Download Theoretical immunology/Microbiology Dept./Vet.Med. 2015--

Theoretical immunology/Microbiology Dept./Vet.Med. 2015---2016.
Assist.Prof.Dr.Ikram Abbas Aboud Al-Samarraae
B-Cell maturation, Activation, Differentiation and Humoral immune
response:-
Acquired immunity has two branches called Humoral Immunity (HI) and
Cell Mediated Immunity (CMI). Its need another cell (Antigen Presenting
Cell , APC) to present the antigen in a suitable form to cell . B-lymphocytes
directly bind to antigen through their surface immunoglobulin receptor (B
cell act as APC to help T cell ) . Upon antigen binding the B cell activated
and divided repeatedly to produce plasma cells and memory cells .
B – Lymphocyte Structure and Characteristics : Lymphocyte are small round cells ranging from 7 – 15 µm in diameter. The
surface of some Lymphocytes are seen to be smooth , where as others are
covered by many small projections . B cell originated in the bone marrow ,
but maturation in the payers patches . They are found in the cortex of lymph
nodes , payers patches and spleen (in the marginal zone of the splenic white
pulp .
B – Lymphocyte Activation :The mature B – Lymphocytes released from bone marrow are in
arresting state and they don’t secrete Abs .The resting B cell which don’t
contact the antigen die within few days, whereas , arresting B cells , which
binds to the antigen through the B cells surface immunoglobulins becomes
activated . The activated B cells produces plasma cells & memory B cells
.The activation of B cells requires two signals & cytokines from T cells:1-The first signal is provided by the binding of Ag to surface
immunoglobulins on B cells surface.
2-Cell to cell contact between B cell & T helper cell provides the second
signal required for B cell activation.
The cytokines secreted by T helper cell bind to their respective receptors on
B cell & help in proliferation & differentiation of B cells.The initial
antibodies secreted by plasma cells in response to antigen always belong to
IgM class .
Memory B Lymphocytes:The memory B cells don’t secrete antibodies immediately .They remain in
arresting state for many months to years .The memory B cells in the lymph
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nodes are waiting for the contact with their specific antigens, upon contact
with a specific antigen the memory B cell becomes activated & then divides
to produce plasma cells & memory B cells.
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Inhibition of B cell activation:Plasma cells die shortly, they have a life span of few days only and the Abs
production is stopped .There is a negative feedback mechanisms that regulate
the Abs production. The inhibition of B cell activation is probably mediated by
the binding of Ag Abs complex to the B cell .The Ag combines with surface
immunoglobulins that may interferring with the signaling mechanism inside the
B cell, leading the suppression of B cell activation & finally new plasma cells
production & Abs secretion don’t occur . The level of Abs induced against an
Ag decreases.
Antibodies- Mediated (Humoral) Immunity:When an individual encounters an antigen for the first time . antibodies to
that antigen is detectable in the serum within days or weeks depending on the
nature , dose of the antigen and the route of administration ( e.g . Oral ,
parenternal ) . The serum Abs concentration continues to rise for several weeks
& then declines; it may drop to very low levels.The first Abs formed are IgM ,
followed by IgG , IgA or both .IgM tend to declines sooner than IgG levels.
The Secondary Immune Response:In the event of a second encounter with the same Ag (or closely cross- reacting
one) months or years after the primary response, the Abs response is more rapid
& rise to higher levels than during the primary response .In the secondary
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response the amount of IgM, produce in qualitatively similar to that produced
after the first contact with Ag .However, much more , such Abs tends to bind
Ag more firmly (to have higher affinity) and thus to dissociate less easily.
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