The basis of allergic reaction is an exaggeration of the
immune response. The reaction can be harmful or
beneficial (McCance & Huether, 2006).  Briefly this
immune response is conducted in the following way. 
Exogenous antigens are inhaled, ingested, injected
(enter the body) where they are taken up by
antigen-presenting cells (APCs).  The APCs (macrophages
or dendritic cells) phagocytose the antigen or the
antigen is engulfed by endocytosis by the B cells.  The
antigen is fragmented in the lysosome to short peptides.
 At the same time MCH II molecule is assembled in the
endoplasmic reticulum and transported to the lysosomes. 
The short peptides or antigen fragments now combine with
MCH II molecule and transported to the surface of the
APC.  At this point the antigen/MCH II complex is
available for helper T cell interaction.

For example: an inhaled antigen – dander of a cat
triggering asthma; ingested –a  peanut that triggers an
allergic response; injected – vaccine). 

In some cases dendritic cells can present an intact
antigen directly to B cells. In this case, the engulfed
antigen is not degraded in lysosomes but is returned to
the cell surface for presentation to B cells bearing
BCRs of the appropriate specificity. (Antigen
presentation, 2008).

Antigen presentation, retrieved on May 14, 2008 from
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AntigenPresentation.html).

McCance, K.L., & Huether, S. E. (2006). Pathophysiology:
 The biologic basis for disease in adults and children. 
Elsevier  Mosby: St. Louis.