-cell generation
The consequence: normal B cells, poor antibody
When CD4 help is absent, B cells are present and can still respond to T-independent polysaccharides with IgM, but they cannot mount a strong class-switched, high-affinity response to protein antigens. Total immunoglobulin may be low or near normal, yet functional antibody is inadequate. This is why a T-cell defect can present with an infection profile that overlaps humoral failure, and why distinguishing the two requires looking at the T-cell compartment rather than only at antibody levels.
Connecting back
The B-cell chapter separated antibody effector functions and showed that normal total immunoglobulin does not guarantee functional antibody. T-cell failure is a second route to the same problem: the B cell is intact, but the help it needs to make useful antibody is missing.