The human immune system A simplified account of how your immune system works

The Humoral Response - Activation Phase antigen The immune response starts with an antigen approaching a cell of the immune system, here a macrophage. macrophage

The macrophage engulfs the antigen by phagocytosis. Antigen engulfed in vesicle The macrophage engulfs the antigen by phagocytosis. Lysosome containing enzymes

The vesicle containing an antigen fuses with a lysosome The vesicle containing an antigen fuses with a lysosome. The enzymes in the lysozome break down the antigen into fragments. This is antigen processing. antigen processing

Inside the macrophage, the processed antigens combine with special class II MHC proteins. These proteins can move to the cell surface membrane. enzymes and proteins combining Class II MHC proteins

The antigen/MHC protein complex is displayed on the immune cell surface membrane. The macrophage is now known as an antigen presenting cell. Processed antigen/ MHC protein complex Antigen-presenting cell

The next stage involves a helper T cell (also know as a T-helper cell) as well as the macrophage receptors which bind to specific antigen/MHC protein complex T helper cell (TH) macrophage (antigen-presenting cell)

The receptors on the helper T cell enable it to bind to the specific antigen-MHC complex of the antigen presenting cell. helper T cell macrophage (antigen-presenting cell)

The binding of the helper T cell with the antigen - MHC protein complex triggers the macrophage to release proteins (cytokines) that activate the helper T cell. Cytokines from macrophage

The activated helper T cell now releases its own cytokines Cytokines from helper T cell

The released cytokines stimulate the helper T cell to reproduce and form a clone of cells. Each new cell has the same receptors as the original helper T cell, so they are specific for the original antigen. Clone of helper T cells

The Humoral Response - Effector Phase Another phase of the immune response begins with a B cell. The B cell has membrane bound globular receptor proteins (called IgM). Some of these are specific for the same antigen presented earlier by the antigen presenting cell. IgM receptor antigen B cell

The B cell’s receptor protein (an IgM) binds to the antigen, and the cell engulfs the antigen by endocytosis. IgM bound to antigen engulfed by cell lysosome

Fused vesicles containing antigen and enzymes from lysosome The vesicle formed inside the B cell fuses with a lysosome. This contains digestive enzymes which break down the antigen. B cell

Fragments of the digested antigen remain after processing within the vesicle. Processed antigen Class II MHC protein

The processed antigen is attached to Class II MHC receptors within the B cell, and is transported to the membrane.

Antigen/MHC protein complex The MHC proteins form a complex with the antigen which is displayed on the surface of the B cell. It has become another type of antigen presenting cell B cell (antigen presenting cell)

A helper T cell from the clone of cells produced earlier specifically recognises the antigen presented by the B cell. Helper T cell clone Antigen-presenting B cell

The helper T cell cell binds to the antigen/MHC protein complex displayed by the B cell. This triggers the release of cytokines from the T cell. Once the cytokines are released the helper T cell no longer binds to the B cell. helper T cell binds to antigen complex on B cell Cytokines released by helper T cell

The cytokines released by the helper T cell stimulate the B cell to divide and form a clone of identical cells B cell clones

The B cells continue to divide and form two groups of clones The B cells continue to divide and form two groups of clones. Some are long – lived MEMORY cells. Most are antibody-secreting PLASMA cells. Plasma cells have extensive endoplasmic reticulum and many ribosomes. Memory cell Plasma cells

Plasma cells are essentially antibody factories Plasma cells are essentially antibody factories. They produce and secrete antibodies identical to those of the surface receptors of the original parent B cell antibodies

Like the IgM surface receptors on the parent B cell, the antibodies can bind to and inactivate the antigens, forming an antibody-antigen complex. This complex makes it easier for other white blood cells to engulf the antigen (phagocytosis). Antibody-antigen complex