Humoral immunity

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Immunoglobulin molecule

→免疫グロブリンとは#分子としての免疫グロブリン immunoglobulin

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)