3.2.4: Cell recognition and the immune system (defence mechanisms)

Cell recognition:

It is important to recognise these as millions of humans die each year from infectious diseases.

To defend the body, lymphocytes (a type of white blood cell) must be able to recognise your own cells from 'bad' cells, such as pathogens. Each type of cell has specific molecules on its surface that identify it. These molecules (for example, proteins) enable the immune system to identify:

• Pathogens
• Cells from other organisms of the same species
• Self-cells (your own body cells)
• Toxins
• Abnormal body cells (e.g tumours/cancers)

Proteins are the most important cell identification molecules as they have a highly specific tertiary structure and enormous variety and therefore make it easy to distinguish one cell from another.

This response is very effective, but may be potentially dangerous to patients who, for example, have had an organ transplant. The immune system may recognise these cells as non-self and attempt to destroy them. To minimise the effect of tissue rejection, donor tissues are matched as closely as possible to the cells of the recipient (for example, an organ from a relative) and also immunosuppressant drugs are often administered to reduce the level of immune response that may occur.

There are millions of types of lymphocytes in out body's. There is a high probability that, when a pathogen enters the body, one of these lymphocytes has a protein on its surface that is complementary ti one of the proteins on the pathogen. When an infection occurs, the one that is present with the complimentary protein to the pathogen will be stimulated to divide, building up its numbers to a level where it is effective in destroying the infection. This is known as clonal selection.

But how to lymphocytes recognise self cells (cells belonging to the body)?

Basically, in the fetus the lymphocytes are constantly colliding with other cells. Here, infection is rare as mama protects baba (and the placenta also protects baba). This means that lymphocytes will collide almost exclusively with self cells. The lymphocytes that have receptors that are complementary to self cells are suppressed or die. This means that the only remaining lymphocytes are those that might be complimentary to foreign material. (also, in adults lymphocytes produces in bone marrow will initially only encounter self cells and any that are complimentary undergo apoptosis before they mature).


An infection: an interaction between a pathogen and the body's defence mechanisms

If the individual recovers from the disease the body's defence mechanisms seem better prepared for a second infection from the sam pathogen - often killing it before the patient is aware they have been infected. This is known as immunity.

The body has a range of different defences to protect itself from pathogens. These defence mechanisms can be split into specific and non-specific responses:

Specific: response is slower and specific to each pathogen

• Cell-mediated immunity (T lymphocytes)
• Humoral immunity (B lymphocytes)

Non-specific: response is immediate and the same for all pathogens

• Physical barrier (skin)
• Phagocytosis

Phagocytosis

Should the physical barrier line of defence fail, next up is phagocytosis. Basically, phagocytes ingest and destroy the pathogen before it can cause harm (well, it doesn't always work hence we have cell-mediated/humoral responses as a back up, but this is worth a try first):

• Chemical products of pathogens attract the phagocytes
• The phagocytes have several receptors on their cell-surface membrane that recognise and attach to chemicals on the pathogens surface
• The phagocyte then engulfs the pathogen forming a vesicle (in this instance, the vesicle is also known as a phagosome)
• Inside the phagocyte, lysosomes move towards the vesicle and fuse with it
• Lysozymes from the lysosome destroy the bacteria by hydrolysing their cell walls
• The soluble products are absorbed into the cytoplasm of the phagocyte

This is pretty cool tbh