The immune system

· Immune system = body defence system that protects against pathogens and infected cells.
· Defence includes non-specific responses and specific immune responses.
· Phagocytosis = immediate, non-specific defence carried out by phagocytes.
· Lymphocytes produce a specific immune response against particular antigens.
· Key cells: macrophages, neutrophils, B-lymphocytes, plasma cells, T-helper cells, T-killer cells, memory cells.

Phagocytes: macrophages and neutrophils

· Phagocytes engulf and digest pathogens by phagocytosis.
· Main phagocytes in this topic: macrophages and neutrophils.
· Sequence: recognition of pathogen → engulfment → pathogen enclosed in a phagosome → lysosomes fuse with phagosome → enzymes digest pathogen → waste removed by exocytosis.
· Neutrophils are short-lived phagocytes that move quickly to infected tissues and destroy pathogens.
· Macrophages are larger, longer-lived phagocytes; after digestion, they can act as antigen-presenting cells.
· Macrophages present antigens from pathogens on their cell surface to help activate T-helper cells.
· Exam focus: phagocytosis is non-specific, but antigen presentation links it to the specific immune response.

Antigens, self antigens and non-self antigens

· Antigen = molecule, often a protein or glycoprotein, that can be recognised by the immune system.
· Antigens are found on the surfaces of cells, pathogens, toxins or other foreign material.
· Self antigens = antigens on the body’s own cells; normally recognised as “self” and not attacked.
· Non-self antigens = foreign antigens on pathogens or infected/abnormal cells; trigger an immune response.
· The immune system must distinguish self from non-self to avoid attacking body cells.
· Specific lymphocytes have receptors complementary to specific antigens.
· Exam focus: do not write that all antigens are pathogens; an antigen is a recognised molecule, not the whole organism.

Primary immune response

· Primary immune response = response after first exposure to a particular non-self antigen.
· It is relatively slow because few lymphocytes with complementary receptors are present at first.
· Macrophages engulf pathogens and display pathogen antigens on their cell surface.
· T-helper cells recognise antigens presented by macrophages and become activated.
· Activated T-helper cells release chemical signals that activate selected B-lymphocytes and T-killer cells.
· B-lymphocytes with complementary receptors bind to the antigen and are activated.
· Activated B-lymphocytes divide by mitosis to form a clone of identical cells.
· Some cloned B-lymphocytes differentiate into plasma cells.
· Plasma cells secrete specific antibodies against the antigen.
· Some cloned B-lymphocytes become memory B cells.
· T-killer cells destroy infected body cells carrying the non-self antigen.
· Some T-lymphocytes become memory T cells.

This diagram summarises the primary immune response, including antigen recognition, activation of lymphocytes, and production of effector cells. Use it to link the roles of macrophages, B-lymphocytes, T-lymphocytes, and plasma cells in one sequence. Source

Roles of B-lymphocytes

· B-lymphocytes are involved in the specific immune response against antigens.
· Each B-lymphocyte has a specific receptor complementary to one antigen.
· When activated by antigen binding and T-helper cell signals, B-lymphocytes divide by clonal selection and clonal expansion.
· Cloned B cells form plasma cells and memory B cells.
· Plasma cells secrete large quantities of specific antibodies.
· Memory B cells remain in the body and enable a faster response during reinfection.
· Exam focus: plasma cells secrete antibodies; memory cells provide long-term immunity.

Roles of T-lymphocytes

· T-helper cells coordinate the immune response by releasing chemical signals.
· T-helper cells activate B-lymphocytes, helping them divide and form plasma cells and memory cells.
· T-helper cells also activate T-killer cells.
· T-killer cells recognise infected body cells carrying non-self antigens.
· T-killer cells destroy infected cells, preventing pathogens from reproducing inside them.
· Memory T cells remain after infection and respond quickly if the same antigen is encountered again.
· Exam focus: T-helper cells activate other immune cells; T-killer cells kill infected body cells.

This diagram shows how T-helper cells become activated by antigen-presenting cells and then stimulate other immune cells. It helps explain why T-helper cells are central to both B-cell antibody responses and T-killer cell responses. Source

Memory cells and secondary immune response

· Memory cells are long-lived lymphocytes formed during the primary immune response.
· Types include memory B cells and memory T cells.
· Secondary immune response = response after later exposure to the same antigen.
· It is faster, stronger and produces a larger number of effector cells than the primary response.
· Memory B cells rapidly divide and form many plasma cells, so antibodies are produced quickly.
· Memory T cells rapidly form active T cells, including T-helper cells and T-killer cells.
· This provides long-term immunity against the same pathogen.
· Exam focus: secondary responses are quicker because memory cells are already present and specific to the antigen.

This diagram compares the first and second responses of B cells to the same antigen. It shows how memory B cells cause a larger and faster production of plasma cells and antibodies during the secondary response. Source

Primary vs secondary immune response

· Primary response: first exposure to antigen; slower; fewer lymphocytes activated at first.
· Primary response: includes production of plasma cells, T-killer cells and memory cells.
· Secondary response: same antigen encountered again; faster and more intense.
· Secondary response: memory cells divide rapidly to form many effector cells.
· Long-term immunity depends on survival of memory cells after the primary response.
· Exam comparison phrase: “The secondary response is faster and stronger because memory cells specific to the antigen remain in the body.”

Common exam mistakes to avoid

· Do not confuse phagocytes with lymphocytes: phagocytes engulf pathogens; lymphocytes produce specific immune responses.
· Do not say antigens are antibodies: antigens trigger recognition; antibodies bind to antigens.
· Do not say T-helper cells kill infected cells: T-killer cells kill infected cells.
· Do not say plasma cells are memory cells: plasma cells secrete antibodies; memory cells remain for future responses.
· Do not describe vaccination, monoclonal antibodies or antibody structure here; those are in 11.2, not 11.1.