See also antibody production methods

B cell activation can occur one of two ways. The first way depends upon TH cells and is called Thymus-dependent (TD). The other way does not and is referred to as thymus-independent (TI).  Antigens which activate the TD route are usually soluble proteins whereas such antigens are usually bacterial cell wall components for the TI route. The vast majority of antigens activate the TD route.

Thymus-independent (TI) Activation: 

The types of antigens which activate the TI route come in 2 types. The first type includes some bacterial cell wall components like lipopolysaccharide (LPS) whereas the second type of antigens are highly repetitious molecules like bacterial cell wall polysaccharides with repeating polysaccaride units. Most of the type 1 antigens that are thymus independent are polyclonal B cell activators (mitogens) in that they are able to activate B cells regardless of their antigenic specificity. Studies involving haptenated polymers have shown that arrays of 20-30 haptens, spaced optimally by 5-10 nm, can activate B cells in the absence of T cell help. In general, these responses are thymus independent type 2 (TI-2) which means that they still depend on some T cell help. The response can become completely T cell independent (TI-1) if determinants are not arranged linearly on a flexible polymer but in a rigid, paracrystalline, two dimensional array. Many viruses examined for induction of TI B cell responses exhibit repetitive, identical neutralizing epitopes that can be defined as highly organized. This includes all nonenveloped viruses that exhibit a highly organized icosahedral structure. Also, many of the enveloped viruses have highly repetitive surface structures.

Thymus-dependent (TD) activation:  

The CD40L (CD40L/CD1154)-CD40 ligand and BAFF-BAFF receptor (BAFFR) cytokine systems, members of the tumor necrosis factor (TNF) superfamily, play critical roles in the homeostatic regulation of B cell functions. The CD40 mediated pathway has been shown to play im portant roles in T cell mediated B lymphoctye activation. Ligation of B cell CD40 by CD40L (CD154) expressed on activated T cells stimulates B cell survival, proliferation, differentiation, isotype switching, upregulation of surface molecules contributed to antigen presentation, development of the germinal ceter (G), and the memory B cell repsonse. Mice deficient in expression of CD40L or CD40 are unalbe to mounta primary or a secondary antibody resposne to a T cell dependent antigen, do not form GCs and do not generate antigen-specific memory B cells. 

Whereas binding of a thymus independent antigen to a B cell provides both signals for B cell activation, a thymus dependent antigen provides signal 1 by cross-linking mIg, but a separate interaction between CD40 on the B cell and CD40L on an activated TH cell is required to generate signal 2. 

The sequence of events for thymus-dependent antigen activation of a B cell is though to involve 1) crosslinking mIg which generates signal 1, which leads to increased expression of class II MHC and costimulatory B7. 2. A TH cell recognizes antigen-class II MHC on B cell membrane and this plus the co-stimulatory signal activates the TH cell. 3) TH cell begins to express CD40L which interacts with the CD40 providing signal 2.

BAFF (also known as BlyS, TALL-1, zTNF4, THANK and TNFSF 13B) is a TNF family member that is important regulator of B cell homeostasis. In BAFF-deficient mice, peripheral B cell survival is severaly perturbed resulting in a complete loss of flollicular and marginal zone B lymphocytes. Conversely, mice overexpressing BAFF display mature B cell hyperplasia and symptoms of systemic lupus erythematosus (SLE). BAFF exerts its effect by binding three receptors: transmembrane activator of and CAML interactor (TACI), B cell maturation antigen (BCMA) and BAFF receptor (BAFFR/BR3). 

CD40 and BAFFFR ligation activate the NFkappa B family of transcription factors, which are critical for the regulation of B cell survival and development. CD40 and BAFFR utilize TRAF (TNF receptor associated factor) molecules as receptor proximal adapters to mediate the activaiton of downstream kinases including IKKs and MAP kinases. 

ACT is an important adapter molecule which is important regulator in signaling pathways mediated by CD40 and BAFF. Endogenous Act1 is recruited to CD40 in B cells upon stimulation with CD40 ligand (CD40L). ACT1 deficient mice show major lymphoid system defects such as lymphadenopathy, hypergammaglobulinemia and production of autoantibodies. A general increase in the numbers of peripheral B cells and CD40 and BAFFR mediated B cell survival is significantly increased in Act1 deficient mice, indicating that Act1 is an important modulator in humoral immune responses by regulating CD40 and BAFFR signaling in B cells. (Li, US2007/0028316). 

What occurs after B cell activation: 

Transduction of activating signals actually involves the Ig-alpha/Ig-beta heterodimers of the BCR. Both cytoplasmic tails of the heterodimers contain several tyrosine residues that can be phosphorylated by 3 members of the src family of tyrosine kinases. When mIg is cross linked with an antigen, each of these protein kinases is activated and they then act to phosphorylate and activate a number of molecules involved in signal transduction.

Germinal centers in lymph nodes arise after exposure to a thymus-dependent antigen. In these centers, B cell differentiation events occur which are 1) affinity maturation, 2) class switching and 3) formation of plasma cells and memory B cells.

Antigen-C3d complexes increase sensitivity to antigen: Antigen-C3d complexes corss-link BCR and CR2-CD19 complex which increases sensitivity of B cells to antigen.

What changes occur after B cell activation

• B cells increase in size and go from G0 to G1 of the cell cycle.

• There is upregulation of MHC classII

• upregulstion costimulatory molecules (B7-2)

• upregulationadhesion molecules (ICAM1)

• upregulationcytokine receptors (IL-2R).

• B cells become receptive to T cell help (protected from fas).

• B cells migrate to outer T zone (altered response to chemokines).

• B cells also enter mitosis if provided with submitogenic does of other stimuli (LPS, CD40L, IL-4)

Activation of Specific Types/Populations of Cells

Rabbit B cells: 

Allison (US 14/776,945, published as (US 2016/003504) discloses obtaining rabit B cells from one or mroe rabbits that have been immunized or exposed naturally to an antigen of interest and separately culturing at least 50 different enriched fractions each comprising rabbit B cells which secrete antibodies against the antigen of interest in the presence of irradiated EL4 feeder cells. 

Endl (US 2014/0315252) discloses co-cultvationg a pool or rabbit os singe deposited rabbit B cells where CD40L expressing CHO cells are used as feeder in the presence of IL-2 and IL-21.