(a) Lymphocytes

• Lymphocytes are the white blood cells involved in the specific immune response.
• Lymphocytes respond to specific antigens on invading pathogens.
• Antigens are molecules, often proteins, located on the surface of cells that trigger a specific immune response.
• Lymphocytes have a single type of membrane receptor which is specific for one antigen.
• Antigen binding leads to repeated lymphocyte division resulting in the formation of a clonal population of identical lymphocytes.
• There are two types of lymphocytes — B lymphocytes and T lymphocytes.

B lymphocytes

• B lymphocytes produce antibodies against antigens and this leads to the destruction of the pathogen.
• Antibodies are Y-shaped proteins that have receptor binding sites specific to a particular antigen on a pathogen.
• Antibodies become bound to antigens, inactivating the pathogen.
• The resulting antigen-antibody complex can then be destroyed by phagocytosis.
• B lymphocytes can respond to antigens on substances that are harmless to the body, eg pollen.
• This hypersensitive response is called an allergic

T lymphocytes

• T lymphocytes destroy infected body cells by recognising antigens of the pathogen on the cell membrane and inducing apoptosis.
• T lymphocytes attach on to infected cells and release proteins.
• These proteins diffuse into the infected cells causing production of self-destructive enzymes which cause cell death.
• Apoptosis is programmed cell death.
• The remains of the cell are then removed by phagocytosis.
• T lymphocytes can normally distinguish between self-antigens on the body’s own cells and non-self-antigens on infected cells.
• Failure of the regulation of the immune system leads to T lymphocytes responding to self-antigens.
• This causes autoimmune diseases.
• In autoimmunity, the T lymphocytes attack the body’s own cells.
• This causes autoimmune diseases such as type1 diabetes and rheumatoid arthritis.

(b) Immunological memory

• Some of the cloned B and T lymphocytes survive long-term as memory cells.
• When a secondary exposure to the same antigen occurs, these memory cells rapidly give rise to a new clone of specific lymphocytes.
• These destroy the invading pathogens before the individual shows symptoms.
• During the secondary response, antibody production is greater and more rapid than during the primary response.

HIV and AIDS

• The human immunodeficiency virus (HIV) attacks and destroys T lymphocytes.
• HIV causes depletion of T lymphocytes which leads to the development of AIDS (acquired immune deficiency syndrome).
• Individuals with AIDS have a weakened immune system and so are more vulnerable to opportunistic infections.