Lymphatics and Immunity 4: Antibody-Mediated Immunity

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So what do B cells do?

B cells are responsible for antibody-mediated immunity, where pathogens are outside of the cell. B cells differentiate into plasma cells, producing specific antibodies to attack pathogens. There are millions of B cell populations, each with different antibody molecules. Antibody-mediated immunity is also called humoral immunity and is present from birth.

Antibody-mediated immunity involves 3 steps:

  1.   Sensitisation

Sensitisation is when antigens bind to antibodies on an inactive B cell, making it sensitised. Recall that B cell membranes also contain Class 2 MHC proteins, which help in sensitisation by presenting the foreign antigen on the plasma membrane. The B cells is then on 'standby' until a helper T cell activates it.

  2.   Activation

Helper T cell binds to the class 2 MHC protein on the B cell, recognising the antigen and releasing cytokines to stimulate replication of the B cell and enhance antibody production.

  3.   Division and Differentiation

Activated B cells proliferate and differentiate into plasma and memory B cells. The plasma cells then produce antibodies. and produce antibodies. The memory B cells don't respond to the initial threat of the foreign antibody, but instead remain in reserve when the body encounters the same antigen in a secondary exposure.


There are 5 antibodies important in antibody-mediated immunity:
  • IgG: largest subclass, majority of defences
  • IgM: first type secreted following primary (first) exposure to an antigen
  • IgA: attacks pathogens before they enter body tissues
  • IgE: accelerates inflammation on exposure to an antigen
  • IgD: binds antigens in the extracellular fluid to B cells
Primary antigen exposure produces an initial primary response, whereas secondary antigen exposure triggers and secondary response which is more extensive and prolonged, with memory cells already primed. In the primary antigen exposure, concentration of IgM increases before IgG, however in secondary exposure they are both secreted at the same time with IgG at a much greater concentration than IgM.

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