Specific immune responseEdexcel International A Level Biology: Revision notes

An antigen is a molecule, usually a protein or glycoprotein on the surface of a pathogen, cell or toxin, that triggers a specific immune response. Your own cells carry 'self' antigens, which are normally ignored.

An antibody is a protein secreted by plasma cells. It is made of four polypeptide chains, with two variable regions that form antigen-binding sites. The shape of the binding site is complementary to one antigen only, so each antibody is specific. When an antibody binds, it forms an antigen-antibody complex. This can cause pathogens to clump together (agglutination) and makes them easier for phagocytes to engulf.

Macrophages engulf pathogens by phagocytosis and digest them. They then place pathogen antigens on their own surface membrane. A cell that displays antigens like this is an antigen-presenting cell (APC). Presenting antigens links the non-specific response to the specific response, because it allows the right lymphocytes to be activated.

Infected body cells can also present viral antigens on their surface membranes.

Each B cell has a different antibody on its surface membrane. When a B cell with a complementary antibody meets its antigen, and is stimulated by T helper cells, it is selected. It then divides by mitosis (clonal expansion) to produce a clone of identical cells, which differentiate into:

• Plasma cells (effector B cells) – short-lived cells with abundant rough ER that secrete large amounts of antibody
• Memory B cells – long-lived cells that remain in the blood and respond rapidly if the antigen returns

Antibody production against an antigen is called the humoral response.

T cells have receptors on their surface membranes that bind to antigens presented by APCs or infected cells. After selection they divide and form:

• T helper cells – release cytokines that stimulate B cells (to become plasma cells), killer T cells and macrophages
• T killer (cytotoxic) cells – bind to infected body cells presenting the antigen and release perforin and other substances that kill them
• Memory T cells – long-lived cells that rapidly form new helper and killer T cells on re-exposure

The action of T cells is called the cell-mediated response.

In the primary response, few lymphocytes have the right receptor, so it takes days for enough plasma cells and antibodies to appear, and symptoms may develop.

In the secondary response, memory cells are already present in large numbers. They divide and differentiate rapidly, so antibody concentration rises sooner, reaches a higher peak and lasts longer. The pathogen is usually destroyed before symptoms appear.