Immunity

• Phagocytes (neutrophils and macrophages) engulf and digest pathogens: the non-specific response.
• An antigen is a molecule that triggers an immune response; self antigens are tolerated, non-self antigens are attacked.
• Primary response: antigen presentation by macrophages, clonal selection and clonal expansion of B- and T-lymphocytes, plasma cells secrete antibodies, T-killer cells destroy infected cells.
• Memory cells give a faster, larger secondary response and long-term immunity.
• Antibody structure (variable and constant regions, four polypeptide chains) explains how antibodies work.
• Monoclonal antibodies are made by the hybridoma method and used in diagnosis and treatment.
• Active vs passive and natural vs artificial immunity; vaccines and vaccination programmes.

Phagocytes: the non-specific response

Phagocytes are made in the bone marrow. There are two types:

• Neutrophils circulate in the blood in large numbers, leave capillaries by squeezing between endothelial cells and arrive first at a site of infection. They are short-lived and die after ingesting a few bacteria (dead neutrophils make up pus).
• Macrophages are larger and longer-lived. They travel in the blood as monocytes, then settle in organs such as the lungs, liver, spleen and lymph nodes. As well as destroying pathogens they act as antigen-presenting cells.

Mode of action (phagocytosis)

1. Damaged cells and pathogens release chemicals (e.g. histamine); phagocytes move towards them by chemotaxis.
2. Receptors on the phagocyte bind to the pathogen (helped if antibodies are already attached to it).
3. The cell surface membrane flows around the pathogen and engulfs it by endocytosis, forming a phagocytic vacuole (phagosome).
4. Lysosomes fuse with the vacuole and release hydrolytic enzymes (e.g. proteases, lysozyme) that digest the pathogen.

Antigens: self and non-self

An antigen is a molecule, usually a protein or glycoprotein on a cell surface, that is recognised by the immune system and stimulates an immune response (the production of antibodies). Each antigen has a specific shape.

• Self antigens are the body’s own molecules. Lymphocytes that would react to them are destroyed or suppressed during development, so they do not normally trigger a response.
• Non-self antigens are foreign: on pathogens, on transplanted cells, or on cells of another blood group. They trigger an immune response.

The primary immune response

Lymphocytes are made in the bone marrow. B-lymphocytes mature in the bone marrow; T-lymphocytes mature in the thymus. Each lymphocyte has receptors of one shape only, so it can respond to one antigen. Many thousands of different lymphocyte types exist before any infection.

1. Antigen presentation. A macrophage engulfs a pathogen, digests it and places its antigens on its own cell surface membrane.
2. Clonal selection.
3. T-helper cells cytokines
4. Clonal expansion of B-lymphocytes. plasma cells