B cells are lymphocytes that make antibodies, proteins that bind one specific antigen. Because antibodies circulate in the blood and body fluids, this arm is called humoral immunity.
Activation
Each B cell carries a surface antibody that fits one antigen. When it binds that antigen and gets help from a helper T cell, it multiplies into plasma cells that pour out antibodies, plus long-lived memory B cells.
What antibodies do
Antibodies neutralize toxins and viruses by blocking them, coat bacteria so phagocytes eat them faster, and switch on complement. They mark germs for destruction rather than killing them directly.
First and second exposure
On first exposure, IgM rises within about a week and IgG follows. On a second exposure, memory cells produce large amounts of IgG within days, which is the basis of lasting immunity.
Antibodies and blood groups
People naturally carry antibodies against the ABO blood group antigens they lack. That is why ABO-mismatched blood can cause a fast, severe transfusion reaction.
| Class | Where it is found | Main role |
|---|---|---|
| IgG | Blood and tissues, crosses the placenta | Main long-term protection |
| IgM | Blood | First antibody in a new infection |
| IgA | Mucus, tears, saliva, breast milk | Protects mucous membranes |
| IgE | Bound to mast cells | Allergy and defense against parasites |
| IgD | B cell surface | Helps start B cell activation |
At the bedside
Before a transfusion, ABO and Rh typing and the crossmatch prevent a reaction from the patient's own antibodies. On serology, IgM points to a recent infection, while IgG alone usually means past infection or immunity.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).