Humoral Immunity

The humoral immune response, also known as antibody-mediated immunity, is part of the adaptive immune system. The immune system is what protects an organism from disease, and is activated when antigens, any sort of substance that triggers an immune response, are found in the body. Once these antigens are detected, B cell lymphocytes, a type of white blood cell, undergo a differentiation process that creates memory B cells and effector B cells.

Since the humoral immune response is part of the adaptive immune system, memory B cells remember the current pathogen, bacteria, or viruses that infect the body, making the production of antibodies faster in the future, and the effector B cells release antibodies.

The types of humoral immunity

There are two types of humoral immunity: active and passive.

Active humoral immunity

Active humoral immunity occurs when an individual produces their own antibodies and memory cells in response to a pathogen. This lasts for decades, as memory cells reside in the body for a long period of time after being produced. This immunity either comes naturally or artificially.

If the active humoral immunity occurs naturally, then it means that the body has fought off the pathogens from a natural infection. Artificial immunity comes from a vaccine. Vaccines are made up of weakened pathogens for the body to fight off, so disposing of full-strength pathogen infections in the future will be quicker and easier.

Passive humoral immunity

Passive humoral immunity occurs when an organism passes antibodies to another. Unlike active humoral immunity, it doesn’t use memory cells and typically only lasts for a few months. It also has natural and artificial types. Two examples of natural passive humoral immunity are the antibodies that a mother passes to her baby through breast milk and the antibodies that a mother passes to a fetus via the placenta.

An example of artificial passive immunity is immunisation, a way someone gains protection against disease due to vaccination. While vaccination provides immediate protection, it doesn’t always last forever, depending on the disease being vaccinated against. People can get immunised through their doctors and healthcare facilities.

Role of B cells in the immune system

B cells, created in the bone marrow, play an important role in the immune system by producing antibodies. Antibodies are important proteins that act to neutralise a pathogen by binding to them. These antibodies are known as neutralising antibodies, and they have a few different ways they can disable a pathogen:

• Block the virus’ attachment to a healthy cell or prevent it from entering the body in the first place.
• Bind to a capsid protein. This is a type of protein where the genetic information of a pathogen is kept.
• Prevent a pathogen from changing shape in order to sneak inside a healthy cell and replicate.

Once the B cells identify antigens, they differentiate into either memory B cells or effector B cells.

Effector B cells, also called plasma cells, are activated in response to antigen exposure. This induces these cells to synthesise and secrete antibodies specific to the antigen. Each effector B cell is complementary to an antigen, which means each effector B cell will produce a unique antibody for one type of antigen. As you can see in the figure below, when B cells can bind to their complementary antigens, they need a T cell to help them divide into plasma cells. Also, they do not live as long as the memory B cells.

Memory B cells help out the secondary immune response by remembering the pathogens that have invaded the body. That way, the other immune cells in the body can produce antibodies faster in the future if they invade again. These cells are considered long-term immunity and can live for a very long time: they can even survive for up to two decades.

Fig. 1 - Steps involved in B cell activation and antibody production

Difference between B and T cells in the immune system

There are only two types of B cells found in organisms: memory B cells and effector B cells. A difference between B and T cells is that B cells recognise antigens on the surface of pathogens and T cells recognize antigens expressed on the membrane of an infected cell.

There are four types of T cells that each perform their own task to fight off pathogens in the body. These T cells are:

• Helper T cells activate any immune system cells needed to fight off disease.
• Memory T cells can remember previous infections to help other cells produce antibodies faster.
• Cytotoxic T cells destroy the pathogens found in the body.
• Regulatory T cells help the body know when to stop the immune response processes.

Fig. 2 - Different types of T cells and their specific roles

Effector T cells are created once freshly created T cells (known as naive cells because they are not ‘assigned’ to fight off a specific pathogen) receive signals from the body to transform. Just like the effector B cells, effector T cells live for a short period of time.

Helper T cells are essential for fighting off pathogens in the body by multiplying and producing cytokines, proteins that are essentially the immune system’s messengers. Helper T cells then summon the cytotoxic T cells to the location.

The cytotoxic T cell response to virally infected cells is to kill the infected cells in the body such as cancer cells

and pathogens. They kill the infected cells by using apoptosis. Apoptosis occurs by cytotoxic T cells releasing proteins, known as perforin and granzymes, that will cause the infected cells to die.

Regulatory T cells make sure helper T cells and cytotoxic T cells stop their tasks once the threats have been taken care of. Otherwise, they will harm healthy cells that are meant to be in the body.

Memory T cells are not effector cells, but they still have an important role to play in the immune system. These long-living cells are able to remember pathogens, so they will be able to fight them faster the next time they are found in the body. Memory T cells typically come from helper T cells who transform into memory T cells once the pathogen is taken care of.