Beginner's Guide to the Immune System: Innate and Specific Responses

Understanding the Immune System

This overview of immunology will help you understand the basic features of the immune system. It will also be useful to revisit this article after you learn more details so you can see how everything fits together regarding how the immune system works.

Innate vs. Specific Immune Response

The immune system has two important parts: the specific immune system and the innate immune system. The innate immune system has components that are already at the infection site and respond immediately with a general response.

The specific immune system includes specialized T and B cells that are located away from the infection site. When they are activated, they launch a detailed response against the specific invading pathogen. The specific immune system takes longer to respond, especially when a pathogen enters the body for the first time.

We will discuss both systems and also cover the complement system, which works alongside the innate and specific immune systems.

The Role of Pathogens

Let’s look at what happens when a pathogen invades the body. First, pathogens must break through physical barriers before an infection can occur. The skin is one of these barriers, and most invaders are blocked by it.

However, sometimes an invader can get through, like when there is a cut in the skin. Other physical barriers include the mucosa of the respiratory system, gastrointestinal tract, and urinary tract.

There are also chemical barriers that destroy harmful pathogens before they can cause an infection. Examples include hydrochloric acid in the stomach, lysozyme in sweat and tears, and lactic acid in the vagina.

Once a pathogen breaks through these barriers, three things happen:

1. Macrophages recognize the pathogen and activate the innate immune system.
2. Dendritic cells are like scouts for the immune system. They gather information about germs and then move through the blood and other parts of the body to find specialized fighters called T and B cells. Once they locate these cells, they help activate them to respond to the infection.
3. When a harmful germ enters the body, it triggers a part of our immune system called the complement system. This system can be activated in a couple of ways to help fight off the invader.

The body's innate immune system delivers a rapid response to infections. The first responders are macrophages. They recognize pathogens by specific traits that only pathogens have. These traits are called pathogen-associated molecular patterns (PAMPs). Macrophages detect PAMPs using different receptors, including toll-like receptors.

Phagocytosis and Its Mechanism

Next, let’s discuss phagocytosis. This is the process that macrophages and other cells, like neutrophils, use to destroy pathogens. When our immune cells spot an invader, like a germ, they wrap around it and pull it into themselves. This invader ends up in a special compartment known as a phagosome.

Then, this phagosome combines with another structure called a lysosome. Lysosomes contain powerful enzymes that break down the germ, destroying it and turning its remains into harmless waste. If the invaders are not too big or complicated, specialized immune cells called macrophages can easily get rid of them, and everything goes back to normal.

However, if there are too many pathogens, macrophages call for help. They release cytokines, which are proteins that signal an infection in the area, similar to local hormones. This triggers inflammation. The main job of these cytokines is to recruit and activate more immune cells, such as additional macrophages, monocytes, and neutrophils

Monocytes are cells that can become macrophages once they enter tissues. Neutrophils also circulate in the blood and can move into tissues to help defeat invading pathogens. Inflammation includes several processes that help control and fight the infection. These include widening blood vessels, increasing how permeable blood vessels are, activating mast cells to release more cytokines, and kicking off clotting and kinin systems.

Inflammation also stimulates macrophages and neutrophils to release more cytokines, especially interleukins. This is called the acute phase response and leads to a wider inflammatory response. It sends interleukin-1 to the brain to produce fever, raising body temperature to levels that many pathogens struggle to handle, while also causing loss of appetite and fatigue. Interleukin-6 goes to the liver to produce acute phase proteins known as opsonins. Opsonins are important for the immune system as they make it easier for macrophages and neutrophils to recognize and engulf pathogens.

Interleukin-8 recruits and activates more neutrophils, while interleukins 2 and 12 activate natural killer cells. Tumor necrosis factor alpha also contributes to these responses on its own.

The Importance of Opsonins

Now, let's focus on opsonins. These are complex molecules that attach to pathogens, helping macrophages and neutrophils recognize and eat them. You can think of opsonins like a fork that spears a pathogen, allowing the macrophage to grab it and consume it. 

One example of a substance that helps the immune system is called C-reactive protein (CRP). This protein is made by the liver when the body responds to certain signals, such as a type of molecule known as interleukin-6. Doctors measure the level of CRP in patients to assess inflammation in the body and gauge the severity of an infection.

To sum up, the innate immune system is a general defense that recognizes and reacts to invading pathogens by causing inflammation and recruiting cells that destroy the pathogens through phagocytosis.

Overview of the Complement System

Let's briefly look at the complement system. This system works with both the innate and specific immune systems to help destroy germs. It consists of a series of proteins named C1 to C9.

When the complement system is activated, these proteins start a process called the complement cascade, where they activate each other. The products of this cascade have important roles, such as marking germs for destruction, triggering more inflammation, and directly attacking pathogens.

Activation and Pathways of the Complement System

The complement system can be triggered in three ways. The lectin and alternative pathways are activated directly by pathogens, while the classical pathway is activated by complexes formed by antibodies and antigens from the specific immune system.

Structure and Function of the Specific Immune System

Now, let's look at the specific immune system. This system consists of two primary kinds of immune cells called B cells & T cells, both of which are lymphocytes. They move through the lymphatic system and blood but mainly stay in lymph nodes and mucosa-associated lymphoid tissue. Think of these lymphoid tissues as bases where soldiers wait for news of an enemy they are trained to fight.

Pathogens have unique molecules called antigens. Each T cell has a receptor that is specific to one type of antigen. Likewise, B cells have antibodies on their surface that are also specific to one kind of antigen. As a result, there are millions of different T and B cells, each targeting a specific antigen.

When a new pathogen infects the body, the specific T and B cells need to be alerted. However, these cells are located in lymphatic tissues, while the infection may happen elsewhere. That’s where dendritic cells come in. They act as messengers, picking up antigens at the infection site, displaying them on their surface, and bringing them to lymphatic tissue.

Once they arrive, T and B cells examine the antigens to see if they recognize them. When specific T and B cells are identified, they become activated. This starts the specific immune response, with the dendritic cell presenting antigens on HLA class II molecules to CD4 cells, which are T cells. These CD4 cells multiply and turn into T helper cells.

T helper cells then present antigens on their HLA class I molecules, which CD8 cells can recognize. They also release cytokines that help CD8 cells multiply and change into cytotoxic T cells. T helper cells stimulate B cells to grow and develop into plasma cells, which release many antibodies, as well as memory B cells that help respond quickly to future infections by the same pathogen.

Additionally, T helper cells travel to infected areas and release cytokines to recruit monocytes and macrophages to the site and activate macrophages to create inflammation and start the process of phagocytosis.

Cytotoxic T cells kill cells that have been infected by pathogens, like viruses. They do this by attaching to the infected cell using their T cell receptor and an HLA class one molecule that shows the relevant antigen protein.

Once attached, they have two ways to kill the infected cell. The first way is called granule exocytosis. In this process, they release enzymes that break down the infected cell’s membrane, leading to its death. The second way is by activating the FASP molecule, which acts like a self-destruct switch and causes the cell to go through apoptosis.

Role of Antibodies and Immune Cells

Plasma cells and antibodies are key parts of the specific immune system. Plasma cells are B cells that have matured into antibody-producing cells. Their role is to produce many antibodies that specifically target the invading pathogen.

Antibodies are proteins shaped like a Y. One end has a variable shape that matches different antigens, while the other end has a fixed shape recognized by other immune system cells. These proteins move through the blood and attach to matching antigens.

Antibodies help the immune system fight pathogens in several ways. First, they can bind to harmful toxins, which are also antigens, and neutralize their effects. Second, antibodies can attach to the surfaces of viruses and bacteria, blocking them from functioning.

For example, they can stop viruses from recognizing and invading cells. Third, antibodies can clump together with pathogens, which slows down the spread of those pathogens. This process is called agglutination.

Finally, some pathogens are difficult for macrophages and neutrophils to recognize. Antibodies can help by acting as markers. When they attach to a pathogen, they make it easier for macrophages and neutrophils to find and destroy that pathogen. This is a summary of how the immune system works.

The Impact of Diet on Our Immune System

Let's talk about diet, which plays a critical role in our immune system, impacting both the innate and adaptive responses. 

The body's primary line of defense against threats is the innate immune system. It quickly jumps into action when something harmful invades, such as by causing a fever to heat up the body or producing special proteins called interferons that help fight off illness. This system is essential for protecting us from viruses like COVID-19 and the flu. 

Our immune system has two main parts. The first responds quickly to germs, while the second, the adaptive immune system also known as specific Immune System, learns to recognize specific germs over time. It produces antibodies that help eliminate these pathogens, improving our defense mechanisms with each encounter.