Lymphocytes, antigens and antibody production

Organisms (Interaction and interdependence) · Defence against disease · note 4 of 8

Spec C3.2.6, C3.2.7, C3.2.8, C3.2.9Download all Defence against disease notes (PDF)Download all notes (PDF)11 pages

Lymphocytes, antigens and antibody productionSpec C3.2.6, C3.2.7, C3.2.8, C3.2.9

In short

Antigens are recognition molecules, usually glycoproteins or other proteins on pathogen surfaces, that trigger antibody production. Lymphocytes in the blood and lymph nodes cooperate to make antibodies. A specific B-cell is activated by its antigen plus contact with a helper T-cell activated by the same antigen, then divides by mitosis into a clone of antibody-secreting plasma cells.

Lymphocytes

Lymphocytes are the cells of the adaptive immune system. They both circulate in the blood and are contained in lymph nodes. An individual has a very large number of different B-lymphocytes (B-cells), each able to make one specific type of antibody. Lymphocytes cooperate to produce antibodies.

Antigens

An antigen is a recognition molecule that triggers antibody production. Most antigens are glycoproteins or other proteins, usually located on the outer surfaces of pathogens. Each antibody has a binding site with a shape complementary to one antigen.

Antigens on the surface of erythrocytes (for example the A and B antigens of the ABO blood groups) may stimulate antibody production if transfused into a person with a different blood group. For example, a Rhesus-negative person given Rhesus-positive blood makes antibodies against the Rhesus (D) antigen. In the ABO system, a person already has antibodies against the A or B antigen they lack (a person with group A has anti-B), so a mismatched transfusion makes the donated erythrocytes clump (agglutinate).

Activation of B-lymphocytes

There are antigen-specific B-cells and antigen-specific helper T-cells. B-cells produce antibodies and become memory cells only when they have been activated. Activation requires both:

direct interaction of the B-cell with its specific antigen, and
contact with a helper T-cell that has also become activated by the same type of antigen (helper T-cells are activated when they bind the antigen, for example displayed on the surface of a phagocyte that has engulfed the pathogen).

Clones of plasma cells

Only a relatively small number of B-cells respond to a specific antigen. To produce sufficient quantities of antibody, activated B-cells first divide by mitosis to produce large numbers of plasma B-cells (plasma cells), all capable of producing the same type of antibody. This group of identical cells is a clone. Plasma cells have extensive rough endoplasmic reticulum and secrete antibodies into the blood.

Flow diagram of B-cell activation: a pathogen with surface antigens is engulfed by a phagocyte, which displays the antigen to a helper T-cell, activating it; a specific B-cell binds the same antigen and is activated by contact with the activated helper T-cell; the activated B-cell divides by mitosis into a clone of plasma cells that secrete Y-shaped antibodies, plus memory cells. — A B-cell is activated by its specific antigen and by contact with an activated helper T-cell, then divides by mitosis into a clone of plasma cells and memory cells.

Common mistake:

Antibodies are made by plasma cells, not by helper T-cells or phagocytes. Helper T-cells activate B-cells but do not make antibodies.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

How do vaccines work?

Vaccines contain antigens, or DNA or RNA coding for antigens, from a specific pathogen. They trigger a primary immune response in which B-cells are activated and memory cells are produced, without causing the disease. If the real pathogen infects later, memory cells respond quickly, producing antibodies before the person becomes ill.

Why can't antibiotics kill viruses?

Antibiotics work by blocking processes found in bacteria but not in eukaryotic cells, such as cell wall synthesis or protein synthesis by 70S ribosomes. Viruses have no metabolism of their own, so there is nothing for antibiotics to block. They replicate using the host cell's processes, which antibiotics do not target.

What is the difference between innate and adaptive immunity?

The innate immune system responds to broad categories of pathogen and does not change during an organism's life; phagocytes are part of it. The adaptive immune system, based on lymphocytes, responds specifically to particular pathogens and builds up a memory of pathogens encountered, so later responses are faster and more effective.

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