<P> Now activated, B cells participate in a two - step differentiation process that yields both short - lived plasmablasts for immediate protection and long - lived plasma cells and memory B cells for persistent protection . The first step, known as the extrafollicular response, occurs outside lymphoid follicles but still in the SLO . During this step activated B cells proliferate, may undergo immunoglobulin class switching, and differentiate into plasmablasts that produce early, weak antibodies mostly of class IgM . The second step consists of activated B cells entering a lymphoid follicle and forming a germinal center (GC), which is a specialized microenvironment where B cells undergo extensive proliferation, immunoglobulin class switching, and affinity maturation directed by somatic hypermutation . These processes are facilitated by T cells within the GC and generate both high - affinity memory B cells and long - lived plasma cells . Resultant plasma cells secrete large amounts of antibody and either stay within the SLO or, more preferentially, migrate to bone marrow . </P> <P> Antigens that activate B cells without T cell help are known as T cell - independent (TI) antigens and include foreign polysaccharides and unmethylated CpG DNA . They are named as such because they are able to induce a humoral response in organisms that lack T cells . B cell response to these antigens is rapid, though antibodies generated tend to have lower affinity and are less functionally versatile than those generated from T cell - dependent activation . </P> <P> As with TD antigens, B cells activated by TI antigens need additional signals to complete activation, but instead of receiving them from T cells, they are provided either by recognition and binding of a common microbial constituent to toll - like receptors (TLRs) or by extensive crosslinking of BCRs to repeated epitopes on a bacterial cell . B cells activated by TI antigens go on to proliferate outside lymphoid follicles but still in SLOs (GCs do not form), possibly undergo immunoglobulin class switching, and differentiate into short - lived plasmablasts that produce early, weak antibodies mostly of class IgM, but also some populations of long - lived plasma cells . </P> <P> Memory B cell activation begins with the detection and binding of their target antigen, which is shared by their parent B cell . Some memory B cells can be activated without T cell help, such as certain virus - specific memory B cells, but others need T cell help . Upon antigen binding, the memory B cell takes up the antigen through receptor - mediated endocytosis, degrades it, and presents it to T cells as peptide pieces in complex with MHC - II molecules on the cell membrane . Memory T helper (T) cells, typically memory follicular T helper (T) cells, that were derived from T cells activated with the same antigen recognize and bind these MHC - II - peptide complexes through their TCR . Following TCR - MHC - II - peptide binding and the relay of other signals from the memory T cell, the memory B cell is activated and differentiates either into plasmablasts and plasma cells via an extrafollicular response or enter a germinal center reaction where they generate plasma cells and more memory B cells . It is unclear whether the memory B cells undergo further affinity maturation within these secondary GCs . </P>

In the adaptive immune system the primary role of b cells is to