Quick, quick, slow- innate and adaptive immunity

In addition to falling into myeloid and lymphoid subdivisions, immune cells can also be classified based on how they function. The cells that act quickly in an immune response, using premade defenses, are part of the innate immune system. On the other hand, cells that respond more slowly and specifically are part of the adaptive immune system.

Innate immunity

Although myeloid and lymphoid cells make up the innate immune system, I’ll focus mostly on myeloid cells like neutrophils, macrophages, and dendritic cells.

Innate immune cells typically act at the site of infection. If you eat something that contains an intestinal pathogen or stomach bug, the innate cells in your gut will launch the first attack against it. Innate cells need to act fast, so they recognize patterns that are common among pathogens rather than pieces of specific bacteria or viruses. That means your innate cells will register “bacteria that use a whip-like structure to move around” rather than “Salmonella enterica Typhimurium.” That pattern recognition results in innate cells releasing their defenses against the bacteria, which are often ready-made and stored in pockets inside the cells.

This early defense mechanism is critical for keeping the vast majority of nasty stuff in check. But sometimes you need a more sophisticated approach.

Adaptive immunity

As an example of the lymphoid cells acting in adaptive immunity, I’ll focus on T cells.

Unlike neutrophils or macrophages, T cells cannot directly recognize pathogens. They need help from other cells in order to respond to a threat. An innate cell, often a dendritic cell, will become activated at the site of infection, then travel to an organ that specializes in immune activation. It will display parts of the bacteria on its surface and present them to a lot of T cells, along with additional information that specifies “Salmonella enterica Typhimurium.” Eventually it will find a T cell that recognizes the part of the bacteria, which will then make copies of itself and tools to fight against the infection. Finally the T cells will travel to the site of infection to fight the pathogen. As you can imagine, this takes considerably longer than the innate immune response.

Innate and adaptive immunity work together to keep us healthy. Before these systems get activated, however, there’s a first line of defense that pathogens have to make it through. Can you name it?

Two flavors of immune cells: myeloid and lymphoid

The distinction between myeloid cells and lymphoid cells

We recently covered the fact that hematopoietic (blood) stem cells can make all the blood cell types in the body, including white blood (immune) cells. The immune cells are broadly broken down into two categories: myeloid and lymphoid.

 

Myeloid cells

Myeloid cells include neutrophils, macrophages, dendritic cells, mast cells, and granulocytes. These cells tend to be found in the tissues- your skin, organs, and other locations where an infection might rear its head. They have three main jobs.

1. Pick up information from their environment, which will tell them if any threats are nearby.
2. Kill infected cells and pathogens like bacteria.
3. Communicate with other immune cells, including lymphoid cells, sounding the alert when danger is detected.

Myeloid cells are part of the innate immune system.

Lymphoid cells

Lymphoid cells, or lymphocytes, include T cells, B cells, and natural killer (NK) cells. As the name suggests, these cells are prominent in the lymph, a fluid that drains from the tissues containing cells and debris. These cells tend to have a large nucleus and few granules (the pockets that contain their ammunition). Lymphocytes mount a more sophisticated, targeted attack against threats than myeloid cells. B cells and T cells are part of the adaptive immune system, while NK cells are known to have properties of innate and adaptive immune cells.

We think about myeloid and lymphoid cells separately, since they are made and live in distinct areas in the body, plus they have different jobs. But as we continue, you’ll see how closely linked they must be to keep us healthy- no immune cell is an island.

Next time we’ll talk about the key differences between the innate and adaptive immune responses. As you may guess from the names, innate immunity uses techniques that cells are “born” with to fight infections. Adaptive immunity requires adaptation. For me, that’s where things start to get really interesting!

Drop me a line in the comments if you have thoughts about which type of cells, myeloid or lymphoid, would respond first if you cut your finger.