Cambridge International AS and A Level Biology 11: Immunity

Immunity is Cambridge International Biology 9700 Topic 11. It links phagocytes and antigen recognition to primary and secondary immune responses, antibody structure, monoclonal antibodies, active and passive immunity, vaccines and population-level disease control. Theory owns immune mechanisms and interpretation; assay execution and experimental evaluation remain in the dedicated practical hub.

1. Phagocytes

Neutrophils and macrophages are phagocytes. They respond to chemical signals, recognise foreign material and surround a pathogen with their cell-surface membrane. The pathogen becomes enclosed in a phagosome.

Lysosomes fuse with the phagosome and release hydrolytic enzymes. These digest the pathogen. Useful products may enter the cytoplasm, while residues can be removed by exocytosis. Phagocytosis is non-specific because the same general process acts against many different foreign targets.

Neutrophils are short-lived cells that can reach infected tissue rapidly. Macrophages are larger, can remain in tissues and can present pathogen antigens after digestion. Both engulf material, but their roles are not identical.

2. Self and non-self antigens

An antigen is a molecule, often a protein or glycoprotein, that is recognised as foreign and can stimulate a specific immune response. Antigens have particular shapes that interact with complementary receptors or antibody binding sites.

Self antigens are normal molecules on a person's own cells and are usually tolerated by the immune system. Non-self antigens are recognised as foreign. They may occur on a pathogen, infected cell, transplanted tissue or other foreign material.

An antigen is not the whole bacterium or virus. It is a molecular feature that can be recognised. One pathogen may carry several different antigens and therefore stimulate several lymphocyte clones.

3. Antigen presentation and T-helper activation

After a macrophage digests a pathogen, it displays fragments of non-self antigen on its surface. A T-helper lymphocyte with a complementary receptor binds to the presented antigen. This selects the matching cell from many lymphocyte specificities.

The selected T-helper cell divides by mitosis, producing a clone. Activated T-helper cells release cytokines that stimulate matching B-lymphocytes and T-killer lymphocytes. Specificity depends on complementary recognition, not on the immune system designing a new receptor after infection.

4. B-lymphocytes and antibodies

A B-lymphocyte with a complementary surface receptor binds the antigen and, with appropriate T-helper signalling, undergoes clonal expansion. Some cells differentiate into plasma cells. Plasma cells have extensive rough endoplasmic reticulum and secrete large quantities of one antibody specificity.