Humoral immunity
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Immunoglobulin molecule
Immunoglobulin as B cell receptor
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Primary and secondary responses of B cell
| Primary | Secondary | |
|---|---|---|
| Type | Naive B cell | Memory B cell |
| Lag time | 4-7 days | 1-3 days |
| Peak | 7-10 days | 3-6 days |
| Intensity | Varies | 100-1000 times higher than primary |
| Ig class | IgM | IgG |
| Antigen | T-cell independent | T-cell dependent |
| Affinity | Low | High |
B cell activation by cross-linking
- Cross-linking of multiple B cell receptors (immunoglobulins) by antigen is needed to activate B cell intracellular signaling
T cell independent antigens
- Antigen with repetitive structure (e.g., lipopolysaccharides) cross-link B cell receptors in wider ranges as well as non-BCR structures on B cell surface, resulting in activation of B cell without T cell (T cell independent antigen)
T cell dependent antigens
- Antigen of proteins cross-link B cell receptors but at the same time needs antigen presentation by TCR of CD4+ T cell onto pMHC class II on B cell via CD28 and/or CD40 co-stimulation
| T independent | T dependent | |
|---|---|---|
| Antigen | Repetitive structure | Protein |
| Ig class switching | Little→IgM main | Induce→IgG |
| Affinity maturation | Doesn't occur | Occur |
| B cell memory | No | Yes |
| B cell longevity | Long plasma cell | Short plasma cell |
Affinity maturation at germinal center
- Somatic hypermutation at dark zone of lymph nodes
- Activation-induced cytidine deaminase (AID) induces random point mutations on V regions of both of heavy chain and light chain
- Affinity maturation at light zone of LN
- Follicular dendritic cells present antigens to select only B cells that randomly obtained affinitive BCR (Ig)
