To secrete antibodies.
Lymphocytes are really smart. They are small cells, so they don’t try to eat up or do a chemical attack on the prey like the granulocytes. Instead, they shoot antibodies from a safe distance.
Antibodies don’t kill directly. They simply target the enemy. Targeted enemies get killed by bigger cells easily.
How does the B cell recognize the enemy?
Membrane bound immunoglobulin (Ig) serves as the B cells receptor for the antigen.
This immunoglobulin is known as the B-cell receptor (BCR) and it can not be secreted.
Why does the B cell need to be activated?
To generate sufficient quantities of effector lymphocytes. Only lymphocytes with the appropriate receptor specificity (The one who recognizes the enemy properly) must be activated & allowed to proliferate.
What does the B cell do before it is activated?
It sits around waiting to be exposed to an antigen. It is called a naive B cell.
(naive adj. Lacking experience, wisdom, or judgment. The definition fits so perfectly!)
What activates the B cell?
Helper T cells.
First, the B cell will internalize the molecule and display peptide fragments on MHC class II molecules.
(It’s like getting a paper and a stick, you make a flag out of it. The paper is the antigen.)
This peptide:MHC class II complex is presented to the antigen specific CD4 T cell.
(It’s like waving a flag to the T cell. “Look what I got! Activate me!”)
There is an interaction between CD40 receptor on the B cell and CD40L on the TH cell.
The absence of this co-stimulatory signal inhibits the B cell response to T-dependent antigens.
(It’s like evaluating the flag. If the symbol on the flag is appropriate, T cells activate you!!!)
Can a B cell be activated directly without a Helper T cell?
Yes. It is called thymus independent B cell activation. But only a few types of antigens can do it. They are called TI-2 antigens.
TI-2 antigens act by simultaneously cross-linking a critical number of B-cell receptors on the surface of antigen-specific mature B cells.
(It’s like getting too many papers & you are no longer interested in making flags. You start making paper streamers instead!)
Dendritic cells and macrophages provide co-stimulatory signals for the activation of B cells.
TNF-family cytokine BAFE secreted by dendritic cell is an example for a co-stimulatory signal.
(It means having the paper work is not enough. Other molecules and cells need to approve activation.)
Many common extracellular bacterial pathogens are surrounded by a polysaccharide capsule that enables them to resist ingestion by phagocytes.
(Macrophages don’t eat slimy paper. It’s not yummy enough!)
So bacteria are not getting ingested by phagocytes and T-cells are not stimulated. (T cells respond to bacteria presented by macrophages.)
At this stage, activated B cells rapidly produce IgM antibodies against the capsular polysaccharide (independently of peptide-specific T-cell help).
(Your paper streamers saved the day!)
This will coat the bacteria, promoting their ingestion and destruction by phagocytes early in the infection.
(Antibodies make slimy tasty & ingestable.)
Did you know?
TI-2 antigens include components of some bacterial cell wall components (e.g., lipopolysaccharide, the slimy guys) or antigens containing highly repetitious molecules (e.g., bacterial flagellin, the guys that give too many similar papers).
What happens after activation?
After getting activated, B cells transform into plasma cells (antibody secreting cells) and memory B cells (the spies who conduct surveillance for similar papers.)
*phew* That’s all!