B-Lymphocytes

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Humoral immunity involves the production of antibodies. Antibodies are soluble in blood and tissue fluid.

B-cells produce a specific antibody that responds to one specific antigen.

When an antigen enters blood or tissue fluid, there will be a B-cell with a complementary antibody to the antigen. Antigen enters the B-cell by endocytosis and gets presented on cell surface.

Helper T-cells bind to processed antigens and stimulate B-cells to divide by mitosis to form clones of identical B-cells. These B-cells produce the antibody that is specific to the foreign antigen - called clonal selection.

Endocytosis :

Process in which a cell takes in materials from outside by engulfing and fusing them with plasma membrane.

Most extracellular materials that cells absorb are large polar molecules - cannot pass through hydrophobic plasma membrane by simple diffusion.

Two types of endocytosis are phagocytosis (cell-eating) and pinocytosis (cell-drinking).

Exocytosis:

Transport of material out of a cell by sac or vesicle that engulfs the material first and then extruded through an opening in cell membrane.

Plasma Cells:

Secrete antibodies into blood plasma.

Lead to the destruction of antigens.

Responsible for immediate defence of the body against infection.

Antibody production and memory cells are known as the primary immune response.

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Memory Cells:

Responsible for secondary immune response - live longer than plasma cells.

Circulate in blood and tissue fluid.

Divide rapidly to produce plasma and more memory cells when encountering a previously met antigen.

Produce the antibodies needed to destroy the pathogen.

Increased quantity of antibodies secreted faster than in primary immune response.

New infection destroyed before it can cause any harm.

The main stages of a B-cell response:

Pathogen invades the body.

Pathogen taken up by macrophage.

Macrophages present antigens to B-lymphocytes.

Antigen binds to receptors on specific B-lymphocytes.

B-cells can also take up surface antigens directly.

Cloning of activated B-lymphocytes by mitosis.

Cloned B-lymphocytes differentiate into cells with different functions.

Plasma cells - responsible for the primary immune response:

Secrete specific antibodies.

Antibodies bind to antigens on surface of pathogenic cell, forming antibody-antigen complex.

Antibodies help to kill cells in various ways. *

* The antibodies help to kill cells in various ways:

Binding to antigens on viruses and bacteria - cause pathogens to clump together (agglutination) which prevents the pathogen from entering the host cell, makes it easier for phagocytes to engulf them.

Neutralising / inactivating toxins by binding to free toxin proteins produced by pathogens - change shape of active region so proteins no longer take part in the reactions that cause disease.

Memory B-Cells:

Continue to secrete antibodies in small quantities for many years.

Can multiply rapidly to produce an instant supply of plasma cells if same pathogen invades again.

This is the secondary immune response.

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