Hello
My answer to this question is yes.
In the thymus-dependent antibody response, antigen presentation by B
cells requires a specific interaction between the antigen and antibody
on the B cell surface, ie the B cells have to recognise the antigen in
order to respond by engulfing it and processing it for presentation.
The B cells then externalise the antigenic fragments in a complex with
a class II major histocompatability molecule. At the same time,
phagocytic cells, eg macrophages, also present antigen in a similar
manner after having engulfed and processed it through a non-specific
reaction. The combination of B cell and macrophage presentation
activates helper T cells. These produce cytokines which activate the
antigen-presenting B cells, causing them to differentiate into plasma
cells which produce antibody against the antigen. The net result, from
the standpoint of the B cells, is an initial recognition and engulfing
pf the antgien, followed by presentation of the antigenic fragments,
which eventually leads to the B cells producing antibodies specific to
that particular antigen.
Moreover, it appears that the MHC-antigen complex itself has a role in
activating the B cells, ie causing a direct response of the B cells to
antigen:
According to a news item which appeared in February 2001: "On their
surfaces B cells express major histocompatibility (MHC) class II
molecules associated with antigenic peptides, which bind to a receptor
on helper T cells. It has long been known that this interaction
induces the helper T cells to promote B-cell activation. In a series
of experiments, Dr. Cambier and his colleagues showed that information
flows both ways. The interaction causes the MHC/antigen complex to
deliver an activation signal to the B cell itself. "It's a
bi-directional signal," said Dr. Cambier. "Upon their association,
both TCR and MHC/antigen transduce signals that activate the T cell
and B cell respectively. The consequence is a proliferation of both
partners." "
In addition, you also have to consider the existence of
thymus-independent antibody responses produced by certain types of
antigens, for example large polysaccharides. These are able to trigger
B cells to produce a low level IgM response without the intervention
of helper T cells. This is therefore a direct recognition and response
of B cells to antigen.
Sources:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AntigenPresentation.html
"Antigen presentation" on Kimball's Biology Pages, an online textbook
of biology by Dr. John W. Kimball - explains the difference between
antigen presentation by phagocytic cells and by B cells.
http://www.njc.org/news/immune.html "New Mechanism Found in Immune
Response, Antibody Production" A summary on the web site of the
National Jewish Medical and Research Center describing the findings of
Dr John Cambier.
http://www.kcom.edu/faculty/chamberlain/Website/MSTUART/lect8.htm CELL
COOPERATION IN THE ANTIBODY RESPONSE Lecture notes by Dr. Melissa
Stuart, Associate Professor, Department of Microbiology/Immunology
Kirksville College of Osteopathic Medicine - describes both
thymus-dependent and thymus-independent antibody responses. |
