Cells of the Immune System: B Lymphocytes Revision notes | A-Level Biology CIE

Cells of the Immune System: B Lymphocytes

This lesson covers:

The different types of cells involved in the humoral response
The role of B cells in the humoral response
The differences between the primary and secondary immune response

The humoral response

The humoral response is one of the specific defence mechanisms used to protect the body from disease. It involves the production of specific antibodies to destroy pathogens.

Flowchart showing defence mechanisms including non-specific and specific responses with humoral response highlighted.

The humoral response involves the use of B lymphocytes or B cells. B cells produce and are covered in proteins known as antibodies. It is called the humoral response as antibodies are found in body fluids, which are otherwise known as 'humors'.

Cells involved in the humoral response

The humoral response involves the following types of cell:

B cells

These cells have antibodies on their cell-surface membrane that bind to complementary antigens.
On doing so, they engulf the antigens and display them on their cell-surface to become antigen-presenting cells.
Once activated, B cells can divide into plasma cells and memory cells.

Plasma cells

These are types of B cell that can produce and secrete antibodies against a specific antigen.
They have a short lifespan of only a few days.

Memory cells

These are types of B cell that provide long-term immunity against specific pathogens.
They have a much longer lifespan than plasma cells.
They rapidly divide into plasma cells if the body is re-infected by the same pathogen.

Helper T cells

These cells bind to antigen-presenting cells to activate the division of B cells.

The humoral response

The humoral response is so-called because it involves antibodies that are found in body fluids (or humors).

The stages of the humoral response are as follows:

Diagram showing the stages of the humoral response including B cell activation and antibody production.

A B cell with a complementary antibody binds to the antigens on a pathogen.

Diagram showing B cell activation and clonal selection during the humoral response.

The B cell engulfs the pathogen and presents its antigens on the cell-surface membrane to become an antigen-presenting cell.

Diagram showing clonal selection and activation of B cell by a helper T cell.

Clonal selection - Activated T helper cells bind to the B cell, causing activation of this B cell.

Diagram illustrating the stages of the humoral response including clonal selection, clonal expansion, plasma cells, and memory cells.

Clonal expansion - The activated B cell divides by mitosis to form plasma and memory cell clones.

Diagram showing the stages of the humoral response including B cell activation, plasma cell formation, and antibody secretion.

The cloned plasma cells produce and secrete the specific antibody which is complementary to the antigen on the pathogen's surface. These antibodies attach to antigens on pathogens and destroy them.

Diagram illustrating the stages of the humoral response, including clonal selection and clonal expansion of B cells.

The memory cells circulate the blood and tissue fluid, ready to divide if the body is re-infected by the same pathogen.

Make sure you know the difference between the following two terms:

Clonal selection - The B cell with the correct antibody is selected for cloning (by being activated by a T helper cell).
Clonal expansion - The division of specific B cells to produce genetically identical clones.

Primary and secondary immune response

The body's response to an infection depends on whether the specific pathogen has been encountered before or not.

There are two types of immune response:

Primary immune response - This takes place when the body is exposed to a pathogen for the first time. This response is slow and the infected individual experiences symptoms of the disease.
Secondary immune response - This takes place when when the body has been exposed to the same pathogen before. This response is much faster and stronger and pathogens are destroyed before any symptoms appear.

The graph below shows how the concentration of antibodies changes as the body is exposed to a pathogen on two separate occasions.

Graph showing concentration of antibodies during primary and secondary immune responses to pathogen exposure.

Primary immune response:

The production of antibodies is slow after the first exposure to the pathogen (longer lag phase).
The concentration of antibodies increases slowly.
This is because there are very few B cells that are specific to the pathogen's antigens.
It takes time for the B cells to divide into plasma cells to produce the correct antibody, so the individual experiences symptoms of the disease.
During this process, some B cells divide into memory cells to make the individual immune to this disease.

Secondary immune response:

The production of antibodies is much quicker after exposure to the pathogen (shorter lag phase).
The concentration of antibodies increases quickly.
This is because memory cells recognise the pathogen's antigens and quickly divide into plasma cells.
These plasma cells secrete larger numbers of antibodies to quickly destroy the pathogen before the individual experiences any symptoms.

Comparing the primary and secondary immune response