What Is an Antigen?
An antigen is any molecular marker that can be specifically recognized by the immune system. Typically, antigens are proteins or polysaccharides (complex sugars) expressed on the surfaces of cells, pathogens, or other foreign substances. Their unique three-dimensional structures function like a molecular “nametag”, allowing immune cells to identify whether the entity is part of the body or foreign.
Antigens can be found on:
- Viruses and bacteria : marking pathogens for immune detection.
- Parasites : signaling infection to the immune system.
- Allergens : proteins capable of eliciting hypersensitivity reactions.
- Tumor cells : altered self-proteins recognized as abnormal.
- Normal host cells : termed self-antigens, which are tolerated by the immune system.
The immune system distinguishes self from non-self antigens. Self-antigens prevent autoimmunity by signaling the immune system to ignore normal body components. In contrast, non-self antigens trigger immune responses to eliminate foreign or pathogenic elements.
Antigens vs Antibodies
Antigens are molecular markers that signal the immune system that a substance is foreign or potentially harmful. They serve as the triggers for immune responses, effectively acting as antibody generators.
Antibodies, on the other hand, are highly specific proteins produced by B cells that recognize and neutralize antigens. Each antibody binds to its corresponding antigen with precision, like a key fitting into a lock, enabling the immune system to eliminate the target.
Types of Antigens
Antigens are classified based on their origin and role in the immune response:
1. Exogenous Antigens
These originate from foreign substances entering the body via inhalation, ingestion, or skin breaches. Examples include: viruses, bacteria, fungi, parasites, and pollen.
2. Endogenous Antigens
These are present on cells within the body. They help the immune system distinguish between self and non-self. Infected cells, such as those harboring viruses or intracellular bacteria, display endogenous antigens to signal destruction. Normal self-markers, including red blood cell antigens and human leukocyte antigens (HLAs), are also endogenous.
3. Autoantigens
These are self-antigens that are mistakenly targeted by the immune system, leading to autoimmune diseases.
4. Tumor Antigens
Tumor antigens are expressed on cancerous cells. They include:
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Tumor-associated antigens (TAA) :normal proteins expressed abnormally in tumors.
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Tumor-specific antigens (TSA) : derived from mutations or oncogenic viruses within tumor cells.
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Neoantigens : newly formed antigens resulting from genetic mutations.
Tumor antigens can arise either from overexpressed normal proteins or mutated proteins that alter immune recognition.
Antigen-Presenting Cells (APCs)
APCs are specialized immune cells that capture, process, and present antigens to T cells, initiating adaptive immune responses. The primary APCs are:
These cells act as detectives breaking down antigens and displaying peptide fragments on their surface via major histocompatibility complex (MHC) molecules. This functions as a wanted poster for T cells. If a T cell’s receptor matches the antigen fragment, it triggers an immune response against the invader.
Immune Response to Antigens
When an antigen enters the body:
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Recognition :B cells detect the antigen using specific surface receptors.
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Activation : Compatible B cells proliferate and produce antibodies shaped to fit the antigen.
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Binding and Neutralization : Antibodies bind to antigens, marking them for destruction.
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Immunological Memory : Some B cells become memory cells, enabling a faster and stronger response upon future encounters with the same antigen.
Challenges: Antigens can mutate, altering their structure. These changes may prevent antibodies from recognizing them, analogous to a lock changing shape so that the original key no longer fits.
