The specific immune responseEdexcel A-Level Biology A: Revision notes

An antigen is a molecule, usually a protein or glycoprotein on the surface of a pathogen or foreign cell, that triggers an immune response. The body recognises its own cells and does not respond to them.

An antibody is a Y-shaped protein made of four polypeptide chains (two heavy, two light) joined by disulfide bridges. The constant region is the same within a class. The variable regions at the tips of the arms form two antigen-binding sites with a tertiary shape complementary to one antigen, which is what makes antibodies specific.

Antibodies bind to antigens and can cause agglutination (clumping of pathogens), neutralise toxins and viruses, and mark pathogens for phagocytosis.

After a macrophage has engulfed and digested a pathogen, it displays pathogen antigens on its surface membrane. It is then an antigen-presenting cell (APC). APCs link the non-specific response (phagocytosis) with the specific response, because the displayed antigens can be recognised by T cells. Infected body cells also display antigens on their surface.

Each T cell has a receptor with a shape complementary to one antigen. T cells are made in the bone marrow and mature in the thymus.

• Helper T cells: bind an antigen on an APC, are activated and divide. They release cytokines that stimulate B cells, killer T cells and macrophages.
• Killer T cells (cytotoxic): bind to infected body cells showing the antigen and release perforin, which makes holes in the membrane, so the cell dies.
• T memory cells: stay in the body and respond rapidly if the same antigen returns.

This is the cell-mediated response, which is vital against viruses inside cells, since antibodies cannot enter cells.

Each B cell has antibody receptors on its surface specific to one antigen. This is the humoral response (involving body fluids).

1. A B cell binds to its complementary antigen.
2. A helper T cell with the matching receptor releases cytokines that activate the B cell.
3. The B cell divides by mitosis (clonal expansion) to make a clone.
4. Most cells become plasma cells (B effector cells), which secrete large numbers of antibodies.
5. Some become B memory cells, which remain and give a faster response next time.

The selection of the one B cell with the right receptor, followed by its cloning, is clonal selection.

• Receptors: B cells have antibody receptors that bind free antigens; T cells have receptors that bind antigens presented on cells.
• Response type: B cells give the humoral response through antibodies; T cells give the cell-mediated response.
• Effector cells: B cells give plasma cells and memory cells; T cells give helper, killer and memory cells.
• Targets: antibodies act on pathogens in body fluids; killer T cells destroy infected body cells.

The two responses cooperate through helper T cells.