Immunology Basics: Self-Markers, MHC Proteins, and Antigens

Protein Markers and Recognition Mechanisms
- Every cell and component within a human body possesses identifying markers that allow the immune system to recognize them as "self."
- These markers are primarily composed of proteins or glycoproteins (which are defined as proteins with sugar molecules attached to them).
- The primary function of these markers is to ensure that immune system cells do not mistakenly attack the host's own body.
Major Histocompatibility Complex (MHC)
- The immune system identifies self-identity through specific markers known as $MHC+1$ and $MHC+2$ (also referred to as MHC class I and MHC class II).
- Self Antigens: While the term "antigen" usually refers to foreign substances, $MHC+1$ and $MHC+2$ are often referred to as "self antigens" because they are the specific structures present on the body's own cells that the immune system identifies to determine a response or lack thereof.
- Distribution of MHC Markers:
  - $MHC+1$ : These markers are present on almost every cell in the human body, with a few notable exceptions.
  - $MHC+2$ : These markers are found specifically on specialized immune cells.
- Critical Exclusion: Red blood cells do not have MHC markers on their surface.

Organ Transplants
- When performing a transplant, medical professionals must "match" the donor and recipient. This matching process specifically targets the self-antigens present on the cells rather than just red blood cell markers.
- The objective of matching is to ensure that the recipient's immune cells do not recognize the transplanted organ as foreign and proceed to attack and destroy the tissue.
Blood Transfusions and Safety
- The administration of the incorrect blood type is highly dangerous and most likely fatal to the patient.
- Historical Perspective: In earlier eras, blood transfusions were attempted after severe injuries using a "trial and error" method. These attempts did not necessarily cure the patient and often resulted in death.
- This history of failure led to research into why certain blood types are accepted or rejected, leading to our current understanding of immunology and blood typing.

Definition of Antigens
- The term "antigen" is derived from the phrase "antibody generating."
- An antigen is defined as anything that stimulates an immune response in the body.
Allergens as a Special Category
- Allergens are technically a type of antigen, though they are unique because the body should not theoretically respond to them at all.
- When a body has an allergic response, it is a specific response to something harmless (an allergen) that triggers the generation of antibodies and an antigenic reaction.
- Allergic reactions are extremely common, with the speaker noting that roughly three-quarters of a standard population may suffer from "atopy" (AP) or some form of allergy.

Question regarding gut bacteria: A student asked how consultants might help gut bacteria.
Response: The speaker deferred this topic, stating it would be covered the following week. At this stage, the focus remains exclusively on the basics of self-recognition and markers.
Dialogue on Blood Transfusions: A student commented on the pain or sickness caused by the wrong blood.
Response: The speaker clarified that it's more than just getting sick; historically, without matching, the process resulted in death, which prompted the research path into immunology.
Pathogen Discussion: The speaker mentioned they have a "favorite pathogen" and encouraged students to find one for discussion in the next session.

Key Vocabulary Summary:
- Antigen: Antibody generating; any substance stimulating an immune response.
- Self-Recognition: The process of flagging cells as "self" using surface markers.
- Markers: Specifically $MHC+1$ and $MHC+2$ .
- Allergens: Substances that should not act as antigens but trigger an immune response in certain individuals.
- Glycoprotein: A protein with a carbohydrate (sugar) attached.
Reading Assignments and Curriculum Path:
- Section 6a: Covers the bulk of definitions and essential background information.
- Section 6b: Contains a significant volume ("bomb load") of information regarding pathogens; students are expected to read and explore this section.
- Section 6c: Focuses on small pathogens.
- Upcoming Topics: The course will eventually cover the specific cell types and chemicals involved in allergic responses once the basics are mastered.