If you’re getting ready for the MCAT test, start by reviewing the subjects outlined in the MCAT syllabus. The immune system is a crucial topic on the syllabus and is part of the Biological and Biochemical Foundations of the Living System sections of the exam. This comprehensive guide aims to give you all the essential information about the immune system for the MCAT exam.

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The Immune System: An Introduction

The immune system is how humans and other species defend against diseases and eliminate harmful agents that enter the body. It identifies foreign organisms like bacteria, viruses, parasites, and fungi, working to either kill or disable them to prevent harm. Additionally, the immune system targets mutated, cancerous cells, and plays a role in autoimmune and autoinflammatory conditions.

Innate and Adaptive Immune System

The immune system is divided into two main components: the innate immune system and the adaptive immune system. The innate system responds immediately when a pathogen enters the body, aiming to eliminate organisms regardless of type. It involves initial actions by cells like natural killer cells and macrophages. The adaptive immune system, led by memory B cells, specializes against specific pathogens, enabling a targeted response to prevent or minimize symptoms of diseases.

Innate Immune Cells

Macrophages

Macrophages are crucial innate immune cells found in various body parts. They engulf pathogens, digest them, and present antigens on the Major Histocompatibility Complex. Macrophages release cytokines, which cause inflammatory reactions and attract more immune cells to the infection site.

Phagocytes

Phagocytes, including monocytes, neutrophils, dendritic cells, and macrophages, are capable of phagocytosis—engulfing and digesting foreign pathogens. Approximately 6 billion phagocytes exist per liter of blood, playing a significant role in the body’s defense.

Adaptive Immune Cells

T-Lymphocytes

T-lymphocytes, divided into killer T cells, T helper cells, and T regulatory cells, are vital in the adaptive immune system. Killer T cells eliminate infected or cancerous cells, while T helper cells activate B cells, leading to antibody production. T regulatory cells regulate T cell differentiation and prevent attacks on self-antigens.

B-Lymphocytes

B-lymphocytes, activated by T cells, produce antibodies targeting specific pathogens. Plasma B cells undergo somatic hypermutation to create antibodies perfectly fitting pathogen antigens. Memory B cells store pathogen memories for rapid responses upon reinfection.

Immune Tissues

Bone Marrow

The bone marrow differentiates immune cells, especially B cells, which undergo maturation before circulating in the blood during infections.

Thymus

The thymus, responsible for T cell differentiation, undergoes positive and negative selection processes. Positive selection leads to the formation of mature T cells, while negative selection eliminates T cells reacting to host antigens.

Spleen

The spleen, an important lymphoid organ, filters blood components, removing red blood cells and antigens. It consists of red pulp, responsible for filtering blood, and white pulp, containing immune cells involved in adaptive immunity.

Lymph Nodes

Lymph nodes, common immune tissues, filter waste from various body parts. They contain lymphocytes distributed to different tissues to fight infection. In illnesses, infections, or diseases, lymph nodes can become inflamed (lymphadenopathy).

Clonal Selection

Clonal selection involves the differentiation and expansion of B and T lymphocytes in response to presented antigens. The lymphocytes binding most strongly to the antigen reproduce rapidly to combat infections effectively.

Recognition of Self vs. Non-Self

Recognizing self-antigens is crucial to prevent autoimmune and autoinflammatory disorders. Failure to distinguish between self and harmful antigens can lead to disorders like Type 1 diabetes and rheumatoid arthritis.

Antigens and Antibodies

Antigens, peptide molecules on pathogens, help the body recognize and differentiate between pathogens. Antibodies, produced by B lymphocytes, bind to antigens, marking them for immune cell interaction.

Structure of Antibodies

Antibodies have a variable Fab region, adapting to antigens, and a constant Fc region. The Fab region attaches to pathogen epitopes, enabling “class switching,” while the Fc region remains constant.

The Role of Antigen Presentation

The effectiveness of the adaptive immune system relies on the ability of pathogens to display antigens, triggering the production of specific antibodies to eliminate the pathogen. Antigen presentation occurs on antigen-presenting cells (APCs), which include all nucleated cells after breaking down the pathogen. These antigens are showcased on the Major Histocompatibility Complex (MHC) and recognized by T cells. Helper T cells aid this recognition by presenting antigens to B cells, leading to B cell activation, antibody class switching, and subsequent immune responses.

Recognition in Antigen-Antibody Interaction

Various antigen-presenting cells, often phagocytic, absorb and break down pathogens, presenting antigens on their surfaces through MHC. This presentation targets immature T and B cells in lymph nodes or Peyer’s patches, prompting a response. For APCs to engage TH cells, CD40 on the APC binds to CD40L on the TH cell. In the case of TC cells, a ligand like CD28 on the APC binds to the T cell receptor on the TC cell, activating both T cell types. Activated T cells then assist B cells in class switching and clonal selection through released cytokines, resulting in the production of suitable antibodies.

To learn more on Antigen-Antibody Recognition, click here.

The Major Histocompatibility Complex (MHC)

The MHC, present on all nucleated cells, facilitates the presentation of antigens on cell surfaces. There are two main types: MHC I, expressed on nearly all cells, presents endogenous non-self-antigens detected by CD8 on TC cells. MHC II, found exclusively on APCs, presents antigens to TH cells through CD4 costimulatory signals, showcasing both self and non-self-antigens, with the latter being exogenous antigens.

Conclusion

Understanding antigen presentation is crucial for comprehending the adaptive immune system. For more in-depth MCAT preparation, explore our blog covering a range of topics, including the digestive system, excretory system, and meiosis. 

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