4.1.5 Independent Revision
Wednesday 23 April 2025
17:08
When pathogens manage to breach the body’s first line of defense — such as the skin or mucous
membranes — the secondary non-specific defenses come into action. These responses do not target specific
pathogens but instead react in the same way to any invading microorganism, making them “non-specific.”
These defenses are crucial as they quickly deal with infections before the adaptive immune system is fully
activated.
A central part of this secondary defense system involves phagocytes, which are white blood cells that engulf
and destroy pathogens. There are two main types of phagocytes: neutrophils and macrophages. Neutrophils
are smaller cells that are the first to arrive at the site of infection. They have a multi-lobed nucleus, which
allows them to squeeze through capillaries and reach infected tissues quickly. These cells are short-lived and
can engulf around five to twenty pathogens before dying. In contrast, macrophages are much larger, live
significantly longer, and can engulf more than a hundred pathogens. They are derived from monocytes in the
blood, which migrate into tissues where they mature into macrophages.
The process through which phagocytes destroy pathogens is called phagocytosis. It begins when the
phagocyte detects antigens on the surface of the invading pathogen. Antigens are unique chemical markers
that signal the presence of something foreign. To make it easier for the phagocyte to recognize and attach to
the pathogen, opsonins (such as antibodies) bind to the antigens. These opsonins act as tags. The phagocyte
then uses receptors on its surface to bind to these opsonin-tagged pathogens. Once attached, the phagocyte
engulfs the pathogen into a membrane-bound vesicle called a phagosome.
Inside the cell, the phagosome fuses with another vesicle called a lysosome, which contains powerful
digestive enzymes known as lysozymes. Together they form a phagolysosome, where the pathogen is broken
down and destroyed. Neutrophils usually die soon after completing this process, often forming pus at the site
of infection. Macrophages, however, have a longer role. After digesting the pathogen, they do something
special — they become antigen-presenting cells (APCs). They place fragments of the pathogen’s antigens on
their own cell surface, which is vital for activating the next stage of the immune response: the adaptive
immune system. This triggers clonal selection, where specific B and T cells are activated to launch a more
targeted and long-term immune defense.
Phagocytes are well adapted for their role. Their flexible cytoskeleton allows them to change shape and
engulf pathogens efficiently, and also helps them move between gaps in the capillary walls to reach infected
tissues. They contain lots of mitochondria to provide the energy needed for phagocytosis, and many
ribosomes for synthesising the enzymes required for breaking down pathogens. This combination of rapid
response, engulfing ability, and the power to initiate further immune action makes secondary non-specific
defenses incredibly effective.
Questions:
Q1. Explain in detail what happens when a pathogen enters the body and is destroyed by a phagocyte. Make
sure to include all important steps and terms involved in this process. (6 marks)
When a pathogen enters the body, phagocytes such as neutrophils and macrophages are attracted to the site
of infection by chemical signals. The phagocyte recognises foreign antigens on the surface of the pathogen.
Molecules called opsonins, such as antibodies, bind to these antigens and make the pathogen easier to
detect. The phagocyte has receptors that bind to the opsonin, helping it attach to the pathogen. The
phagocyte then surrounds and engulfs the pathogen, forming a vesicle called a phagosome inside its
cytoplasm. This phagosome fuses with a lysosome that contains enzymes, forming a phagolysosome. The
enzymes inside break down the pathogen. In the case of macrophages, the digested antigens are then
presented on the surface of the cell, turning it into an antigen-presenting cell that activates the next stage of
the immune response.
Q2. Describe the differences between neutrophils and macrophages in terms of their structure, function, and
lifespan. (6 marks)
Neutrophils and macrophages are both phagocytes, but they have several differences. Neutrophils are smaller
and have a nucleus with several lobes. This lobed shape helps them squeeze through gaps in blood vessels to
reach infection sites quickly. They respond rapidly to infections but are short-lived, dying soon after engulfing
a few pathogens. In contrast, macrophages are larger cells with a single round nucleus. They live much longer
than neutrophils and can engulf more pathogens. Macrophages also become antigen-presenting cells, which
display fragments of pathogens on their surface to activate the immune system. This makes macrophages
important for linking the non-specific and specific immune responses.
Q3. Why are mitochondria and ribosomes important in phagocytes? (3 marks)
Phagocytes need a lot of energy to engulf and digest pathogens, which is why they contain many
mitochondria. The mitochondria release ATP through respiration to power movement, shape changes, and
phagocytosis. Ribosomes are also essential because they make proteins, including enzymes that are needed
to break down pathogens inside lysosomes.
Q4. What is an antigen-presenting cell, and why is it important in the immune response? (4 marks)
An antigen-presenting cell is a cell, such as a macrophage, that displays fragments of a pathogen's antigens
on its surface after digesting it. This is important because it allows the immune system to recognize the type
of pathogen that has entered the body. The displayed antigens are recognised by T helper cells, which then
activate other parts of the immune system, such as B cells and cytotoxic T cells. This helps to launch a more
specific and effective immune response.
Q5. Explain how phagocytes reach the site of infection and how their structure helps them carry out their
function. (6 marks)
Phagocytes move towards infection sites by following chemical signals released by damaged cells or
pathogens, a process called chemotaxis. Their flexible cell membranes and cytoskeleton allow them to change
shape and squeeze between the cells of capillary walls. Neutrophils have a lobed nucleus, which also helps
with this movement. Inside the phagocytes, many mitochondria produce the energy needed for engulfing
pathogens, and ribosomes make enzymes needed for digestion. These adaptations allow phagocytes to
effectively travel to and destroy pathogens at the site of infection.