Antibodies (Basic science review)

1. Blood cell lines
2. Lymphocytes
3. Antigens
4. Structure of antibodies
5. Classification of antibodies
6. Passive vs. active immunity
7. Autoimmune diseases
8. Monoclonal antibodies
9. Conjugation
10. Uses for antibodies
    1. ELISA
    2. Rapid Antigen Tests
    3. Immunofluorescence
    4. Purification
    5. Monoclonal antibody therapy

Blood cell lines

Blood cells are produced in the bone marrow, starting with a stem cell that is able to turn into various “blast” cells. When the blood cells are mature, they move from the bone marrow to the blood stream.

Although this diagram looks more complicated, blood cells can be thought of as having three different types:

Blood Cell Type	Function
Erythrocyte (Red blood cell)	Carries oxygen
Leukocyte (White blood cell)	Fights infection
Thrombocyte (Platelet)	Forms blood clots

White blood cells are classified as either myeloid cells or lymphocytes.

Lymphocytes

There are three types of lymphocytes: B cells, T cells and natural killer cells. The ones we are interested in are B cells. B cells produce antibodies.

The human body contains millions of B cells which all produce a different type of antibody. Before an antigen attaches to the B cells, the B cell only has antibodies on its surface. Once an antigen binds to the B cell, the B cell is activated and converts into a plasma cell, producing large amounts of antibodies that it puts into the blood stream.

Antigens

An antigen is any molecule that can cause an immune response. Typically, an antigen is a protein or polysaccharide molecule¹. Very small molecules cannot be antigens. For instance, you cannot develop an immune response against a water molecule.

Structure of antibodies

An antibody is a protein that is made of 4 strands of amino acids. There are two bigger polypeptides (called heavy chains, yellow and blue in the diagram) and two smaller polypeptides (light chains, green and red in the diagram). These 4 polypeptides bind together to make one protein. The antibody has two binding sites where an antigen can attach.

To simplify drawings, an antibody is typically drawn as a Y-shaped structure with two longer lines and two smaller lines.

Classification of antibodies

There are 5 classes of antibodies in humans:

Class	Function
IgA	found on mucosal surfaces (wet surfaces) like the inside of your nose, mouth, lungs, gut and urinary tract. These fight off infection before it actually invades.
IgD	these act as B cell receptors on B cells before they become activated and form plasma cells
IgE	cause allergic reactions, but also helps fight parasitic infections
IgG	this is the main class of antibodies that fight off infection in the blood stream
IgM	these are formed immediately after infection and then drop off as IgG becomes the dominant antibody later

Antibodies may stick together to form dimers (IgA) or pentamers (IgM).

Passive vs. active immunity

If a person is exposed to an antigen and they begin producing antibodies against the antigen, this is called active immunity.

If instead, a person is given an antibody, this is called passive immunity. These antibodies will eventually break down and the person is not able to make more of those antibodies because they were never exposed to the antigen.

Examples of passive immunity:

• IgG passes through the placenta from the mother to the baby to give the baby an immune system when it is first born
• IgA is passed from the mother through breast milk to the baby
• Immunoglobulin against rabies is given along with a rabies vaccine to protect a patient until they are able to develop their own antibodies to the vaccine
• Rattlesnake antivenom is produced by injecting rattlesnake venom into a horse which then produces antibodies that are injected into a snakebite victim to neutralize the venom.

Examples of active immunity:

• A person gets a viral infection and then develops antibodies against the virus so they will not be susceptible to the infection again
• A person is given a vaccination against a disease and develops antibodies against the antigens in the vaccine

Autoimmune diseases

Usually, your immune system is able to distinguish between antigens that are part of your body (“self”) and antigens that come from outside your body (“foreign”). This ability of distinguishing “self” from “foreign” is very important to keep your immune system from attacking your own body. Unfortunately, sometimes things go wrong, and a person can develop an autoimmune disease, in which they develop antibodies that attack their own cells.

One example of this is type I diabetes mellitus². The level of glucose (blood sugar) in the blood is controlled by insulin which is produced by specialized cells in your pancreas called beta cells. If all of the beta cells are destroyed, then a person cannot produce insulin, and they develop type I diabetes. One way that these cells can be destroyed is by developing antibodies against the beta cells.³

Monoclonal antibodies

Monoclonal antibodies are antibodies that are all produced by one type of cell that is created in the lab for a specific use.

Steps to producing monoclonal antibodies:

1. A mouse or rabbit is injected with an antigen which stimulates it to develop plasma cells that produce antibodies against that antigen.
2. The plasma cells are removed from the animal.
3. The plasma cells are merged with cancer cells to produce hybridomas, which continue to produce antibodies, but now survive and reproduce in the laboratory like cancer cells. These many different cells are making many different antibodies that all bind to the same antigen. These antibodies would be “polyclonal”.
4. A specific cell that makes antibodies against the antigen is selected and cloned into millions of cells. All of these cells will produce identical antibodies. Because these antibodies are produced by clones of a single cell, they are called “monoclonal”.

There are currently over 500 different monoclonal antibodies that have been developed.⁴

Conjugation

The antibodies are often linked to another molecule. This linking process is called conjugation.

Antibodies are often conjugated to:

• a fluorescent molecule
• an enzyme that causes a color change due to a chemical reaction
• a molecule containing a radioactive atom

The molecules that are conjugated to the antibodies are often used to be able to visually detect the presence of the antibodies.

Uses for antibodies

Because of the unique ability for antibodies to be very specific about what they attach to, and to be made to attach to almost anything, they have many uses in laboratory science and medical therapy.

Some of these uses include:

• Testing
• Purification
• Treatment

ELISA

ELISA stands for enzyme-linked immunosorbant assay.

This test allows for rapid testing of many samples at one.

Rapid Antigen Tests

Common rapid antigen tests include:

• Covid infection
• Strep throat
• Pregnancy

Immunofluorescence

Immunofluorescence is a microscopy technique that allows specific structures within a cell to be visualized by binding antibodies that are conjugated to a fluorescent molecule. After the antibodies attach to the structure in the cell, the structure can be visualized by shining ultraviolet light on it, causing the fluorescent molecule to give off light that can be seen under a microscope.

Purification

One method of using antibodies to isolate and purify a sample is called immunoprecipitation.

Monoclonal antibody therapy

Monoclonal antibodies can be used as medical therapy in many diseases including:

• Infection
• Autoimmune disease
• Cancer

Drug names of monoclonal antibodies have an invented root “-mab” at the end of their name to indicate they are monoclonal antibodies. For instance, infliximab is a monoclonal antibody that targets an inflammatory molecule called TNF-α⁵ and is used to treat autoimmune diseases such as rheumatoid arthritis, ulcerative colitis, Crohn disease and ankylosing spondylitis.