Responses to infection (6.7, 6.8, 6.9)

An overview of responses to infection (6.7, 6.8, 6.9) from Edexcel A level Biology A including: phagocytes, lymphocytes and antibodies

4 min

Inflammation: increases blood flow and capillary permeability allowing immune cells and antimicrobial substances to reach infected tissue.
Lysozyme action: breaks down bacterial cell walls.
Interferon: prevents viruses replicating / spreading.
Phagocytosis: pathogens are engulfed and digested by immune cells.

Phagocytes are a type of white blood cell that play a key role in the non-specific immune response by engulfing and destroying pathogens.

Neutrophils are the most abundant type of phagocyte. They rapidly respond to infections and die after engulfing pathogens.

Macrophages are larger, longer-lived phagocytes that develop from monocytes (a type of white blood cell). Macrophages act as antigen-presenting cells (APCs) to activate specific immune responses.

Phagocytosis is the process by which phagocytes (e.g., neutrophils, macrophages) engulf and destroy pathogens.

Phagocytosis involves the following steps:

Chemotaxis – phagocytes use chemical signals to move toward pathogens.
Recognition and attachment – receptors on the phagocyte membrane bind to antigens on the pathogen surface.
Engulfment – the pathogen is enclosed in a phagosome (a membrane-bound vesicle).
Digestion – lysosomes fuse with the phagosome to form a phagolysosome, releasing enzymes to break down the pathogen.
Antigen presentation – macrophages display the pathogen’s antigens on their surface membrane to activate the immune response.

Diagram illustrating the process of phagocytosis. A green cell labeled 'Phagocyte' surrounds a bacterium. The bacterium is labeled 'Bacterium' and is shown being engulfed by the phagocyte, labeled 'Bacteria being engulfed'. The bacterium is then enclosed within a structure labeled 'Bacterium enclosed within a phagosome'. Arrows indicate the movement of the bacterium into a vacuole. The vacuole is labeled 'Lysosomes, containing digestive enzymes, fuse with phagocytic vacuole', where purple circles represent lysosomes. The bacterium is digested, and labeled 'Digested bacterium'. The final stage shows the digested bacterium leaving the phagocyte, labeled 'Digested bacterium leaves phagocyte by exocytosis'.

An antibody is a Y-shaped globular protein made of four polypeptide chains (two heavy and two light), held by disulfide bonds.

The variable region at the tips form two antigen-binding sites; they have a specific tertiary structure that is complementary to a specific antigen, meaning specific binding will occur at these sites.

The constant region is the same in all antibodies of that class and interacts with other components of the immune system, helping to recruit immune cells.

The hinge region provides flexibility, allowing the antibody to bind to antigens at different angles.

The image depicts a schematic diagram of an antibody structure. The top left shows an 'Antigen' labeled in red, with an arrow pointing to the 'Epitope region: binds to the antibody.' The antibody structure is Y-shaped, with two 'Variable regions' labeled at the arms, shown in purple and orange. Annotations on the diagram highlight different regions: 'Constant region' at the base of the Y, 'Light polypeptide chain' on the outer arms, and 'Heavy polypeptide chain' on the inner arms. The 'Hinge region' is labeled on the central part of the Y, described as 'flexible to allow the antigen binding site to take different positions when binding antigens; not all immunoglobulins have hinges.' There are '-S-S- (disulphide bridges)' indicated between the heavy chains. Text at the top right reads: 'Antigen binding sites match the epitope of an antigen and vary from antibody to antibody; they have unique amino acid sequences for specific antigens.'

Antibodies bind to antigens on pathogens (or to toxins). The binding is specific, like a lock and key and forms an antigen–antibody complex.

Once the antigen-antibody complex is formed then there are several different ways in which the antibody itself or the immune system will destroy the pathogen.

An antigen is a molecule, usually a protein or polysaccharide, that triggers an immune response by stimulating the production of antibodies or activating immune cells.

Antigens are usually found on the surface of pathogens (e.g., bacteria, viruses) but can also be on toxins or foreign substances.

Each antigen has a specific shape that is recognised by complementary antibodies or T-cell receptors.

This recognition enables the immune system to distinguish between self (the body’s own cells) and non-self (foreign invaders).

Some pathogens (e.g., viruses and bacteria) change their antigens over time through mutations or genetic recombination.

This allows them to evade immune recognition, because previously produced antibodies and memory cells may no longer recognise the altered antigens. This makes it harder for the immune system to respond effectively.

B lymphocytes are a type of lymphocyte that mature in the bone marrow and play a key role in the specific (humoral) immune response. They are responsible for targeting pathogens in body fluids such as blood and lymph.

When a B cell detects a specific antigen, it becomes activated and divides to form plasma cells (B effector cells), which produce antibodies that bind to and neutralise the pathogen.

Some B cells become memory cells, which remain in the body and provide long-term immunity by responding quickly if the same antigen is encountered again.

The diagram illustrates the maturation process of B cells. At the top left, immature B cells are shown dividing, indicated by arrows branching downward. Text labels include 'Immature B cells' and 'In bone marrow, immature B cells divide by mitosis' next to a bone illustration. An arrow points to a labeled 'B cells mature in bone marrow' cell, which has a nucleus and cytoplasm depicted. Another arrow leads to a cell labeled 'Production of antibody receptors,' showing surface projections. This cell is connected by an arrow to another cell labeled 'Antibody receptors in cell surface membrane.' Below, 'Mature B cells circulate and concentrate in liver and spleen' is labeled near illustrations of a liver and spleen. At the bottom, several cells with varying antibody receptors are labeled 'Mature B cells each with a different antibody receptor.'

The image illustrates the mechanisms of helper and killer T lymphocytes. At the top, a helper T cell is shown. An arrow points to a division into two smaller cells labeled 'Divide by mitosis.' Below, a killer T cell is depicted interacting with an infected tissue cell displaying antigens. A label states 'Toxin-filled vacuole in killer T cell,' and an arrow indicates 'Divide by mitosis.' On the right, another helper T cell labeled 'Helper T cells release cytokines that stimulate B cells to reproduce to form plasma and memory cells.' Below this, an enlarged view of an infected cell shows 'Toxins are secreted into the infected cell by the T cell,' with 'Hole in tissue cell membrane' indicated. At the bottom, a note says, 'Killer T cells can punch holes in the outer membrane of infected tissue cells to destroy them.'

There are several types of T lymphocytes:

T helper cells (Th): release cytokines to activate B cells, phagocytes, and other T cells
T killer (cytotoxic) cells (Tc): destroy infected or abnormal cells by releasing substances that cause cell lysis.
T memory cells: provide long-term immunity by responding quickly to reinfection.

Responses to infection (6.7, 6.8, 6.9)

On this page

Phagocytes (6.7)

Antibodies (6.8)

Lymphocytes (6.9)