Humoral Immune Response

Results in production of proteins called “immunoglobulins” or “antibodies”.

Body exposed to “foreign” material termed “antigen” which may be harmful to body: virus, bacteria, etc.

Antigen has bypassed other protective mechanisms, ie, first and second line of defense.

Primary immune response◦ Latent period◦ Gradual rise in antibody production taking days to

weeks◦ Plateau reached ◦ Antibody level declines

Antibody production◦ Initial antibody produced in IgM◦ Lasts 10-12 days◦ Followed by production of IgG◦ Lasts 4-5 days◦ Without continued antigenic challenge antibody

levels drop off, although IgG may continue to be produced.

a lag phase where no antibody is detected a log phase in which the antibody titer rises

logarithmically a plateau phase during which the antibody titer

stabilizes a phase (decline) during which the antibody is

cleared or catabilized

Second exposure to SAME antigen. Memory cells are a beautiful thing. Recognition of antigen is immediate. Results in immediate production of

protective antibody, mainly IgG but may see some IgM

APC

APC

APC

TH

TH

B

APC

The Microbial antigen is ingested by an APC and partially digested. Fragments from microbe bind with the MHC II to form a MHC II /Ag complex on the surface of the APC

APC

APCTH

A Helper T cell, specific for the presented antigen, binds to the MHC II/Ag complex

Memory cells do not react right away but are held in reserve for later infections. The secondary response that is carried out by memory cells is different in 3 ways.

Memory cells produce antibodies that bind with greater affinity to their antigens than the antibodies produced in the initial response.

The response time is much faster than the primary response

A greater number of antibodies are produced.

Virgin lymphocyte pool Virgin lymphocyte pool

PRIMARY RESPONSE PRIMARY RESPONSE

SECONDARY SECONDARY RESPONSERESPONSE

effector cells effector cells memory cell pool memory cell pool

effector cells effector cells memory cell pool memory cell pool

antibodantibody titery titer

time time

IgGIgG IgMIgM

Time course Antibody titer Antibody class Antibody affinity

Antibodies function in 6 ways to protect the body◦ Agglutination: Enhances

phagocytosis and reduces number of infectious units to be dealt with

◦ Opsonization: Coating antigen with antibody enhances phagocytosis

◦ Neutralization: blocks adhesion of bacteria and viruses to mucosa. Also blocks active site of toxin

Activation of complement

Increases inflammation through the byproducts of the complement system (C5a and C3a)

Antibody dependant cell mediated cytotoxicity: Antibodies attached to target cell cause destruction by non specific immune system cells.

Antigenic modulation is distinct from antigen shaving in which the antigen/antibody complex is skimmed from the surface of the malignant B cell by macrophages, usually in the spleen. With antigenic modulation the antibody is internali

zed by the malignant B cells. Note that both these mechanisms consume the monoclonal antibody and may account for the more rapid than expected clearance of antibody from the circulation.