Immune Response

What does the innate immune response do?

The innate immune response, aka non-specific immune response, defends the body against non-specific pathogens and is considered the first line of defence. Parts of the body such as skin and mucus form the innate immune response by keeping out pathogens. The mucus in our nose traps the pathogens from getting inside our bodies, and they also use their form of bacteria to destroy the stuck pathogens. In addition, various other cells are involved in the innate immune response as they carry out 'cell eating' – phagocytosis. These additional cells include neutrophils, macrophages, and monocytes.

What do the cells in the innate immune response do?

The cells in the innate immune response are white blood cells that include neutrophils, macrophages, and monocytes.

• Neutrophils are the most abundant type of white blood cell and are the first to fight back against a pathogen. They can do this by releasing enzymes that digest and kill pathogens.
• Macrophages are another type of white blood cell made in the bone marrow. These macrophages play a role in inflammation by releasing cytokines – small proteins that help with inflammation and act as messengers between other cells.
• Monocytes are another type of white blood cell and can turn into macrophages. They also play a role in the adaptive immune system.

Despite their many differences, they are all able to perform phagocytosis

What does the adaptive immune response do?

While the innate immune system remains the same from birth, the adaptive immune system can learn over time. The adaptive immune response works to fight off pathogens and is especially effective against those it has destroyed before. Each time it encounters a pathogen, it will remember it in its memory, and it will be faster to defeat each time it sees it.

There are only two types of cells in the adaptive immune response, T and B cells. T and B cells come from stem cells located in the bone marrow, and they are both classified as lymphocytes, which means they are the primary type of cell found in the lymph.

Fig. 1 - T cells split up into other T cells or B cells after being attacked by antigens

Although it is effective, it is slower to respond than the innate immune response due to it needing to identify the antigen before responding. This immune response splits into two types – humoral and cellular immunity. The adaptive immune response also requires fewer cells than the innate immune response.

What is humoral immunity?

Humoral immunity, or antibody-mediated immunity, is mediated by B cells. B cells move through the lymphatic system, where they encounter an antigen. Suppose the antigen matches the specific antibody on their cell surface, a membrane-bound antibody. In that case, they either split into a memory B cell, or an effector B cell occurs when plasma cells produce antibodies and allow them to move throughout the body.

Humoral immunity splits into both primary and secondary immune responses.

What are primary and secondary immune responses?

Primary immune responses occur when the body encounters a pathogen for the first time. These immune responses use the naive B and T cells. As the B and T cells have not transformed into their effector and memory cells, they are slow to respond to the threat to the point where it takes several days to react.

In contrast, the secondary immune response uses the memory forms of the B and T cells. Therefore, they respond in about half the time the primary response takes. They also produce higher levels of antibodies compared to the primary immune response.

This quicker response is why someone can never be infected with chickenpox again for the rest of their life.

What is cellular immunity?

Cellular immunity involves the activation of lymphocytes upon encountering a 'non-self' material and is mediated by T cells. T cells are created in the bone marrow but travel to the thymus, a small organ located in the chest to get transformed into T cells. In the thymus, they develop T cell receptors where they will get a T cell receptor named CD4 or CD8. The different receptors allow the cells to perform different tasks.

Therefore, there are three types of T cells. Their jobs depend on what T cell receptor they receive. The Helper T cells have CD4 and help B cells and other cells found in the immune response. Cytotoxic T cells have CD8 and destroy pathogens plus infected self cells.

The receptors also need to perform recombination of genes that express these receptors. It is called rearrangement, and it allows for a large amount of diversity in the genes. Still, occasionally a mistake happens, and the gene is rearranged into an identical gene found in a person. This mistake leads to the body attacking itself. To prevent this response, T cells are tested twice by the body. The first test is to make sure they can tell the difference between their body's proteins and the proteins of a pathogen. The second test is to see how it will bind. If it attaches to something that is not an antigen or itself, it will be eliminated.

How do vaccines help the immune response?

Vaccines help the immune response by pretending to be a pathogen. This makes the immune system believe that it is sick and needs to produce antibodies. Now that the immune system remembers how to fight off this pathogen, it can fight it off faster if the body ever actually becomes infected. The figure below shows a quick diagram of how it works.

Fig. 2 - Vaccination immune response steps