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🔅 Antigens are molecules that can trigger an immune response by binding to specific antibodies or T-cell receptors. They can be proteins, peptides, polysaccharides, lipids, or nucleic acids.
– Antigens can originate from outside the body (foreign antigens) or from inside the body (self-antigens).
🔅Self-antigens are normal components of the body's own cells that are recognized as "self" by the immune system and do not elicit an immune reaction. However, sometimes the immune system may mistakenly attack self-antigens, causing autoimmune diseases.
TYPES OF ANTIGENS
• There are different types of antigens based on their origin, structure, function, and immunogenicity. Some of the common types are:
▶️ Exogenous antigens: These are foreign substances that enter the body through inhalation, ingestion, injection, or skin contact. They include bacteria, viruses, parasites, fungi, allergens, and transplanted organs.
▶️ Endogenous antigens: These are generated inside the body due to viral or bacterial infections, cellular metabolism, or tumor growth. They include infected cells, cancer cells, and red blood cell antigens.
▶️ Autoantigens: These are self-antigens that become targets of the immune system due to genetic or environmental factors that impair immunological tolerance. They include proteins or nucleic acids that are involved in autoimmune diseases such as rheumatoid arthritis, type 1 diabetes, and multiple sclerosis.
▶️ Tumor antigens: These are antigens that are expressed on the surface of tumor cells and can induce an immune response against cancer. They include tumor-associated antigens (TAAs), tumor-specific antigens (TSAs), neoantigens, and oncogenic antigens.
▶️ Immunogens: These are antigens that can induce an immune response on their own. They have the ability to activate the immune system and trigger antibody production or T-cell activation. They are usually large, complex, and foreign molecules such as proteins or polysaccharides.
▶️ Haptens: These are small, non-protein molecules that cannot induce an immune response by themselves. They need to be attached to a carrier molecule such as a protein to become antigenic. They can bind to specific antibodies once they are produced. They include drugs, toxins, hormones, and metals.
REPRESENTATION OF ANTIGENS
• Antigens are represented by their molecular structures and their antigenic determinants or epitopes.
– Epitopes are the distinct regions of an antigen that can interact with specific antibodies or T-cell receptors. Each antigen can have multiple epitopes that can elicit different immune responses. Epitopes can be linear (continuous) or discontinuous (conformational) depending on whether they are composed of sequential or non-sequential amino acids in a protein antigen.
• MHC self-antigens are molecules that present peptides derived from self-proteins or pathogens to T cells for recognition and activation. MHC stands for major histocompatibility complex, which is a group of genes that encode for these molecules. MHC molecules are essential for adaptive immunity and tissue compatibility.
CLASSES OF MHC SELF ANTIGENS
• There are three classes of MHC self-antigens: MHC class I, MHC class II, and MHC class III.
🔅MHC class I molecules are expressed on all nucleated cells and present peptides from endogenous antigens (such as viral or tumor proteins) to cytotoxic T cells (CD8+). They consist of a polymorphic heavy chain and a non-polymorphic beta-2 microglobulin chain.
🔅 MHC class II molecules are expressed on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells and present peptides from exogenous antigens (such as bacterial or parasitic proteins) to helper T cells (CD4+). They consist of two polymorphic chains: alpha and beta.
🔅 MHC class III molecules are not involved in antigen presentation but rather in other immune functions such as complement activation, inflammation, and cytokine production. They include proteins such as C4, C2, factor B, TNF-alpha, and TNF-beta.
TYPES OF BLOOD ANTIGENS
• Types of blood antigens are molecules that determine the blood group of an individual based on their presence or absence on the surface of red blood cells. There are several blood group systems that classify blood antigens into different categories. The most common ones are:
🔅 ABO system: This system is based on the presence or absence of two antigens: A and B. There are four blood types: A (has only A antigen), B (has only B antigen), AB (has both A and B antigens), and O (has neither A nor B antigens). The ABO system also involves antibodies that can react with the opposite antigens: anti-A, anti-B, anti-AB, and anti-O.
🔅 Rh system: This system is based on the presence or absence of a protein called the Rh factor or D antigen. There are two blood types: Rh positive (has the Rh factor) and Rh negative (does not have the Rh factor). The Rh system also involves antibodies that can react with the Rh factor: anti-D.
🔅Other systems: There are many other blood group systems that involve different antigens such as Kell, Duffy, Kidd, Lewis, MNS, and P. These antigens can also cause transfusion reactions or hemolytic disease of the newborn if they are incompatible between the donor and the recipient or the mother and the fetus.
CHARACTERISTICS OF ANTIGENS
– Characteristics of antigens are the features that determine their ability to induce an immune response and to interact with specific antibodies or T-cell receptors. Some of the important characteristics of antigens are:
▶️Foreignness: Antigens should be foreign to the host to elicit an immune response. The more different they are from the host's own molecules, the more immunogenic they will be.
▶️ Molecular size: Antigens should have a sufficient molecular size to be recognized by the immune system. The optimal size is between 10,000 and 100,000 Da. Smaller molecules (such as haptens) need to be conjugated to larger carriers to become immunogenic.
▶️ Chemical composition: Antigens should have a complex and diverse chemical composition to provide multiple epitopes for antibody or T-cell receptor binding. Proteins and polysaccharides are the most immunogenic molecules, while lipids and nucleic acids are less immunogenic.
▶️ Heterogeneity: Antigens should have a high degree of heterogeneity or variability to avoid immune tolerance and escape. Antigens that change their structure frequently (such as influenza virus) can evade the immune system by generating new epitopes that are not recognized by existing antibodies or T cells.
