Uptake mechanisms
Cells take up extracellular material via a process called endocytosis, during which the cell membrane encloses the extracellular material and brings it into the cell in a membrane bubble, called a vesicle. There are different endocytic mechanisms that allow the APC to take up material depending on its size and type.
Phagocytosis is responsible for the uptake of large particles including bacteria, apoptotic cells and the fragments of necrotic cells, as we saw in the innate module.
Pinocytosis is a process whereby the cell forms an endocytic vesicle around soluble molecules and proteins, an example of which would be when a baby takes up antibodies from breast milk. By this method almost anything that is small enough can be taken up by a cell. Take a look at this video of pinocytosis. What is the meaning of the word pinocytosis?
Most PRRs are found in our immune cells. However, other cells at the interface with the external environment, like epithelial cells, also express some PRRs. The PRR expressing cells expose their PRRs at all times regardless of their life cycle and also express many different types of PRRs with differing PAMPs specificities. The different types of PRRs also locate to sites on the cell surface or within the cell where their specific PAMPs are most likely found. The importance of the location of our PRRs is the main focus of the following pages. However, all the above PRRs features help maximise detection of invading pathogens.
Receptor-mediated endocytosis is yet another uptake mechanism that is shown in the next video clip with the uptake of cholesterol. How does receptor-mediated endocytosis contrast to pinocytosis? The clue is in identifying what the cell uses as tools for getting the cholesterol inside itself.
Did you notice how the LDL receptor assured specific uptake of cholesterol? Receptor-mediated endocytosis, in contrast to pinocytosis, is very specific for the uptake of molecules and proteins triggered by their binding to specific surface receptors. Many other receptors can be found on the cell surface to aid in the uptake of proteins in this highly specific manner. Examples include the high affinity IgE receptor (FcεRI). When IgE encounters an allergen to which it is specific it binds it tightly. The IgE part of this complex can then bind to FcεRI present on the surface of antigen presenting cells called dendritic cells (DCs). The allergen- IgE complex is internalised into the DC via FcεRI in an extremely efficient manner, very similar to the cholesterol uptake you have just seen. This specific uptake mechanism of allergen via FcεRI is known as allergen focussing. How do you think allergen focusing affects the amount of allergen that is taken up by an APC? Do you agree that it is almost like a magnet finding those rare needles in the haystack?
While you were watching the last video did you noticed that the clathrin coated pit fused with an endosome. What does this vesicle do? Let's find out in the next section on Processing and Presentation...