I'll end this journey through the innate immune system with a discussion of two more cell types: mast and dendritic cells. That's not say that there aren't additional innate components that I could describe. Leafing through Abbas et al. I discovered several others, both cellular and molecular, but as far as I can tell they are invoked rarely or play relatively minor roles. For example, eosinophils are white blood cells that help fight off parasites. And basophils look like, and function similarly to, mast cells, although they originate from different precursors. They're present in small numbers and its unclear how much they contribute to the overall innate immune response.
Born in the bone marrow and housed in the skin and the lining that covers the surface of our internal organs, mast cells are sentinels on the lookout for a variety of invaders. Their main claim to fame is that upon activation, they quickly disgorge a variety of granules located within their cytoplasm into the surrounding extracellular space. Among the chemicals contained in these granules are histamine and heparin. The latter is a potent anticoagulant. The former is a small molecule that increases the permeability of capillaries so that other innate cells can move from the blood into the surrounding tissue. Histamines also promote itching, and the increased permeability they induce causes runny noses and watery eyes. We take antihistamines to relieve these conditions. (How antihistamines operate is beyond the scope of this blog. That's another way of saying that I have no idea how they work).
One of the hallmarks of the mast cell response is rapidity. When they're challenged by a culprit that they recognize (recognition can be mediated by the kind of innate receptors that I discussed in a previous posting), they can release their load of granules within minutes. But they also can associate with some proteins that are part of the adaptive immune system. This allows them to respond quickly and specifically to a variety of substances that come in contact with our skin, respiratory tract, and digestive system and that the innate system doesn't recognize. We're all familiar with this kind of response. It's called an allergic reaction. I hope to get to the topic of allergy in a later post.
I've left these cells for last not because they're unimportant. On the contrary, I saved them until now because they are a critical bridge between the innate immune system and the adaptive immune system that I'm going to discuss in the remainder of these posts. Named for the tree-like projections of their cytoplasm (dendron is the Greek word for tree), dendritic cells are found in many tissues, but are principally located in the skin and respiratory tract lining. They come in two varieties. One type is a sentinel cell similar to macrophages, but with an even larger range of receptors, including many toll-like receptors that I discussed previously. They are particularly adapt at identifying viruses and their byproducts. When they do come across evidence of a virus, they produce and secrete a set of cytokines that have potent antiviral activity.
The other type of dendritic cell is specialized for antigen presentation. Making use of the major histocompatibility system discussed previously in association with natural killer cells, it is the principle way of letting the adaptive immune system know the exact chemical nature of an intruder. I'll define the terms "antigen" and "antigen presentation" in later posts.
OK. It's time to sum up what I've learned about the innate immune system. I'll leave this important task to the next post.