B Cell

B cell activation
T cell dependent

1. B cell receptors (BCRs) cluster by recognizing epitopes of a pathogen. To promote clustering, complement receptor type 2 (Cr2) on the B cell can anchor invaders that are opsonized by C3b, bringing the pathogen closer to the BCRs. This is an example of how the innate immune system identifies the invaders for the adaptive.
2. CD40L of T cells ligate CD40 of B cells, providing the signal for cell activation. (Therefore, APCs activate T cells activate B cells).

T cell independent (faster activation, recognizes a range of antigen other than protein)

1. Repeated epitopes cluster BCRs, which substitutes for CD40L costimulation.
2. Battle cytokines, such as IFN-γ, fully activate B cells.

What is clustering?
When BCRs are brought close together, internal signaling proteins are concentrated enough to begin an enzymatic chain reaction.

Activated B cells

Fully activated B cells have the choice to undergo class switching (from IgM) or somatic hypermutation.

Class switching
Changes of the gene encoding the tail or heavy chain of the antibody affect the antibody type. This is done by the cutting and pasting of constant region segments. Remember that "VJC" is the light chain, and "VDJC" is the heavy chain.

Somatic hypermutation
"VDJ" gene segments undergo mutation at a high rate to create and circulate more fine-tuned B cells.

Antibodies

IgG/IgE isotype

IgM

• First evolved aB. 
• Appearance of five IgGs stuck together.
• Good at fixing complement. C1 proteins bind to the Fc region. Two C1 clusters activate the complement cascade to form C3b. This allows the complement system to direct an attack on any bacteria, even those that resist complent protein attachment.

IgG

• Unique in that it passes the placenta. 
• Longest lived aB. 
• Abundant in blood. 
• Can fix complement with at least two IgGs, but not well. 

IgA

• Most abundant in body, in mucosal linings.
• Secreted in milk. 
• Has a clip that holds together two IgGs at the tail region. The clip helps transport IgA in and out of the intestine, and makes IgA resistant to acid and enzymes. 
• Does not fix complement.

IgE

• Loaded onto mast cells when the body is first exposed to an allergen. 
• Degranulates mast cells upon second exposure where IgE binds to the same allergen. This causes an allergic reaction.